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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May 28, 2018

Subtropical Storm Alberto Makes Landfall on Florida Panhandle

Subtropical Storm Alberto made landfall near Laguna Beach, Florida, around 5:00 PM on Monday, with maximum sustained winds of 45 MPH. Laguna Beach is about halfway between Fort Walton Beach and Panama City. The storm will continue to produce heavy rain, gusty winds, and the chance for isolated tornadoes as it pushes inland and moves toward the Great Lakes over the next couple of days.

An additional two to three inches of rain are possible across the Mid-Atlantic and Southeast as Alberto and its remnants move north toward the Great Lakes through Thursday. The heaviest rain will follow the core near the center of the storm as it tracks north through Alabama and Tennessee. There's an elevated risk for flash flooding across areas expecting the heaviest precipitation totals—the greatest risk for flash flooding beyond today will exist in the Appalachian Mountains of Georgia and the Carolinas on Tuesday.

There's also a risk for isolated tornadoes in the southeast on the eastern half of the system. Some of the stronger discrete thunderstorms that develop will have the opportunity to produce small, quick tornadoes. Small tornadoes are dangerous nonetheless, and the quick nature of tropical tornadoes can reduce warning lead time to just a few minutes.

The forecasts for Alberto were decent considering the nature of the storm. Alberto did max-out with winds of 65 MPH on Sunday evening before weakening before landfall. The landfall point has been within the cone of uncertainty since Friday morning. Several inches of rain have fallen across the southeast over the past few days and the rain is continuing north as it comes ashore.

Alberto never made the expected transition from subtropical to tropical. A subtropical storm is a cyclone that has both tropical and extratropical characteristics, a sort of hybrid between the two. Subtropical storms derive some of their energy from upper-level winds rather than thunderstorms around the center of circulation. The storm's maximum winds were also displaced from the center for most of its life. The storm also wasn't totally warm from top to bottom, a requirement of tropical systems.

The storm remained an organized mess for its entire life, more closely resembling a nor'easter than a tropical storm. The above satellite image shows Alberto's interesting appearance late on Sunday night. It's not clear whether or not that technical designation confused people in the path of the storm's rain and wind—meteorologists and weather reporters did a pretty good job explaining "subtropical" and hammering away at the risks rather than the name.

Unfortunately, social media is packed this weekend with photos and videos of people ignoring the red flags and beach patrol warning people to stay out of the water. Drowning from flooding is the number one cause of death in tropical cyclones, but people routinely drown in rip currents even when storms are hundreds of miles offshore. They just don't listen to the warnings. They think they know better and they think it won't happen to them.

There's not much we can do to get people to heed the warnings other than constant education and pounding away the message that it's too dangerous to swim in the ocean. I wrote for Forbes a few years ago that meteorologists can't help people who don't want to be warned. I wrote the post in the context of people driving through flooded roads despite signs, warnings, and common sense, but it could just as easily apply to rip currents. 

June 1 marks the beginning of hurricane season in the Atlantic Ocean. Storms commonly form closer to land during the early part of the season and start forming farther out in the Atlantic as the summer wears on. The historical peak of hurricane season's activity is the second week of September.

[Maps: Dennis Mersereau | Infrared Satellite: College of DuPage]

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May 27, 2018

Soaking Rains for Southeast as Florida Prepares for Strengthening Tropical Storm

Subtropical Storm Alberto is looking more organized tonight as it moves into the southeastern Gulf of Mexico. The storm's outer bands are overspreading the Florida peninsula this evening and bringing heavy rain, gusty winds, and the chance for tornadoes to the southern part of the state. Tropical storm warnings are in effect for coastal parts of Florida and Alabama ahead of Alberto's expected landfall on Monday.

The National Hurricane Center still expects Alberto to strengthen into a strong tropical storm before it comes ashore on the Florida panhandle near Pensacola in the afternoon or evening hours on Monday. The latest forecast shows the storm reaching maximum sustained winds of around 65 MPH by landfall.

Alberto is still a subtropical storm due to its interaction with upper-level winds; thunderstorms aren't completely driving the storm's development yet. The center has reformed a couple of times over the past day or so as new thunderstorms pop up near the center. This has caused the storm's minimum central pressure to fluctuate every couple of hours. The storm went from 1005 mb on Saturday morning to 999 mb in the afternoon before coming back up to 1001 millibars when the center relocated yet again on Saturday evening.

The storm certainly looks healthier tonight than it did last night. Infrared satellite imagery around 11:30 PM EDT shows deep thunderstorm activity near the center of the storm and a big, curved outflow of cirrus clouds stretching east into the Atlantic Ocean. It's getting the "look" you'd expect of a tropical system rather than an amorphous blob of clouds that even experienced weather enthusiasts would have a hard time picking out of a lineup.

Thunderstorms should root themselves over the center of circulation as the environment becomes more favorable for tropical development, helping Alberto become a fully-tropical storm and allowing it to gradually strengthen through landfall.

Alberto is lopsided and it'll likely remain asymmetrical through the rest of its life. This will affect the distribution of heavy rain over the next couple of days. The greatest threat with this storm remains flooding from heavy rain.

The Weather Prediction Center predicts a widespread area of 2-4" of rain across the southeast with more than 7" possible where the storm makes landfall. I overlaid the storm's predicted track over the rainfall forecast to show the uneven distribution of rain—the cutoff between flooding rains and a nuisance will be just a few dozen miles.

While rain is the greatest threat, we can't ignore the potential for wind damage. Downed trees and power lines will likely cause power outages in some spots. There's also the possibility of trees falling on buildings and cars. The greatest risk of bodily harm during Alberto will be due to flooding, rip currents, and falling trees.

A storm surge is possible as Alberto makes landfall. The above map shows the National Hurricane Center's "reasonable worst case scenario" for storm surge along the coast.

Most of the surge will be focused east of Alberto's center where the strongest winds are pushing water against the coastline. Here's a zoomed-in view of the part of Florida's Gulf Coast where the greatest surge is expected:

A storm surge of 2-4 feet above ground level is possible along the coast at high tide, especially in the Big Bend region of Florida. Thankfully, this is one of the least-populated stretches of coast in Florida, but people still live there and storm surge flooding is dangerous even if it's a handful of homes at risk.

In addition to the rain, wind, and storm surge, tornadoes are a concern across Florida on Sunday and Monday. The tornado threat will stretch into Alabama and Georgia as Alberto comes inland on Monday and Tuesday.

Small, quick tornadoes are common in the outer bands of tropical cyclones as they interact with land. These tornadoes can happen so quickly that they afford forecasters little or no time to issue tornado warnings for the affected areas. It's extremely important to have a reliable way to receive severe weather warnings as Alberto approaches over the next couple of days.

[Maps: Dennis Mersereau]

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May 25, 2018

Tropical Storm Watches Issued for Northern Gulf Coast Ahead of Alberto

Tropical storm watches are now in effect for portions of the northern Gulf Coast ahead of Subtropical Storm Alberto's expected arrival on Monday. Forecasters expect the system to be a strong tropical storm by the time it makes landfall. While there's a chance that the storm could be near hurricane strength around landfall, the biggest story with this storm will be the rain.

The 5:00 PM EDT advisory from the National Hurricane Center doesn't make many changes to the forecast beyond some fine-tuning of the track and the issuance of tropical storm watches. The watches are now in effect from Grand Isle, Louisiana, to Indian Pass, Florida, including the cities of New Orleans, Gulfport, Mobile, and Pensacola. These watches will likely be upgraded to warnings later this weekend.

A tropical storm watch means that tropical storm conditions (sustained winds between 39 and 73 MPH) are possible within the next 48 hours. The onset of wind and rain will precede Alberto's landfall by a day or so. Rain from this system will begin across Florida tomorrow and Sunday, reaching inland parts of the southeast late Sunday through the first half of next week.

A storm surge watch is also in effect from the mouth of the Mississippi River in Louisiana to Horseshoe Beach, Florida. A storm surge is water sea water pushed inland by strong, persistent winds. Storm surge inundation of 2-4 feet is possible in areas currently under the watch.

Alberto has that just-rolled-out-of-bed look to it on satellite imagery. The storm is quite disheveled as it battles wind shear and it has a ways to go before it organizes into a respectable-looking system. An Air Force Reserve Hurricane Hunter aircraft investigated the storm this afternoon and found that it remains a weak subtropical storm.

A subtropical storm is one that contains characteristics of both a tropical and extratropical storm. In Alberto's case, the storm is pulling some of its energy from an upper-level trough rather than solely through thunderstorm activity around the low-pressure center at the surface. It should become fully tropical as it grows more organized over the next day or so.

The storm is lopsided and it will likely remain that way for much of its life. The majority of the wind and heavy rain will occur along and to the east of the path of the center of circulation. The track is important in determining who sees the heaviest rain and the worst winds. Alberto is also a sizeable system and it will have widespread impacts across the southeast.

The Weather Prediction Center's updated rainfall forecast for the next seven days continues to show at least 3" of rain is possible across most of the southeastern United States, with the heaviest rain focused on Florida, Alabama, and Mississippi. There's now an area of 7"+ of rainfall predicted right around where the National Hurricane Center expects Alberto to make landfall. This bullseye will change as future forecasts adjust the predicted track as needed. Flooding is likely across the southeast where heavy rains have fallen for the past couple of weeks. Flash flooding will be a problem with persistent bands of thunderstorms.

While the rain is by far the greatest threat with Alberto, we can't discount the potential for wind damage, especially since the storm is expected be within shouting distance of hurricane strength by the time it makes landfall.

Water in the northern Gulf of Mexico is abnormally warm for this time of the year, running between 79°F and 82°F. The combination of warm waters and lower wind shear should allow Alberto to strengthen as it slows down in the northern Gulf on Sunday and Monday. The only saving grace is that it looks like dry air will impinge on the storm's ability to organize and ramp itself up. If that doesn't happen, though, there's a chance Alberto could flirt with hurricane status if everything comes together just right.

What can you do to prepare? The most important thing to do is to make sure you're prepared for power outages. Take stock of your ready-to-eat food—stuff like canned ravioli, Spam, breakfast bars, those tasty little cups of fruit—and keep some bottled water on hand in case there's a boil water advisory issued for your area and you can't, well, boil water. Batteries and battery-operated flashlights are a must. Your cell phone's flashlight is great in a pinch but keeping your phone charged is a must.

Take a look around outside and tie up or bring in anything that could fall over or blow around in high winds. Trim tree branches and limbs away from your home if possible. If you have to drive during high winds or heavy rain, search out some alternate routes in case roads are covered by water or downed trees and power lines.

It's the little things—easy food, a reliable flashlight, knowing an alternate route—that really help when you're faced with a storm and you haven't had much time to think and prepare for the season.

The next full update from the NHC comes out at 11:00 PM. We'll know more about exactly who will see the worst of the storm tomorrow and certainly by Sunday.

[Maps: Dennis Mersereau | SST Map: NOAA/ESRL/PSD]

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Alberto Will Likely Threaten the Northern Gulf Coast as a Strong Tropical Storm Next Week

And so it begins. The National Hurricane Center has issued its first advisory on newly-minted Subtropical Storm Alberto in the western Caribbean. The storm could intensify into a strong tropical storm as it approaches the northern Gulf Coast early next week. Heavy rains, flooding, and rip currents are likely across the southeast through next week, an especially tricky situation given the influx of tourists for the holiday weekend.

The first advisory from the NHC on Alberto shows the storm making landfall somewhere between New Orleans and Mobile during the day on Monday. Everyone from central Louisiana to the Florida Peninsula is within the cone of uncertainty. The cone is the historical track error in the previous forecasts for a storm. The center of a storm historically stays within the cone 66% of the time. The wind, rain, and storm surge can extend hundreds of miles from the center of a storm, however, and that's why the entire southeast is in play for foul weather from this storm.


Don't let the term "Subtropical Storm Alberto" trip you up. A subtropical storm is a cyclone that has both tropical and extratropical characteristics. Tropical cyclones are driven by thunderstorms around a low-pressure center at the surface and their wind field is usually compact. Subtropical storms derive some of their energy from upper-level winds and their surface wind fields are usually more spread out or detached from the center of circulation.

The difference between a tropical storm and subtropical storm is technical and has little practical use for people in the path of the storm. It's the NHC's job to adhere to the "well, technically" aspect of storms like this. Don't focus on the name. Alberto will likely transition to a fully-tropical system this weekend and it'll still have the same threats and hazards no matter what you call it.


Heavy rain is the greatest concern with this storm as Alberto taps into deep, tropical moisture flowing across the southeastern United States. The Weather Prediction Center's latest rainfall forecast (issued early Friday morning) shows a slug of 3-5" of rain covering most of the southeast.

Rain associated with Alberto will start this weekend in Florida and gradually work its way inland as the storm pushes inland early next week.

The heaviest rain will likely be focused on the eastern side of Alberto when it makes landfall. The storm will likely slow down when it approaches the coast, exacerbating the heavy rain in areas where rain bands start training (repeatedly moving over the same areas). Some spots caught under training bands could see rainfall totals approach double digits by the middle of next week.

I talked a bit yesterday about how the southeast has already been soaked by heavy rain over the past couple of weeks. Most of us have seen several inches of rain and some areas have seen more than half a foot of rain from recent thunderstorms. The ground is already saturated and it won't take much of a downpour from Alberto to cause flooding concerns in low-lying areas.


The threat for wind damage is a growing concern along the northern Gulf Coast where Alberto makes landfall. Alberto is expected to be a high-end tropical storm when it makes landfall on Monday with sustained winds somewhere around 65 MPH. Forecasters will fine-tune the storm's intensity as we get closer to landfall and models get a better handle on the storm.

It's important to point out that the NHC's forecast discussion with the first advisory calls this forecast "conservative" in its intensity as the storm approaches landfall on Monday. The storm will slow down as it enters an environment favorable for strengthening once it reaches the northern Gulf of Mexico on Monday. There's an outside chance that Alberto could push hurricane strength at landfall if current forecasts hold true.

The risk for power outages will grow as the storm grows stronger. That sounds like an unnecessarily obvious thing to say, but all the rain over the past few weeks has loosened the soil enough that it won't take much for trees and power lines to start falling down. Widespread power outages are possible. It's a good idea to make sure you're prepared with batteries, battery-operated flashlights, food that doesn't need to be cooked (ravioli and Pop Tarts are your BFFs in the dark), and bottled water. Even a little bit of cash wouldn't hurt if you're lucky enough to have it—your credit/debit card does no good when the power's out.

Storm Surge

A storm surge is possible at the coast when Alberto makes landfall. The threat for a deeper surge will grow as the storm slows down and grows stronger near landfall. The northern Gulf Coast is an especially sensitive spot for storm surge. New Orleans is firmly within the cone of uncertainty and it would only take a small nudge to the west in Alberto's track to make flooding in New Orleans a serious concern.

The National Hurricane Center started issuing storm surge watches and warnings last year to make residents aware of the threat for coastal flooding independent of the storm's intensity. There aren't any watches or warnings in effect for the United States right now, but they'll almost certainly be issued this weekend as the storm draws closer.

Surf/Rip Currents

It's a holiday weekend and thousands of people are heading to the beach to mark the beginning of the summer season. There will be lots of people at the beaches even as the storm approaches, and many of them will still go in the water even though the risks of rough waves and rip currents will be well advertised.

I'll crib what I wrote yesterday about the threat for rip currents and how to deal with them:

Rip currents will be a growing threat as the winds pick up ahead of Alberto. Rip currents are fast currents of water that pull water away from the beach. Rip currents occur when waves crash directly on the beach rather than at an angle—you can sometimes spot these currents by looking for calm spots mixed in with the waves.

Don't go in the water if you're at the coast for the holiday weekend and there are red flags flying at the beach. Rip currents don't suck you under like they show in the movies. People drown in rip currents because they can't swim, panic, or can't tread water anymore due to exhaustion or waves. If you're ever caught in a rip current, either tread water and calmly signal for help or swim parallel to the shore until the pull stops, and then swim back to the beach.


Tornadoes are a common part of landfalling tropical cyclones. The greatest threat for tornadoes exists in the right-front quadrant of a landfalling storm—in Alberto's case, this will be the eastern side of the storm. Tornadoes in tropical cyclones happen quickly but they're dangerous because they can touch down without much warning at all.

The Storm Prediction Center has issued a marginal risk for severe weather in Florida on Saturday as Alberto passes into the Gulf of Mexico and its rain bands start overspreading the peninsula. The tornado threat will continue early next week farther north when Alberto makes landfall.


The National Hurricane Center issues full forecasts every six hours (5 AM/PM and 11 AM/PM Eastern Time) and will issue intermediate updates every three hours (2 AM/PM and 8 AM/PM Eastern Time) as long as there are watches and warnings in effect.

As always, I'll keep you updated with maps and analysis here on DAMWeather and on Twitter @wxdam. Thank you as always for your continued support.

[Maps: Dennis Mersereau | Satellite: NOAA | Wind Prob. Map: National Hurricane Center]

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