June 29, 2018

How the June 2018 Heat Wave Triggered Three Derechos in One Day

The word "derecho" is usually not allowed on this wholesome website. I try not to use the word unless we're talking about an event that's already happened or the SPC outright uses it in one of their forecasts. That being said, yesterday we saw three derechos in one evening as a result of the growing heat wave. The storms resulted in more than 500 reports of wind damage across a dozen states, claiming one life in Alabama and knocking out power to hundreds of thousands of people. All three storm complexes were strong "mesoscale convective systems," a common type of thunderstorm event during the summer and one you can almost expect to see during a heat wave.

A derecho is a strong, long-lived squall line that produces lots of wind damage along a path stretching hundreds of miles long. The derechos that formed on Thursday were fraternal twins. They were separated by more than a thousand miles, but they all came into existence because of the same ridge producing the heat wave.

The first destructive line of storms moved southwest through Georgia and Alabama on Thursday afternoon. (Another distinct line formed to its southeast in Georgia, but it only produced scattered wind damage reports.) The second derecho began life as a single supercell over Nebraska on Thursday morning that eventually grew into a squall line that raced south into Mississippi before fizzling out after midnight on Friday. A third derecho over the northern Plains produced a wind gust as high as 96 MPH in North Dakota and flipped a small airplane at an airport in Minnesota.

A mesoscale convective system, or MCS for short, is an organized line of thunderstorms that can produce damaging winds across hundreds of miles. The strongest MCSs can last for 12 hours or longer, traversing half the length of the United States before finally petering out.

Most MCSs are easy to spot on radar imagery because they're thick squall lines with a well-defined shield of rain trailing them. The storm comes on suddenly. The hurricane-like winds and rain can last up to 15 minutes before finally subsiding and giving way to a couple of hours of light to moderate rain. The winds can be so violent that people who go through an intense MCS swear they were hit by a hurricane or tornado instead.

What makes an MCS different from other types of thunderstorms?

Thunderstorms "breathe" in the sense that they inhale warm, unstable air in an updraft and exhale cool, stable air with the rain in a downdraft. The vast majority of thunderstorms end up choking on their own downdrafts as the rush of cool air, known as a cold pool, cuts off the updraft and robs the storm of the instability it needs to survive.

When upper-level winds are favorable, though, thunderstorms can merge and share one large cold pool. The storms attach themselves to the leading edge of the cold pool as it races downwind, allowing the storms to thrive as they move into more unstable air. The line of storms doesn't die until they either run out of unstable air or the cold pool "escapes" ahead of the storms, robbing them of the lift they need and causing them to choke on cool, stable air.

The intense winds that can accompany an MCS comes from the "rear inflow jet." Friction and pressure differences within an MCS cause the air to begin circulating within the complex of storms. This circulation leads to the development of a sharp jet of winds that moves from the back of the storms to the front. This rear inflow jet gets shoved to the ground by the downdraft at the leading edge of the thunderstorms, causing the sudden, intense burst of winds that gives a severe MCS its bite.

You can see a mesoscale convective system any time of the year, but they're pretty common around June and July as the intense summer heat starts building over the United States.

I mentioned the possibility when I had a conversation with the little voice in my head on Tuesday:
Q: What kind of severe weather is favored during heat waves?
A: Mesoscale convective systems are a nasty habit of heat waves, especially along the outer edge of the ridge where the dynamics for such thunderstorm events are best. An MCS is commonly known as a squall line or sometimes even a d******.

Q: What's a d******?
A: I'm sorry. That word is censored on this good, moral blog.
Even without ever looking at the Storm Prediction Center's forecast, it's a decent bet to say that severe squall lines are possible when there's a heat wave like this. All of the storm complexes we saw on Thursday formed along the outer periphery of the ridge of high pressure that's allowing it to get so darn hot this week.

Heat waves are a breeding ground for this type of severe weather because of the dynamics involved in their creation. There's usually a sharp temperature and moisture gradient along the edge of a heat-producing ridge. This gradient, a stationary front, serves as the focus along which thunderstorms can develop and organize. Strong instability allows thunderstorms to grow and thrive, and upper-level winds along and north of the ridge of high pressure allows the storms to merge and organize into a single line.

If you overlay Thursday afternoon's radar over surface temperatures at the same time, you can actually see how the thunderstorms are riding the ridge around the heat.

It's not too common to see three separate lines of storms on the same day meet the definition of a derecho, the "d-word" that meteorologists hesitate to use in public for fear of whipping their audience into a panic.

A derecho produces lots of wind damage over a path that's at least 250 miles long. 200 miles? Not a derecho. 250? Ring the alarm. The, uh, specificity of that definition is about as arbitrary as anything else in meteorology; a squall line that only lasts for 50 miles could level a town's power grid and rip the roof off the high school, but it wouldn't technically be a derecho. The term came into the national hype vocabulary after an intense derecho trashed the Washington D.C. and Baltimore metro areas on this date in 2012.

Derechos are serious business. A study conducted in 2005 found that some derechos resulted more costly damage than some of the worst hurricanes to hit the United States. While some meteorologists find endless joy in debating whether or not a squall line really meets the definition, or whether we should use the term at all due to its hypetastic reputation these days, all three of the squall lines that formed yesterday seem to fit the definition for a derecho based on the sheer number and spatial coverage of damage reports each one generated. (If you want to be a stickler and include the requirement of several 75+ MPH wind gusts, as some academic studies do, the one in North Dakota definitely passes muster.)

The Storm Prediction Center is calling for a slight risk of severe weather around the edge of the heat wave over the next couple of days. While the dynamics should be less favorable than Thursday, any threat for severe weather is worthy of attention.

[Maps: Dennis Mersereau]

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June 26, 2018

The June 2018 Heat Wave: Frequently Asked Questions, Answered

Q: Oh, great, another one of these. What's going on?
A: A formidable heat wave building across the Plains this week will spread through the southeast and the rest of the eastern United States through early next week. The map shows forecast high temperatures from the NWS between Wednesday, June 27, and Monday, July 2.

Q: Where do I live on that map?
A: You know what? If Rachel Maddow can find the point after a 20-minute monologue that begins in Victorian England and ends by name-dropping a senator who's been dead for 80 years, I believe in your ability to find your house on a map.

Q: How hot will it get?
A: High temperatures will climb into the 100s across a good portion of the Plains starting around the middle of this week and last until a brief reprieve this weekend. Parts of the southeast will see the mid- to upper-90s through the middle of next week. Highs climbing well into the 90s will be common as far north as New England and the Great Lakes as the ridge moves east through the weekend.

Q: Holy moly! Has it ever gotten this hot?
A: The internet is fun because it seems to erase our memory of past weather. Yes, it gets this hot, and no, it's not totally unprecedented. It's unusual to see such intense heat so far north and a little more unusual to see it this early in the season. But when it comes to raw temperatures, even the forecast high of 98°F in that famous tropical resort town of Burlington, Vermont, would be the 16th time they've seen a reading that hot since records began in 1940. Uncommon, yes, but not unprecedented.

Q: Why such a hullabaloo over heat?
A: Heat kills more people every year in the United States than tornadoes and hurricanes combined. We issue warnings and go on red alert television coverage for those hazards, but heat gets brushed off until it's too late for some. It's not a photogenic hazard. The most appealing heat-related images you'll get on the news are shirtless joggers in the park. The lack of frenetic coverage instills a sense of complacency in people. Hype is a double-edged sword.

Q: Will it be humid?
A: It will be humid and it will be miserable. Dew points will easily climb into the 70s from the eastern Plains all the way to the Atlantic coast. Dew points could push the upper 70s or even reach 80°F in spots, especially in the Midwest or Ohio Valley where crop sweat—no, really!—will add to the humidity to make it almost unbearable outside. The humidity will push the heat index over 100°F for several days in many spots.

 Q: Is the heat index fake? I heard it was fake.
A: No. If the heat index was a conspiracy theory it would be a heck of a lot more exciting. There would be chemtrails and aliens and stuff. The heat index comes from scientific studies that looked at how a wet heat affects your body and found that the combination of heat and humidity has the same effect on your body as a much hotter temperature. You can suffer heat-related illnesses a lot faster when it's humid.

Q: Why does humidity make the heat feel so much worse?
A: Humidity keeps you from cooling off effectively because the moisture in the air slows down the evaporation rate of sweat from your skin.

Q: Why is it getting hot?
A: It's June.

Q: Why is it hotter now than normal?
A: A big ridge of high pressure will park itself over the U.S. and Canada east of the Rockies. Air generally sinks under a ridge. Sinking air results in calm(ish) and warm weather. Ridges also allow gross tropical air to flow north from the Gulf and Caribbean and suffocate us. The term "heat dome" is annoying but I like it because it just feels...right.

Q: Will the heat break?
A: The heat in the Plains should break in about three months, but for everyone else, we'll go from "straight-up miserable" back to "deeply uncomfortable" around July 4th. The last day of the ample heat will be closer to the weekend farther west and after July 4th in the southeast.

Q: When will it get cool again?
A: I would say "Christmas" but even that's not a sure thing these days.

Q: Will it cool off at night?
A: Lows will stay in the mid- to upper-70s for the duration of the heat wave. That's not as bad as it could be, but it won't make for much relief when combined with the humidity. That could be dangerous for susceptible folks who don't have access to fans or air conditioning.

Q: What are some heat safety tips?
A: The NWS has a whole bunch of heat safety tips on their site. Let's be honest, though. If someone's gonna jog five miles or go 8 hours without drinking water when it's 98/74 outside, listening to some smart person say "don't!" or "how 'bout a Dasani, buddy?" isn't going to stop them.

Q: How can I protect my children from the heat?
A: Move the television away from the window.

Q: What's the best way to keep my dog safe from the heat?
A: Shorten the duration of their walkies, stick to shade, and keep their toe beans off the hot concrete.

Q: Is exercising outside okay?
A: I'll let you know if I ever try it.

Q: How much water should I drink to stay hydrated?
A: Lots. It's easy to underestimate how much you need to drink when it's hot...so, how 'bout a Dasani, buddy? But, rest assured, no matter how much you drink, you'll have to pee the second you leave the house.

Q: Does the heat make thunderstorms worse?
A: Does Spatini make spaghetti sauce taste better?

Q: What kind of severe weather is favored during heat waves?
A: Mesoscale convective systems are a nasty habit of heat waves, especially along the outer edge of the ridge where the dynamics for such thunderstorm events are best. An MCS is commonly known as a squall line or sometimes even a d******.

Q: What's a d******?
A: I'm sorry. That word is censored on this good, moral blog.

Q: I thought d e r e c h o had a specific definition and you couldn't say it until—
A: Nice work fooling the censor. Yeah, that cat left the bag in 2012. The apocalypse really happened and we're stuck in jargonistic purgatory now where every bad squall line is a...d-word...and nothing matters. A severe squall line is bad whether it lasts for a few miles or a few hundred. The term is just something people latch onto because Facebook is the WebMD of the weather world.

Q: I'm moving to Canada.
A: Enjoy drinking your milk from a bag.

[Temperature Map: Dennis Mersereau (forecast via NWS) | Heat Index Chart: NWS]

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June 22, 2018

Here's How to Make Beautiful Maps Without Any Training at All

I love my maps. It sounds conceited, but...oh, heck. Who am I kidding? I'm proud of my work. Everyone should like what they make. Creating my own maps compliments my writing and gives my posts a visual style that sets them apart from other weather websites and blogs.

I have fun with it. It wasn't fun when I first started, though. I have no formal training in GIS software. I started tinkering without knowing what "GIS" stood for and it took years of trial and error to find a style that I like. While it takes some work, making maps is basically plug-and-play. You just have to know where all the pieces go.

From Bad to...Less Bad

I'm terrible at making weather maps by hand because I'm not particularly good at drawing and my hands are shaky. To give you an idea of how awful I am at drawing, here's a map I created for my Facebook page a few years ago:

See what I mean? It's cartographic argle-bargle.

My total inability to draw maps was frustrating as someone who liked to write about the weather. The maps produced by the Storm Prediction Center and The Weather Channel are just fine to post on the fly, but anyone can republish those. It's impersonal. It feels like cheating off the smart kid's homework if you do it too often.

I tried to figure out how to make maps with a computer for a long time. It did not go well. I tried to make a snowfall totals map for a blizzard in New England a few years ago and downloaded some weird little contouring program to try to create something that looked halfway decent.

It did not look halfway decent.

So, fast forward a few years and I land a gig at Gawker running my own weather blog (RIP The Vane). Pretty cool for some awkward dude still in college. I suddenly found myself with hundreds of thousands of people reading my posts every month and I knew that copying and pasting other people's maps wasn't going to cut it.

My first adventure in mapmaking was a fine program called MapWindow. It wasn't particularly powerful at the time but it did a good job letting you create nice, quick maps in a hurry.

That was my first map. I created it on June 23, 2014. It's nothing to write home about, but it's a heck of a lot better than the nonsense I created before it. It took a week or two to get the hang of plugging in data and playing around with different colors and styles and layers.

Here's a map of tornado reports and tornado warnings I made about four days after I started playing around with mapping software:

...and a forecast map for Hurricane Arthur that I made about a week after that...

...and this lovely, completely easy-to-read map, which could generously be described as a severe weather forecast verification:

I took screenshots of the map from the program, plunked them into Paint, and created legends using Microsoft PowerPoint. It was sloppy and embarrassing now that I look back on them, but it worked for me while I was still learning.

Jump ahead a few more years and this is what my maps look like now:

It takes time, but you do get the hang of it.

Here's how you can make your own maps without any previous experience.

The Basics

'GIS' stands for Geographic Information System. GIS allows you to map out data about the world around you. Professional GIS software can cost thousands of dollars, but there's plenty of free software available to anyone who wants to make a map. More on that in a bit.

The two data files you'll use most often in GIS programs are shapefiles and rasters. A shapefile is basically—and, oh man, I can hear geographers breathing heavily as they read this sentence—a souped-up Microsoft Excel spreadsheet. A shapefile tells the GIS program where to place lines, points, and polygons to create a map. Shapefiles also contain different variables for those features so you can make sense of the data after you create your map.

I quickly created the above map, showing state outlines, state names, and each state's median rent in 1997, using the data included in a single shapefile.

Shapefiles (.shp) are bundled with other important files (.dbf, .prj, .shx), but you don't have to worry about those. You have to keep them with the .shp file in the same location—the software needs them—but all you'll need to plug into your GIS software is the .shp file itself.

Rasters are images that are pinned to certain coordinates so you can create maps on top of the images. The most common rasters you'll come across are satellite imagery and terrain data. You can layer rasters and shapefiles on top of each other to create complex and visually appealing maps. The above raster shows terrain data for the eastern United States and Canada before it's colorized and shading is added for texture.

When it comes to making weather maps, you'll be dealing with shapefiles if you want to analyze things like severe weather warnings or tornado tracks, and you'll have to use rasters if you want to map out observed rainfall data or Doppler radar images


QGIS (download here) is a powerful GIS program and it's the best software you'll find for free. I use QGIS to make all of my maps. Getting used to all the buttons and settings in the program is a learning curve, but it's worth it if you want to make professional-quality maps without the skills or income involved.

Most of the things I do in QGIS are saved as templates to make things easy. I've spent dozens of hours tinkering with styles and templates so that it only takes me five or ten minutes to make a map from start to finish.

The screenshot above shows the home screen for QGIS. Most of the tools you'll use are already along the left and top toolbars. Your recently saved projects will appear in the window on the home screen for easy access.

Instead of awkwardly trying to point out each individual feature in a confusing jumble of screenshots, let's just dive right into an example to show how it works. We're going to make a map of tornado tracks in the U.S. from 1950-2016.

Here are direct links to download the files we'll use, courtesy of Natural Earth and the Storm Prediction Center.

I have an entire page of GIS resources here on my blog that lists out just about every public source of data I use. It's mostly weather, though there are some basic GIS resources listed at the bottom of the page. If you ever lose the link, it's right at the top of this site.

(Update 6/26/2018: I just realized that I originally linked to a different cities shapefile than the one I used in my examples below. I've corrected the link above. I apologize for my error and hope it didn't cause any headaches. Here's a link to the city shapefile I originally included, featuring lots of cities around the world. )

Starting Out

Download all of the shapefiles linked above and move all of the files from their .zip folders into one common folder. The names of the files are all pretty straightforward. When you have everything on your computer, open up a new project in QGIS and click the "vector" button on the left toolbar.

Clicking the vector button will bring up the data source manager. You can search for files by clicking the [...] button under the source section. Search for the shapefiles on your computer. You can filter the shapefiles by selecting "ESRI Shapefiles (*.shp, *SHP)" from the bottom-right dropdown menu in the file window.

For now, let's load the ocean, lakes, and states shapefiles.

Layering your data is important. All of your shapefiles and rasters will appear in the Layers window on the left side of the screen. (If the window isn't there, you can open it by going to View > Panels > Layers.) Features appear in order on your maps, so if you put your lakes beneath your states, you won't see the lakes.

The map is going to look wonky at first because QGIS randomly assigns colors to new shapefiles. You'll have to go in and change the fill and line colors of each shapefile. You can do that by double-clicking each file in the Layer window to bring up the Layer Properties window.

Pretty Colors

For this map, I chose to use hex code #6BAEFF for the fill color on the oceans and lakes and #C1C1C1 for the fill color of the land. This will make your map a little less psychedelic.

The good news is that you can save style files for easy designing later on. These style files are extremely useful for repetitive maps—severe weather outlooks, snowfall forecasts, hurricane tracks, all that fun stuff. You can save styles by clicking the "Style" dropdown menu on the bottom-left side of the Layer Properties window.

Now let's add the cities and tornado tracks. Click the Vectors button again on the left toolbar and open the two remaining shapefiles. Make sure they're above the other shapefiles or the states and lakes will block them.
You can style the tornado tracks however you like. The Storm Prediction Center includes F/EF-Scale ratings, tornado width, path length, monetary damages, and human casualties for each tornado where the data is available. For the purposes of this map, though, I chose to make all tornado tracks the same color red (#FF000). 


To style the cities, double-click the file in the Layer window just like the other files and style the points on your map to your liking.

In order to get labels to show up, we have to click the "Label" option in the list on the left side of the Layer Properties window. Click the dropdown menu to change "no labels" to "show labels for this layer."

Next, click the "Label with" dropdown and select NAME. You can label your cities with any of the variables available within the shapefile, but the NAME variable will show you the name of the city. The variable you want is obvious most of the time, but the names of variables in some shapefiles aren't always obvious.

Most of the formatting is straightforward. You can add a buffer to your text to give the letters a thick outline. This helps them stand out more. You can also add a background to your label (sometimes it's good to house your label in its own little box so it's easier to read) or include a drop shadow for aesthetics.

Now your map should look something like that. Let's finish it up.

Print Layout

The Print Layout window is your best friend (and biggest pain in the butt) in QGIS. This is the part of the program that lets you finish up your map and publish it as an image. You can use the print layout window to include things like legends, title boxes, and distance scales on your final map.

I have dozens of layout templates saved to speed up the process. Style files and pre-made templates are the reason I'm able to turn around maps so quickly. You can create a new template by clicking the New Print Layout button on the top toolbar, and you can access your Print Layout Manager by clicking the button (featuring a wrench) to its right.

Now that we're in a new layout window, the fun begins.

The white box is your canvas. Click the "draw map" button on the left toolbar to begin. You can click and drag to create a map window that covers as much or as little of the canvas as you'd like. If you fill the whole thing up, the map will be enormous when you publish it—something like 4500x3500 pixels. That's fine. You don't lose many details if you resize it after publishing.

If you want to make your map smaller, bigger, or a different aspect than the canvas, you can drag the map to your desired size and then click the "resize layout" button under the Layout tab on the right side of the window. This fits the canvas to the features on your screen. You can adjust the margins to create a neat buffer around your map.

Once you add your map to the canvas, you can adjust the view by clicking the "move map" button on the left toolbar and clicking and dragging the map. You can use your scroll wheel to zoom in and out, but be careful—each click of the wheel is a big zoom step. Holt the CTRL key while you scroll in and out to refine your zoom levels.

I chose to add a thick frame to my map using the Item Properties tab in the window on the right side of the screen. The Item Properties tab works for whatever feature is currently highlighted/selected on the canvas.

I added a legend to the bottom-left corner of my map. You can edit the names in the legend by deselecting the "Auto Update" tick box in the Item Properties tab. You can double-click each legend item and use the text box that pops up to change its name. You can use the blue up and down buttons to arrange the order of items on the legend.

I added a scale to the bottom-right corner of the map and adjusted it to display distance in miles.

Finally, I added a text box to the top of the map and included a frame so it matches seamlessly with the frame surrounding the map. Placement is easy because the program automatically snaps the edges of features together.

When you're happy with your work, click the publish button along the top toolbar and save the file where you want on your computer. You can resize in any image processing program or keep it in its full-size beauty.

There you have it. It's a nice, easy beginner map. The sky is the limit with how you can design the template and what you can do with all the data you plunk into the program.

If you have any questions, comments, concerns, hate mail, please feel free to email me or ping me on Twitter @wxdam and I'll be glad to help if I can.

Other Tips

It's worth noting that you'll probably have to adjust the thickness of lines and font sizes once you get your map finalized in the print layout. If you look at the map before and after the print layout, the lines are considerably thicker and the font is much larger than what appears in the program itself. Remember that you're sizing features for publishing, not what you see as you're manipulating data.

Playing with different projections is a whole different headache. You can view/change projections with the button on the bottom-right corner of the main QGIS window. It usually has "EPSG:" followed by a number. I hardly ever use the standard, flat WGS 84 projection we used for the example in this post. For national overviews I use U.S. Albers Equal Area Conic and different projections when I focus on smaller regions.

Save your work frequently. QGIS has a nasty habit of crashing at the worst possible moment.

If you're trying to colorize a raster, open the file's Layer Properties and change "Render type" to singleband pseudocolor. This brings up the option to add color ramps and colorize your data. It can be a pain, especially for beginners.

Some files are a bit sloppy around the edges. Some county and state line files bulge out beyond the coast as they cover territorial waters. The SPC's severe weather outlooks include coastal waters, which leads to unsightly skirts of color hanging out offshore (see also: one of my awful maps above). You can remedy this by layering your ocean on top of the problem files.

[Maps: Dennis Mersereau | Raster: USGS]

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June 18, 2018

A Tropical Wave Could Drop More Than 10 Inches of Rain on Coastal Texas This Week

A slow-moving tropical disturbance over the Gulf of Mexico will bring several days of heavy rain to eastern Texas this week. Some areas could see double-digit rainfall totals by the end of the event. The persistent heavy rain will likely lead to at least some flooding issues near the coast and in areas prone to flash flooding. A moderate risk for flash flooding exists from Corpus Christi to the Houston metro area on Tuesday and Wednesday.

It's already raining in southeastern Texas and there's more where it came from. The latest forecast from the Weather Prediction Center shows 5-10" of rain—with higher totals possible in spots—falling over the next couple of days. Forecasters painted a bullseye of 10"+ across a significant stretch of real estate between Corpus Christi and Houston. Not everyone in the 10" zone (or 7", or 5"...) will see 10" (or 7", or 5"...) of rain, but it's certainly possible where thunderstorms begin training, or repeatedly moving over the same areas.

The WPC's flash flood forecast has parts of coastal Texas under a moderate risk for flash flooding on Tuesday and Wednesday. The threat for flooding exists, but the fact that the rain will be steady over a few days rather than falling all in one horrendous downpour should lessen the risk to an extent. The threat for street flooding and natural waterways rising out of their banks will exist where heavy, persistent rains fall, especially in a short period of time. Flooding is flooding whether it's widespread or localized, and it's dangerous no matter what.

Why will the rain stick around in Texas without moving? You can thank the heat that'll roast the eastern part of the country for the next few days.

You usually don't have to worry about heavy rain or thunderstorms during a heat wave—the very pattern that allows it to get so hot is also stifling thunderstorm development. However, you often run into those problems on the outer periphery of heat waves. The high pressure in a heat wave acts like a dome that shunts active weather around it. This is why severe thunderstorms tend to move north toward the Canadian border around this time of the year. The ridge is also why tropical disturbances can stall out when they affect the southern United States.

Heat waves are products of stagnation, both at the surface and in the upper levels of the atmosphere. The lack of movement allows a tropical disturbance like the one enveloping eastern Texas this week to just sit there and rain for several days. There's nothing to move it along. The rain will come to an end once the ridge—and with it, the stagnation—breaks on Wednesday and Thursday.

The possibility of heavy rain in Texas this week has been advertised for more than a week thanks in large part to hair-on-fire social media pages that talked about a strong hurricane smacking into the Gulf Coast this week. Some weather models kept trying to turn the disorganized disturbance into a strong storm; spinning up phantom hurricanes is not an uncommon flaw in certain weather models (*cough* the GFS *cough*).

As a result of that irresponsible hype-mongering on social media, Texas meteorologists worth their weight have had to spend much of the past 10 days trying to calm down people whose anxiety is through the roof. Storm anxiety is a big issue in Texas as we're less than a year removed from the record-breaking devastation of Hurricane Harvey. The category four hurricane made landfall near Corpus Christi, Texas, in August 2017, and its remnants meandered over the Houston area for more than a week. The storm dropped two to three feet of rain and led to widespread flooding that dwarfed the inundation caused by Tropical Storm Allison in 2001.

Thankfully, this won't be like any of those record-breaking storms that come to mind when we think of tropical rainfall in Texas. If you stay alert for warnings and stay aware of your surroundings this week, you should be okay.

[Model Animation: Tropical Tidbits | Map: Dennis Mersereau]

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June 14, 2018

Bud Could Bring Rain to the Parched Desert Southwest This Weekend

The remains of tropical cyclones that have long since kicked the rain bucket are usually headaches for people who already live in areas where the walls drip from the humidity. But that's not always the case. The remnants of Tropical Storm Bud in the eastern Pacific will bring unusually high moisture to a part of the world that's absolutely parched. June is historically the driest month of the year in much of the American southwest, but it doesn't look like that'll be the case for some this year. Rain—possibly heavy at times—is expected in parts of Arizona, New Mexico, and Colorado as the moisture from Bud moves through the desert southwest this weekend.

The eastern Pacific Ocean has seen two named tropical cyclones this year. Both storms became major hurricanes. Aletta was a picture-perfect category four that twirled harmlessly out to sea late last week. Bud, however, formed much closer to land and wound up making landfall on the Baja California Peninsula on Thursday evening as a weak tropical storm.

Bud, and the mess of atmospheric moisture that succeeds it, will enter northwestern Mexico on Friday, bringing up to four inches of rain to the region's desert mountains. The moisture will then move into Arizona and New Mexico where an extreme-to-exceptional drought is underway. The moisture will continue north and bring the chance for flash flooding into northern New Mexico and southern Colorado by Sunday.

The latest forecast from the Weather Prediction Center calls for an inch or more of rain as showers and thunderstorms form across the southwest this weekend. The greatest threat for heavy rain is likely to exist in eastern Arizona and western New Mexico, but thunderstorms that pop up and drop heavy rain for any period of time could present a flash flood threat for nearby areas.

These tiny rainfall totals don't seem like much, but this is the driest month of the year for the southwest and many of these areas are in an extreme or exceptional drought. The entire states of Arizona and New Mexico are mired in some level of drought, and more than half of each state is in the drought monitor's worst two categories.

The drought adds insult to injury when you take into account the fact that these regions see virtually no rainfall during the month of June. Annual rainfall totals in cities like Phoenix and Tucson almost completely level out during the months of May and June before monsoon season kicks in a few weeks into July.

Dumping heavy rain on arid ground that's even more dessicated due to drought is a recipe for flash flooding. Areas that see heavy rainfall—even just a little bit—could deal with flash flooding in urban areas as well as dry rivers (arroyos). Arroyos are popular among hikers and can be especially deadly when there's heavy rain upstream since the flash flood can travel many miles away from the storm into areas where the sky shows no sign of danger.

You can follow the plume of atmospheric moisture from ex-Bud on weather models by looking at precipitable water, or PWAT for short. The above animation shows the GFS model's guidance for PWAT normalized anomalies through Sunday afternoon.

Imagine a column of the atmosphere right above your house that extends from the surface straight up to the top of the atmosphere above you. PWAT measures moisture by telling you how much rain would fall if you were to wring out every bit of water vapor in the atmosphere above you. A PWAT of 1.51" means that if all the moisture present above your house fell as rain, you'd have 1.51" in your rain gauge.

Higher values indicate higher moisture and a greater chance for heavy rain. A high region of PWAT is the reservoir that thunderstorms can tap to produce excessive rainfall. The normal range for PWAT values for any one location varies—low moisture over Miami, for instance, might represent record-breaking moisture levels in Phoenix.

The GFS model shows a PWAT value over Tucson, Arizona, of 1.45" around 6:00 PM MDT on Friday and 1.35" at 6:00 AM on Saturday. These values, while common in June just about anywhere east of the Rockies, would smash daily PWAT records at Tucson dating back to 1951. The above model loop shows how PWAT values over the southwest could be as high as five standard deviations above normal as ex-Bud moves through.

Once the remnants of Bud are gone, the region should go back to its characteristically hot and dry weather for at least a few more weeks. Monsoon season usually starts around the second or third week of July.

[Satellite: RAMMB/CIRA | Model Loop: Tropical Tidbits | Maps/Chart: Dennis Mersereau]

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June 11, 2018

Hurricane Bud Expected to Rapidly Intensify Off Mexico's West Coast This Week

The Eastern Pacific Hurricane Season woke up last week and it's off to a formidable start. Hurricane Bud is the season's second hurricane—and the second storm to form in the past week—and it could affect Mexico's west coast over the next couple of days. The storm could make landfall as a tropical storm on the Baja California Peninsula toward the end of the week.

Hurricane Bud is a rapidly strengthening storm. Sea surface temperatures in the eastern Pacific off the coast of Mexico are extremely warm—approaching 90°F in areas to the southeast of the hurricane. This balmy water is miserable for swimmers but it can act like rocket fuel for hurricanes in the right environment. The combination of warm water, low wind shear, and ample moisture will allow Hurricane Bud to strengthen into what is likely to be a major hurricane over the next few days.

The hurricane will move into cooler waters and a less favorable environment later this week, allowing the storm to weaken as it approaches the Baja California Peninsula. The storm will likely bring foul weather to Cabo San Lucas whether or not the center makes landfall. Rough waves, rip currents, gusty winds, and heavy rain are all likely as the storm passes over or near the popular resort town at the southern tip of the peninsula.

Bud is the second hurricane to form in this part of the world in the past week. Hurricane Aletta was the first named storm of the basin's hurricane season and it was quite the looker. The storm unexpectedly strengthened into a category four hurricane with maximum winds of 140 MPH before hitting cooler waters and slowly weakening.

Hurricane Aletta was the perfect hurricane to admire on satellite imagery without a pang of guilt. Most of the storms that form in the eastern Pacific are great to gawk at because, for the most part, they form near Mexico and swirl west without ever affecting so much as shipping lanes let alone populated areas. A storm like Bud, on the other hand, is more problematic.

We routinely ignore the eastern Pacific's tropical cyclones in the United States because they rarely affect us, but the basin can occasionally spin up devastating storms for western Mexico. 2015 saw the strongest hurricane ever recorded in Hurricane Patricia—the storm packed maximum sustained winds of 215 MPH (really!) before weakening to "only" a borderline category five storm at landfall. The worst of Hurricane Odile's category three winds devastated Cabo San Lucas when the storm came ashore in September 2014.

Hurricane season is fully underway in both oceans that surround the United States and Central America. I published a hurricane season primer last week to help you know what to expect this summer and how to keep track of all the forecasts and products issued when a storm forms.

[Satellite: CIRA/RAMMB | Maps: Dennis Mersereau]

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June 5, 2018

Here's a Hype-Free Rundown to Help You Keep Track of Storms This Hurricane Season

The long-lasting impacts of last year's Atlantic hurricane season makes the arrival of this year's storm season feel unsettlingly abrupt. The 2018 Atlantic hurricane season officially began on June 1, but it really started with the formation of Subtropical Storm Alberto back on May 25. You may have heard of it referred to as a "season of suspense" on The Weather Channel, which is angering on exactly eight different levels. Here's a hype-free rundown of what you can expect this season and what you need to know to stay informed.

Peak Heating

The traditional dates of hurricane season follows tropical cyclone climatology. The most likely time to see tropical cyclones in the Atlantic Ocean is between June 1 and November 30, but it's not unheard of to see storms form outside of that time frame.

The most likely time to see some sort of tropical activity in the Atlantic—the peak of the season—occurs during the second week of September. This is when the ocean's warmth and favorable atmospheric factors like high moisture, low winds, and regular tropical waves, are usually most abundant.

Storms tend to form closer to the United States and Caribbean early in the season and activity slowly shifts east toward Africa in August and September as tropical waves take hold.

Of course, it doesn't take much to see an impactful storm early or late in the season. Tropical Storm Allison formed in early June 2001 and served as the benchmark for immense tropical flooding until last year's Hurricane Harvey. Hurricane Sandy didn't make landfall until a few days before Halloween.

The cliché goes that it only takes one storm to make a mess of things, and the general lack of storms or long quiet periods doesn't necessarily mean that coastal regions are out of the woods. The Atlantic's first named storm of the 1992 season didn't form until the middle of August. It was Hurricane Andrew.

Class, Rank, and Name

What's in a name? A tropical cyclone refers to any low-pressure system that has tropical characteristics—a warm-core low-pressure system powered by thunderstorms tightly packed around a closed center of circulation at the surface. A tropical depression, tropical storm, and hurricane are all tropical cyclones. Those three names are assigned based on the maximum sustained winds in a storm.

A tropical cyclone with sustained winds of less than 39 MPH is a tropical depression. Winds between 39 and 73 MPH make a tropical storm. Winds greater than 74 MPH are hurricane strength.

These seemingly-random numbers make a little more sense when read in knots instead of MPH—tropical depression starts at 20 knots, a tropical storm starts at 35 knots, and a hurricane is 64 knots or greater, otherwise known as the top of the Beaufort scale, a system of wind measurement that saw widespread use starting in the 1800s. (The deeper you dig into meteorology, the more you find that things are completely arbitrary but there are fun reasons behind the arbitrariness.)

The Saffir-Simpson Hurricane Wind Scale, a real mouthful to type, does a decent job in quickly conveying the severity of a hurricane's winds. Over the years, though, it's turned into a pox on threat communication ahead of a landfalling storm. It's seared into our minds to focus on a phrase like "category three hurricane" rather than "enormous storm surge" or "record flooding from heavy rain." Thankfully, meteorologists and reporters are getting better at talking about hazards like storm surge and freshwater flooding and not putting so much focus solely on wind speeds.

Meteorologists began a system for assigning storms human names to keep track of them and make it easier to talk about the storms in public forecasts and coverage. The six lists of storm names for the Atlantic basin are maintained by the U.N.'s World Meteorological Organization (WMO) and are used on a rotating basis. This year's names are:

2018 Atlantic Tropical Cyclone Names
Alberto Florence Kirk Patty William
Beryl Gordon Leslie Rafael
Chris Helene Michael Sara
Debby Isaac Nadine Tony
Ernesto Joyce Oscar Valerie
The name of a particularly destructive or deadly storm and replaced with a new, lazily-chosen name of the same gender and letter to take its place. The list of names in 2018 was last used in 2012 and features a newcomer, "Sara," filling in for now-retired Sandy. The replacement names are sometimes so similar to the retired name that it makes you wonder why they even bothered; most notably, Katrina was replaced with Katia and Rita replaced with Rina.

In the unlikely event that this hurricane season decides to go on steroids and we exhaust the usable parts of the English alphabet (we skip Q, U, X, Y, and Z for lack of expendable names), protocol states that we switch to the Greek alphabet after the "W" storm. This only happened during the hyperactive 2005 hurricane season, which saw 28 named storms—the 2005 hurricane season ended with the dissipation of Tropical Storm Zeta, named after the sixth Greek letter, on January 6 of the following year.

We've Already Had Alberto

Storms can and do form outside of our scheduled hurricane season. This is the fourth year in a row (!!) that we've seen a named storm form in May. Subtropical Storm Alberto formed in the western Caribbean on May 25 and made landfall on the Florida Panhandle on May 29. The storm managed to survive for several days over land, looking more impressive over Indiana than it ever did over the Gulf of Mexico, before finally losing its structure over the famously-tropical state of Michigan.

A subtropical cyclone is one that has both tropical and extratropical characteristics. A subtropical storm gets some of its energy from upper-level winds, its thunderstorms and wind field are displaced from the center, and it's not fully warm throughout the storm. It's not fully tropical, so it's subtropical. (Clever!) Subtropical and tropical systems are close enough to one another that they deserve the same treatment despite the different structure.

Despite its less-than-threatening title, Alberto left flooding in its wake (especially in western North Carolina) and took several lives through rip currents and a downed tree.

Alberto counts toward the season's total number of storms, so the next named storm will be Beryl.

Seasonal Forecasts

Most forecasts for this season call for average or slightly above-average hurricane activity. We're currently in an ENSO-Neutral pattern in the eastern equatorial Pacific Ocean, meaning that there's neither an El Niño nor a La Niña present. NOAA expects these neutral conditions to persist through most of the hurricane season. El Niño (abnormally warm water) and La Niña (abnormally cool water) can each affect tropical activity in the Atlantic by slowing it down (Niño) or allowing more storms to form (Niña).

Without either of those conditions present, it looks like the water around the equator in the eastern Pacific Ocean shouldn't play much of a role in the Atlantic's tropical activity. So what could?

It'll be interesting to see how much the water here in the Atlantic basin serves to hurt tropical activity later this season. NOAA's current analysis of water temperature anomalies in the tropical Atlantic show waters a degree or two Celsius below normal. This could change as we head deeper into the summer months, but cooler-than-normal water could slow down tropical activity and make intensification harder for storms that do form.

It's important to qualify any discussion of seasonal forecasts with the fact that every situation is unique and relies on so many individual factors that it's hard to tell beforehand what's going to happen. We could have nothing but a string of sloppy, weak storms this year or go the entire year and see two storms that take every opportunity they get to thrive. It's hard to tell in advance.

A Flood of Forecasts

The National Hurricane Center has the sole responsibility of issuing forecasts for storms in the Atlantic Ocean. They issue so many different forecasts and graphics and alerts that it can get confusing even for a weather geek.

I often make my own maps when I write about storms and post about them on social media. My maps are very similar to the National Hurricane Center's. For these examples, though, I'll post the NHC's own graphics.

The NHC issues a tropical weather outlook four times a day explaining their thinking for tropical weather over the next five days. These forecasts are issued in terms of the probability of tropical cyclone formation. A < 30% chance is considered low, 40-60% chance is medium, and >70% is a high chance of tropical development.

An advisory is an update on the storm that includes current conditions (position, wind, pressure) and a five-day forecast. Each advisory includes a public advisory, which is written and formatted in plain English, a forecast advisory containing technical details, a forecaster's discussion outlining the scientific justification for each forecast, and a whole suite of forecast graphics.

The cone of uncertainty is the most common forecast graphic we see from the NHC. The cone of uncertainty is the margin of error in the agency's storm track forecasts. Historically, the center of a storm stays within that cone 66% of the time. Storms can and do venture outside of that cone, especially when it's weak or in a complex environment. The cone only applies to the center of a storm—the rain, wind, and storm surge can extend hundreds of miles away from the storm's center track.

Wind speed probabilities show the chance of seeing tropical storm or hurricane force winds along the track of the storm. It's a good way to get a quick idea of who's at risk of seeing strong winds from a storm. This differs from the cone of uncertainty as it incorporates the anticipated size of the storm's wind field at each point along its forecast track. Even a low probability of strong winds is relatively high.

The most likely time of arrival for tropical storm force winds is a new graphic that helps you plan for when conditions will be too dangerous for travel or storm preparation. The potato-looking graphic shows when strong winds could arrive based on the storm's forecast size, track, and speed. The earliest reasonable time of arrival graphic shows the earliest you could expect strong winds if the storm sped up or grew in size.

Watches and warnings are always tricky. A watch means that dangerous conditions are possible while a warning means that dangerous conditions are expected. A tropical storm or hurricane watch is issued 48 hours before the possible arrival of strong winds. A tropical storm or hurricane warning is issued 36 hours before the expected arrival of strong winds.

Tropical storm and hurricane watches/warnings only apply to wind speeds. These alerts don't speak to the potential for storm surge, flooding, or tornadoes.

A new product that started in 2017 was the addition of storm surge watches and warnings. A storm surge is sea water pushed inland by strong, persistent onshore winds. These new storm surge alerts are issued when a life-threatening storm surge is possible or expected at a particular location.

[Satellite: NOAA | SSTs: NOAA/ESRL/PSD | Storm Graphics: NHC]

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