July 19, 2019

Hot Nights Will Make This Weekend's Excessive Heat And Humidity Even More Dangerous

Parts of 19 states are under an excessive heat warning over the next couple of days as the combined effect of high heat and stifling humidity climbs to dangerous levels. The heat baking the Midwest this week will grow more intense and spread east toward the Mid-Atlantic and Northeast through the weekend. It's not the worst heat wave we've ever seen, but it'll get hot enough—and stay warm enough at night—that it could take a serious toll on vulnerable populations.

The Heat Index





The above map shows the National Weather Service's heat alerts as of about 9:30 PM on Thursday night. Excessive heat warnings cover a vast area—the widest-reaching heat wave we've seen in a couple of years—including the entire state of Ohio and several of the smaller states on the East Coast, as well as just about every major city between Kansas City and Boston.

These alerts are in place because of the heat index.

Humidity affects us on a hot day because the excess moisture in the air hampers your body's ability to cool itself off. Meteorologists and other scientists worked together to create the heat index to measure the impact that the combination of heat and humidity has on your body.

The heat index isn't made-up for clicks or ratings.

The heat index isn't a scare tactic to make people think the heat is worse than it is.

People who say stuff like that are trying to act cool and hip when they're really just waving their ignorance around for everyone to laugh at.

The heat index is a real metric that tells you what the combined heat and humidity feels like to your body. If it's 93°F with a heat index of 105°F, the moisture in the air is making your body work as hard as it would if the actual air temperature were 105°F. That means heat-related illnesses will set in even faster than you would expect given the air temperature.

Heat alerts are relative to where you live—it's harder to get an excessive heat warning in Wilmington, North Carolina, than it is in Syracuse, New York, because people in the south are used to and equipped for sustained hot and muggy weather. I wrote a post for Outside last summer explaining how each NWS office issues these alerts, complete with a map showing the heat alert criteria for almost every office east of the Rockies.

Here's the National Weather Service's forecast (as of Thursday evening) for the heat index at 5:00 PM EDT on Friday, July 19:

 ...and here's the same forecast for the same time on Saturday, July 20:



...and the maximum heat index forecast for Sunday, July 21, from the Weather Prediction Center:


(Note: The WPC issues maximum heat index forecasts between 3 and 7 days out. I tried to make my own map of their forecasts, but it...uh...didn't turn out well, so I used their map here.)

It's going to feel like the 110s again across much of the Midwest on Friday, with those heat indices moving into the I-95 corridor by Saturday and Sunday. Some models are trying to put Friday's heat index readings into the 120s across the corn fields in places like Iowa. That may be extreme—but not unheard of!—but it underscores just how darn hot and muggy it's going to get.

It won't take long to come down with a heat-related illness in this kind of weather. Folks who work outside or work-out outside are most susceptible to feeling the effects of the daytime grossness. Keep drinking water even when you're not thirsty. You lose liquid faster than you realize when it's hot out.

It's Going To Stay Unbearably Warm At Night


It's bad enough that it'll be dangerously hot and humid during the day, but the temperature at night is extremely concerning in a situation like this. The effect is most pronounced in cities and communities up north where home air conditioning isn't as common as it is down south.

I know folks in the Midwest get salty when we focus on the East Coast when a heat wave affects such a large area, but there are hundreds of thousands of elderly people and folks with health problems in cities like Washington and Baltimore and Philadelphia who will need air conditioning over the next few days but won't have access to it or simply can't afford it.

Those vulnerable populations who can't access air conditioning will be at great risk for illness over the next couple of days if they can't find relief from the heat and humidity. Heat doesn't necessarily kill people because it gets ridiculously gross during the day. A prolonged heat wave kills people because it stays unbearably warm at night, preventing folks who suffer during the day from feeling any relief at night. The stress from the heat compounds day after day until it's too much to take.

Here are the predicted low temperatures on Friday morning:


And Saturday morning:


And Sunday morning:


Some of those temperatures, especially in the Mid-Atlantic and Northeast, are hardly low temperatures at all. 82°F for the low in Washington D.C. and 81°F in New York City on Sunday morning? That's horrendous, and even worse when you account for the humidity that doesn't go away when the sun goes down.

The Heat Will Break Next Week


We're dealing with this heat wave because the jet stream is firmly wedged north of the Canadian border, creating a large ridge of high pressure over the eastern two-thirds of the United States. Ridges of high pressure during the summer are often called "heat domes" because they foster heat waves like the one we're seeing right now.


The ridge will begin to break on Sunday in the middle of the country and by Monday on the East Coast, bringing relief to those who need it the most. It's not going to get cool—hey, it's the middle of July—but we'll go from "dangerously warm" to just "miserably warm," and the latter is at least bearable.


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July 11, 2019

Life-Threatening Flooding Likely As Tropical Storm Barry Crawls Toward Louisiana

Extremely heavy rain associated with Tropical Storm Barry will lead to life-threatening flooding across southern Louisiana and the lower Mississippi River Valley over the next couple of days. The slow-moving storm could reach hurricane strength before it makes landfall on the Louisiana coast on Saturday. Latest forecasts show double-digit rainfall totals across the eastern part of the state, compounding ongoing flooding issues across the region.

NHC Forecast

The disturbance we've been watching for the last couple of days developed a closed surface circulation on Thursday morning, allowing the National Hurricane Center to upgrade the system to Tropical Storm Barry. The NHC's 7:00 PM CDT advisory shows the storm slowly organizing over the next two days, possibly strengthening into a hurricane before making landfall in southern Louisiana on Saturday morning.

Conditions across the northern Gulf of Mexico are conducive to strengthening if the storm can get its act together fast enough to take advantage of its environment. Barry's structure on Thursday afternoon was...lacking...with an exposed low-level circulation south of the mouth of the Mississippi River, and almost all clouds and convection fanned-out on the southern side of the system.

The window for strengthening will begin to close the longer the storm remains disorganized. That's great news in terms of wind and storm surge, but Barry is set to bring flooding rains to the northern Gulf Coast regardless of its strength at landfall.

Rainfall Forecast



While wind speeds and the phrase "hurricane warning" will get the most attention, the real story of this storm is the water. Barry threatens to wring out a tremendous amount of tropical moisture over the northern Gulf Coast through this weekend.

The latest rainfall prediction from NOAA's Weather Prediction Center shows the potential for 20" or more—yes, that's twenty inches or more—of rain across southern Louisiana. A wider swath of half a foot or more of rain spreads up the Mississippi River toward the Memphis area.

Not everyone will see all of the rain in the forecast. But the storm will move slow enough that rain bands and thunderstorms will be able to tap into a deep reserve of tropical moisture and produce copious amounts of rain in a short period of time.

It's important to note that these predicted rainfall amounts will change as the storm gets closer to land and forecasters get a better handle on the structure and future track of the storm. Small changes in intensity, organization, and track will shift the bullseye for heavy rain with time. The overall point is that everyone in Louisiana should prepare for a potentially significant flooding event.

Flooding Potential

NOAA/NWS
The combination of accumulated water from heavy rain, the Mississippi River rising from excess upstream runoff, and a potential storm surge could severely strain the ability of New Orleans and surrounding areas to stave off floodwaters.

The Mississippi River is already in flood from months of heavy rain across the central United States. Add that on to the flash flood emergency that played out across New Orleans on Wednesday—dropping more than half a foot of rain in a couple of hours—and it won't take much heavy rain to cause major flooding along the area's already-strained waterways.

River flooding forecasts from the National Weather Service show the Mississippi River in New Orleans cresting at 19 feet if current precipitation forecasts hold up, which would be the highest crest recorded there since February 1950. The levees along the Mississippi in New Orleans are only 20 feet tall, so the water would only be about a foot away from the top.

The entire city of New Orleans sits below sea level. Not only does it face a threat from the bodies of water that surround the city, but rainwater has to be pumped out of the city because it can't seep into the ground. The pumps can handle rainfall rates of 1.00" in the first hour and 0.50" in every subsequent hour after the rain stops. Rain that falls faster than that will cause flooding in spots around the city until the pumps can catch up with the excess water.

It's not just New Orleans, either. The excessive rainfall amounts forecast across Louisiana and throughout the lower Mississippi River Valley will cause extensive and life-threatening flash flooding across the region.

A scenario like this played out not too long ago. A disturbance over the northern Gulf of Mexico brought a widespread swath of 10"-20" of rain to central and eastern Louisiana back in the summer of 2016. The resulting flooding was some of the worst in Louisiana's modern history, killing more than a dozen people and causing more than $10 billion in damage. Many homes destroyed by the floods in 2016 didn't have flood insurance, as the owners thought they were safe from flooding.

Winds

Flooding is the greatest threat, but we can't ignore the winds. Hurricane warnings are in effect for the central Louisiana coast for the potential that Barry could strengthen into a hurricane before reaching land. In practical terms, though, the difference between a 70 MPH tropical storm and a 75 MPH hurricane is negligible.

Wind gusts above 70 MPH and soggy ground will allow trees and power lines to fall with relative ease. Widespread power outages are likely where the strongest part of the storm makes landfall. Strong winds will easily snap tree limbs and blow around small debris—stuff like trash cans and yard decorations.

Tornadoes

Tornadoes are always a threat in the right-front quadrant of any landfalling tropical system. Folks along and to the east of Tropical Storm Barry's track will stand the greatest threat for tornadoes. Tropical tornadoes are different from tornadoes you'd see in a "regular" thunderstorm. They can happen so quickly that forecasters can miss them between radar sweeps. Eastern Louisiana, Mississippi, and Alabama will have to be on the lookout for tornadoes as the storm makes its way inland.

[Satellite Image: NOAA]


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July 9, 2019

Tropical System In Gulf Likely To Strengthen; Greatest Threat Is Flooding From Heavy Rain

A tropical disturbance entering the Gulf of Mexico has a high chance of growing into a named storm by the end of the week. Some models have the system intensifying before it makes landfall on the northern Gulf Coast this weekend. Flash flooding from heavy rain is far and away the greatest threat with this system, no matter what it's called at landfall.

Tuesday afternoon's update from the National Hurricane Center gave the disturbance, dubbed "Invest 92L" for now, an 80 percent chance of developing into a tropical depression by Thursday morning. We've been watching this trough of low pressure mosey southward across the southeastern United States for a couple of days. The disturbance emerged in the northeastern Gulf of Mexico on Tuesday morning, opening the window for potential development over the next day or two.


Conditions over the Gulf of Mexico are capable of supporting a tropical system. Sea surface temperatures in the northern Gulf are in the mid- to upper-80s, which is plenty warm enough to support tropical development. Low wind shear will allow thunderstorms to develop without getting ripped apart, and ample moisture should stave off intrusions of dry air from suffocating the storm.

The major complication here is the structure of the disturbance itself. It's pretty disorganized at the moment. Assuming the system develops, exactly where it takes root and how strong it gets will ultimately determine how far west it travels across the northern Gulf. A stronger storm may be able to tap into winds that could steer the storm farther west across the Gulf.

Water is the greatest threat with this system no matter what it does. The best case scenario right now is that this system remains disorganized and weak. The worst case scenario is that the system gets its act together in a hurry, posing a threat for wind damage and power outages in addition to lots of heavy rain.


Either scenario would bring heavy rain to the Gulf Coast. The latest forecast from the Weather Prediction Center shows more than five inches of rain falling across a wide swath of the southern United States, with a maximum near the coast where the system ultimately makes landfall. This swath of heavy rain will shift in location and intensity as forecasters get a better handle on exactly what will happen. Everyone along and inland of the Gulf Coast is at risk of seeing flooding rains through next weekend.

We've seen so many storms in recent years—from tropical storms to major hurricanes—leave behind horrendous flooding in their wake. Folks in Texas and North Carolina are intimately aware of the threat for flooding in a landfalling tropical system. Even so, it's still a major battle to get folks in harm's way to appreciate the threat of water over the threat of wind.

The threat of winds can't be completely ignored, of course. Strong winds could cause damage if this system reaches shore as a tropical storm or hurricane. Tree damage, power outages, and some structural damage would be possible in that scenario. Those are significant hazards, made even worse by the potential for flooding from heavy rains.

Forecasters (and weather models!) will have a better idea of what's going to happen over the next couple of days once—and if—the system develops and there's actually something there to analyze. It's a good idea to prepare for an extended period of heavy rain if you live in any of the southern states. Good questions to ask yourself right now include "do I have multiple routes to get around if roads are flooded out?" and "do I have food and supplies to get through a couple of days without power?"

The National Hurricane Center will begin issuing advisories on this system every 3-6 hours if/once it develops into a tropical depression. In the meantime, they issue tropical weather outlooks every six hours at 2:00 and 8:00 AM/PM Eastern Time.


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July 7, 2019

A Trough Over Georgia Could Become A Tropical System In The Gulf—And It's Not As Weird As It Sounds!

The National Hurricane Center is monitoring a trough of low pressure over the southeastern U.S. for signs of tropical development once it reaches the Gulf of Mexico in a couple of days. A disturbance turning into a tropical cyclone once it moves from land to sea isn't as weird as folks are making it sound on Facebook and Twitter—after all, where do you think Cape Verde hurricanes come from? (It ain't the stork!)

Sunday's 8:00 PM EDT update from the NHC gives the disturbance over northern Georgia a 60 percent chance of developing into a tropical system once it reaches the northern Gulf of Mexico later this week. Conditions are favorable for the system to slowly organize by late next week as it meanders in the northern Gulf.



Even if the storm doesn't develop, it looks like a good bit of rain will fall along the northern Gulf Coast and points inland once this whatever-it-is starts moving inland next weekend. The latest rainfall forecast from the Weather Prediction Center shows a boatload of rain (technical term!) falling across parts of Florida, Alabama, and Georgia, through next Sunday, and these totals could easily tick higher if the system organizes and grows stronger once it reaches the Gulf. Flash flooding is likely in areas that see too much rain too quickly.

Everything I've seen about this system on social media so far makes a hullabaloo about the National Hurricane Center mentioning it in their outlooks while the trough was still over Tennessee. If that seems weird, it shouldn't. Lots of tropical systems begin as clusters of storms that form over land and move over the ocean.

You know how we talk about Cape Verde hurricanes and "tropical waves" moving off of Africa in August and September? Those are disturbances that begin over western Africa and move over Atlantic Ocean. Most of the big hurricanes we remember—Katrina! Andrew! Rita! Charley!—started as troughs or thunderstorms over the African continent.

Most folks just aren't used to hearing about this process occurring over the United States. Well, that, and the fact that it's a little unsettling to see a giant X over Tennessee on a map produced by the National Hurricane Center. But this is certainly one method of tropical development this early in the season, and the Gulf of Mexico is a prime location for storms to develop in July.

Oh, and if you're wondering..."Barry" is the next name on this year's list of tropical cyclone names for the Atlantic Ocean. (We used the name Andrea back in May.)


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July 3, 2019

Anchorage Could Break Its All-Time High As An Intense Heat Wave Builds Over Alaska

High temperatures will soar into the 90s near Anchorage over the next week as a historic heat wave builds across Alaska. The actual air temperature is astounding—90s in southern Alaska!—but the duration of this heat wave will take an especially hard toll on residents not used to such warm temperatures. The heat isn't forecast to break until the middle of next week.
The upper-level ridge over Alaska this Saturday, per Wednesday afternoon's run of the GFS model. | Source: Tropical Tidbits


This heat wave is the result of a strong ridge of high pressure building over Alaska this week. The ridge will peak this weekend before waning early next week. Calm, sinking air beneath the high will allow temperatures to climb into record territory for an extended period of time.
An animated map showing the NWS's forecast highs across Alaska between July 4, 2019, and July 9, 2019.
The latest forecast from the National Weather Service shows highs easily climbing into the 80s across most of central and southern Alaska. The Anchorage area will see the worst of the heat. The official high temperature in Anchorage*, recorded at Ted Stevens International Airport on the west side of the city, could meet or exceed the all-time high temperature of 85°F set in June 14, 1969. Temperatures will climb into the 90s for the duration of the heat wave in the valley north of Anchorage, which is home to communities like Wasilla. The average high in Anchorage at the beginning of July is about 65°F.

*Note: The official temperature in Anchorage is recorded at the international airport, which is right on the water. The rest of Anchorage gets warmer as it's farther inland. The temperature label for Anchorage on the maps in this post are for the forecast point very close to downtown Anchorage, which more accurately reflects the temperature that residents will experience.

It can get quite warm in low-lying parts of Alaska that are on relatively flat land. Fairbanks, for instance, climbs into the upper 80s or low 90s at least a few times each summer. The state's all-time high temperature was 100°F set in Fort Yukon, north of Fairbanks, during the historic heat wave of 1915.

A long-duration heat wave of this caliber will pose a risk for heat-related illnesses in vulnerable populations. Most homes and businesses here don't have air conditioning. It's Alaska, after all. They rarely need it.

The lack of cooling systems, combined with the intense high-latitude summer sunshine and homes built "to hold onto every molecule of warmth," will lead to a long period of abject misery for most folks affected by this heat wave. It'll be hard or impossible to stay cool at home, leading to a heightened risk of heat exhaustion or heat stroke in the elderly and those in poor health.

Annual average temperature trends in Alaska since 1925. | Source: NOAA/NCDC
This event is an unsettling reminder that Alaska is steeped in the effects of our changing climate. Climate Central sums it up well:
Alaska is the fastest-warming state by far, as the Arctic is warming twice as quickly as the rest of the world. And what happens in the Arctic, doesn’t stay in the Arctic. Melting glaciers are fueling sea level rise and permafrost thaws are releasing more heat-trapping gases. According to a recent UN report, winter temperatures over the Arctic Ocean could rise 5-9°F by midcentury unless the world’s emissions reduction commitments are strengthened. 
One heat wave isn't a Day After Tomorrow-esque manifestation of climate change, but the warming trend in Alaska over the last couple of decades is undeniable. 2014, 2015, 2016, 2017, and 2018 were five of the ten warmest years ever recorded in Alaska. Climate change accentuates the extremes, making heat waves like this more likely in the future.


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July 2, 2019

Hurricane Barbara Grew From A Tropical Storm To A Cat. 4 Hurricane In 21 Hours

Hurricane Barbara rapidly strengthened into a category four hurricane on Tuesday as it spun over the eastern Pacific Ocean. "Rapid" isn't an exaggeration in this case—the storm grew from a tropical storm to a category four hurricane in just 21 hours. The hurricane is likely near its peak strength and will weaken over open waters later this week. 

The latest advisory from the National Hurricane Center shows the storm at or very near its peak strength. The hurricane will soon start losing steam as it moves over cooler waters to its north. While Hawaii is in the frame on the forecast map above, take note of the scale on the bottom-right—even by Sunday, the storm will be hundreds of miles from Hawaii, and the storm or its remnants should be weak (or even non-existent) by the time it reaches Hawaii.


Barbara was quite the healthy hurricane this afternoon. The storm has a well-defined eye surrounded by a thick eyewall. The classic "buzzsaw" appearance of the storm—those cirrus clouds radiating clockwise from the center of the storm—are indicative of good upper-level outflow, which is an essential to maintaining a storm's strength.

The storm took advantage of calm winds, warm waters, and decent organization to get going in a hurry. Barbara was a tropical storm with 70 MPH winds at 11:00 AM PDT on Monday. The storm nearly doubled in strength by the following morning. This is another point on a long list of hurricanes that underwent rapid intensification. Thankfully, this event occurred well away from land, but rapid intensification is a prime reason folks along coast need to pay extremely close attention to the latest forecasts when future storms threaten land.

The eastern Pacific hurricane season got off to an unusually late start this year. I wrote over at Forbes last week that the season's first tropical depression—which eventually strengthened into Alvin—was tied for the latest first tropical system since reliable records began in the mid-1900s. As we've learned so many seasons before, a late start doesn't necessarily mean a quiet season. There's likely another storm forming on Barbara's heels, and, like Barbara, it should also head out to sea without affecting land.


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June 24, 2019

Summer Is Squall Line Season In The United States—Here's How These Storms Form

It's squall line season in the United States. The majority of the exciting weather we'll see over the next couple of months will come from these lines of storms that bubble up and race across hundreds of miles in one shot. We see lots of interesting severe weather in this country, but squall lines—often referred to as mesoscale convective systems (MCS)—are impressive both for their strength and sheer resilience. Here's a look at how these fascinating systems develop.

Not all thunderstorms are the same. They may share similar features, sure, but their structures can be very different and it's that structure that determines their fate.

Single-Cell Thunderstorms

Single-cell thunderstorms are far and away the most common storms out there. They begin with a single updraft and dissipate when the downdraft of cool air chokes off the storm's access to instability. The above radar image shows single-cell storms bubbling across central Alabama on a hot summer afternoon in 2015.
 
Supercells

Supercell thunderstorms get top billing when it comes to severe weather—and for good reason. Supercells feature rotating updrafts that can power a single thunderstorm for hours. The structure of a supercell can produce strong tornadoes, enormous hail, and very strong winds. The classic supercell above produced the infamous F5 tornado that hit Moore, Oklahoma, on May 3, 1999.

Multicell Thunderstorms

And then there are multicell thunderstorms, which are complexes of thunderstorms that are all linked to one another. The interconnected nature of multicell convection can keep these storms going for hours, leading to a threat of destructive winds or flash flooding (or both!). A cluster of storms that merge into a well-developed line of storms—like the one above from last summer—is a mesoscale convective system (MCS). The terms MCS and squall line are interchangeable.

A strong and long-lived MCS that produces wind damage over a path of about 250 miles or longer is known as a derecho. The above MCS last summer was a derecho...in fact, one of three derechos that formed that day. That word can whip-up fear in a flash after the bad derecho of 2012. Lots of meteorologists—fully aware of the fear factor involved—hesitate to use the term now, and some even get angry when others use it. However, it's a valid meteorological term, and it's perfectly fine to use when a storm meets the criteria.

The July 11, 2011 Derecho


My favorite example of an MCS is the central Iowa derecho of July 11, 2011, because we got such a detailed view of the structure of this storm on the Des Moines radar. I've mentioned this storm in passing in some past explainers I've written on derechos (links here and here).

The bow echo pattern on radar imagery is often a sign of strong winds...an understatement in this case, as it turns out. The above image shows the storm just as it's starting to produce a wide swath of straight-line winds in excess of 100 MPH. The white line through the middle of the storms is the cross-section I use in the images that follow.


The storm that tore through central Iowa started out as a couple of run-of-the-mill thunderstorms west of Omaha, Nebraska. Just about every MCS starts as a handful of separate thunderstorms.

Thunderstorms breathe in warm, unstable air and exhale cool, stable air. This outflow of stable air beneath a storm causes cold air to pool up at the surface. This cold pool spreads out from the parent thunderstorm like a ripple on a pond. The leading edge of a cold pool, called an outflow boundary, acts like a mini cold front as it scoops up unstable air and triggers new thunderstorm activity.

If thunderstorms form close enough together, their cold pools can merge into one entity. This merger causes the thunderstorms to become interconnected, moving and strengthening in unison. The thunderstorms latch on to the leading edge of that cold pool, forming into a line as they begin to race downwind.

The forward motion of the newly-minted line of storms causes their updrafts to tilt backwards, allowing unstable air to get scooped directly into the thunderstorms. This structure allows the storms to consume unstable air without getting choked off by the stable air beneath them.

The radar cross-section above shows the winds throughout the MCS just as it's starting to produce those 100 MPH straight-line winds. The image is oriented so that southwest is on the left and northeast is on the right. Des Moines is just off-screen to the left. The green colors show wind blowing southwest toward the radar, while the warmer colors show wind blowing northeast away from the radar.


You can see the tilted updraft on radar, clear as day. The cold pool at the surface is lifting up all that unstable air over central Iowa and feeding it right into those storms.



Friction begins to take its toll on the cold pool after a while. We wind up seeing lots of horizontal and vertical rolling motions within an MCS due to friction between the moving air, the calmer air around the storm, and the ground below. It's kinda like the little whirls that form in a swimming pool when you run your hand through the water.

You can't really see the horizontal rotation (diagrammed above) on radar, but the vertical rotation shows up as those little curly ends on the edges of squall lines. These curls are known as "bookend vortices." This storm had a pretty hefty bookend vortex, but it's hard to see on radar. These features are exceptionally pronounced in some squall lines, like the May 8, 2009, derecho depicted at the very top of this post.


These friction-induced areas of rotation create a feature known as a "rear inflow jet" that feeds air into the front of the storm. It's the rear inflow jet that creates the powerful straight-line winds in a squall line. The rear inflow jet roars from the back of the storm to the front, getting shoved into the ground by the thunderstorms along the leading edge of the squall line. In stronger squall lines, the abrupt onset of violent winds is partially why people remember these storms so well.

Death of an MCS

Not all squall lines are powerhouses of doom. Most of them remain below severe levels and only produce a breezy rain. But whether it's the strongest storm in living memory or one that no one will ever remember, they all have one last thing in common—they must die.


There are two main ways a squall line can die. The first is that they can run out of instability and just gently power down like a toy when its batteries start to fail. The above animation shows a squall line slowly losing power as it survives crossing the North Carolina mountains only to find an environment with absolutely no instability.

The second cause of death is when the cold pool outruns the thunderstorms, which is much more interesting to watch on radar and experience in person. Thunderstorms have to keep up with the leading edge of the cold pool in order to maintain their strength and keep rolling along. After all, the storms are relying on the cold pool to feed them that instability.

If the storms start to fall behind, it gets harder and harder for them to ingest the unstable air they need to survive. Eventually, the cold air runs too far ahead and the storms choke and start to dissipate. This process can sometimes happen very quickly. The remnant outflow can serve as the focus for more storm development the following day.

The southern end of the June 29, 2012, derecho (above) is a great example of the cold pool far outrunning its thunderstorms. The outflow boundary is the thin green line a dozen or so miles ahead of the thunderstorms. This derecho was so strong, though, that the outflow boundary continued to produce 60+ MPH winds deep into North Carolina. Some folks lost all the trees in their yard long before it even started raining.

Focus...


The most common spot to see an MCS develop is along a sharp boundary. The outer edge of a heat-wave-producing ridge of high pressure, often called a "ring of fire," is a common track for a strong MCS to develop during the summer months. Stationary fronts, old outflow boundaries, and sharp instability gradients are all breeding grounds for MCS development.

The map above shows radar imagery superimposed over a temperature map. Remember I mentioned earlier that three derechos formed in one day last summer? The map above shows a radar image of those storms in progress, superimposed over a temperature map at the same time. You can see the storms riding along the heat ridge parked over the southern United States at the time.

[all radar images taken with Gibson Ridge's GR2Analyst]


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June 21, 2019

Here's a Look at CAPE, the Fuel That Powers Thunderstorms



The National Weather Service tweeted out a chart on Wednesday showing how incredibly unstable the atmosphere was over the Dallas-Fort Worth area on Wednesday afternoon. The region had CAPE values above 7,000 j/kg, which is an extreme amount of instability. Meteorologists and weather enthusiasts gawked over the chart while lots of other folks asked, with a touch of nervousness, exactly what it meant. CAPE is one of those terms we use so often in the weather that it's easy forget it's inside baseball to folks who only casually follow the forecast.

CAPE, an acronym for Convective Available Potential Energy, is a measure of how much instability there is in the atmosphere. Greater CAPE values indicate a greater potential for powerful thunderstorms. We measure CAPE in joules per kilogram (ex: 1,234 j/kg), but the unit doesn't really matter. It's all about the number.

Rising Air

Much of what we know about instability in the atmosphere relies on parcel theory. Imagine a bubble of warm air rising from the surface. Since the warmer bubble of air is less dense than the surrounding cooler air, the warmer air will continue to rise like a balloon.

Rising air cools off slowly, so it stays warmer than the air around it as it ascends. The greater the difference between the temperature inside that rising bubble of air and the air around it, the faster that bubble of air will rise. The air doesn't stop rising until it cools down enough that matches the temperature of the surrounding air and loses its positive buoyancy.

That rising bubble of air is the updraft that produces thunderstorms. When you watch a huge cumulonimbus cloud billowing on the horizon, you're watching parcel theory in action. Warm air continues racing upward until it cools off and stops rising. (Air spreading out when it stops rising creates the anvil we see at the top of storms.)

SKEW-T Charts



Meteorologists use SKEW-T charts to visualize temperature and moisture throughout a column of the atmosphere. This data can be collected by radiosondes attached to weather balloons or simulated by weather models. Some of these charts are simple and only feature a couple of lines, while others (like the graphic above) are rich with more data analysis than most people ever need. 
  • The solid red line shows the air temperature measured by the radiosonde attached to the weather balloon.
  • The solid green line shows the dew point measured by the radiosonde attached to the weather balloon.
  • The dotted red line on the right shows the temperature inside a bubble of air rising from the surface.
  • Heights are measured using air pressure along the y-axis; the pressure decreases quickly from bottom to top, mirroring how rapidly the atmosphere thins out with height.
  • Temperatures are plotted along the x-axis using diagonal lines that stretch from bottom-left to top-right. I highlighted a couple of the lines in purple to make them easier to spot.
  • Winds are plotted at various heights on the right using traditional wind barbs.

All we have to worry about here is the difference between the environmental temperature and the temperature of that rising bubble of air. The big gap between the two temperatures is CAPE. The taller and fatter that blank space gets, the faster the air can rise and the stronger any thunderstorms can grow.

That environment produced numerous reports of significant severe weather, including multiple instances of softball-size hail. It takes a strong updraft to be able to keep such a huge chunk of ice suspended in the air, and CAPE values greater than 7,400 j/kg were, uh, plenty robust enough to allow that to happen.

You don't always get the whole story on instability from CAPE. Another important factor is the rate at which the environmental temperature decreases with height, which is known as a lapse rate. You could have decent CAPE—but a mediocre lapse rate—and wind up with a situation where updrafts struggle to get going.

Not All Environments Are The Same

If everything else is favorable, CAPE is usually sufficient to get a decent thunderstorm going once it rises above 1,000 j/kg. The atmosphere is very unstable once CAPE rises above 2,500 j/kg, and the environment is supportive of big-time thunderstorms once it's above 3,500 j/kg. It's rare to see values as high as we saw in northern Texas this week, but it can happen during the hot and humid summer months.

Not all bad thunderstorms require huge CAPE values in order to make a mess of things. Instability is only one part of an equation, and there are plenty of scenarios where thunderstorms can produce damaging winds and tornadoes in environments with relatively low instability.

It's worth noting that a day with big CAPE can see exactly zero thunderstorms—it is potential energy, after all. Temperature inversions are the most common cause of CAPE failing to produce any thunderstorms. An inversion occurs when temperatures suddenly warm with height. These are often referred to as "capping inversions" because the shallow layer of warm air acts like a bottle cap keeping all that air from rising beyond a certain point, stifling potential thunderstorm activity.

We saw that scenario play out this past May when one of the most dangerous severe weather setups in recent years failed to produce much thunderstorm activity—likely due, at least in part, to an unexpected cap over central Oklahoma.


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