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.

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.


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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June 12, 2019

Parts Of Southern Georgia Saw 7 Inches Of Rain In Just Three Hours On Tuesday

A large cluster of thunderstorms that popped up in southern Georgia on Tuesday evening produced more than 7.00 inches of rain in just a couple of hours, prompting a flash flood warning as local waterways and storm sewers were inundated by the abrupt surge of water. The sudden nature of the storms highlights the flooding risk that summertime thunderstorms can pose in the moisture-laden southeastern United States.

Folks who live in the southeast are no strangers to a drenching afternoon thunderstorm. It's not uncommon for a thunderstorm to pop up and drop a quick inch or two of rain before moving on an hour later. The storms north of Valdosta, Georgia, however, are an example of how quickly things can get serious when your run-of-the-mill summertime thunderstorms sit in the same spot for too long.

The cluster of thunderstorms that put down the torrential rain were the result of converging outflow boundaries. An outflow boundary is the rush of cool air that descends out and away from a thunderstorm. Outflow boundaries often act like little cold fronts that scoop up unstable air ahead of them and trigger the development of more thunderstorms as the afternoon wears on. This domino effect can continue until the unstable air is exhausted—usually around sunset.

Outflow boundaries were responsible for the flooding rains over southern Georgia on Tuesday. Imagery from the Valdosta radar showed multiple outflow boundaries colliding almost head-on across the counties north of Valdosta. A cluster of thunderstorms bloomed when the boundaries collided and the unstable air had nowhere to go but straight up.
Radar-estimated rainfall amounts on Tuesday evening. Source: GREarth/AllisonHouse

Since there weren't any prevailing boundaries or strong steering currents to drive the storms out of the area, they just sat and poured over the same communities for several hours at a time as they very slowly drifted toward the south. A weather spotter near Weber, Georgia, reported 5.81 inches of rain between 5:54 PM and 7:54 PM. NWS Tallahassee reported on Twitter that one community—possibly that same weather spotter—saw more than 7.00 inches of rain by the time the storm wound down. Radar estimates indicate that several counties saw 5-7 inches of rain during the storm.

The Weather Prediction Center warns that there's a chance for more flash flooding across coastal sections of Georgia, South Carolina, and southeastern North Carolina on Wednesday. It's hard to say who will see the heaviest rains, but any thunderstorms that pop up in the region have the potential to produce lots of heavy rain in a short period of time.

[Radar Imagery: GR2A/Gibson Ridge]

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June 11, 2019

Typically-Comfy San Francisco Hits 100°F As West Coast Heat Wave Continues This Week

Hoowee. It hit 100°F at San Francisco International Airport on Tuesday, the warmest temperature ever recorded so early in the year. The heat wave baking the West Coast with unusually toasty temperatures for early June will continue through the end of the week, forcing some residents to deal with almost unbearable indoor temperatures.

Temperatures easily soared into the 100s as far north as northern California's Central Valley on Monday and Tuesday, with 90s at lower elevations through eastern Washington. Temperatures at 4:00 PM PDT on Monday are shown in the map at the top of this post. Even downtown San Francisco, which is typically rather cool given the influence of the chilly Pacific waters, made it all the way up to 97°F on Monday, breaking the station's record for June 10 by one degree.

Things didn't cool down much on Tuesday. Portland, Oregon, hit 95°F at 3:00 PM PDT on Tuesday, and temperatures were right up around 100°F again in the San Francisco Bay area. 

San Francisco's high on Monday was one of only seven times SFO Airport's temperature reached 100°F or warmer, and the earliest it's ever done so. Every other triple-digit reading at the city's airport occurred during the month of September, according to data pulled from xmACIS2 and professionally compiled on the lovely PowerPoint chart above.

Other record highs on Monday include 113°F in Thermal, CA; 105°F at Salinas Airport in Monterrey County, CA; 105°F in Stockton, CA; 104°F in El Cajon, CA; and 101°F in Redwood City, CA; and 101°F in Santa Rosa, CA. Many of the records broken in California, especially around the San Francisco area, have stood since at least 1994.

500mb height anomalies on Monday afternoon. Source: Tropical Tidbits

A large ridge of high pressure parked over western North America is responsible for the prolonged heat wave. Ridges tend to foster calm, hot weather—stronger and more anomalous ridges can bring about stronger and more anomalous heat waves. Models show the ridge sticking around for at least a couple more days, which means temperatures will be slow to cool down through the end of the week.

An animated loop of expected high temperatures across the western U.S. between Tuesday, June 11, and Friday, June 14.
The National Weather Service's forecast on Tuesday afternoon called for high temperatures at or near 100°F to persist across most of California's Central Valley through Friday. Things will progressively start to cool down at the coast in places like San Francisco and Los Angeles—the high in downtown San Francisco will only hit the mid-60s by the end of the week, but even inland the warmth won't be anywhere near as brutal as we saw on Monday.

It's bad enough to have to deal with hot temperatures when you're not used to them, but many homes and businesses in the western United States—especially near the coast—aren't equipped with air conditioning, which makes a days-long heat wave an uncomfortable and potentially dangerous prospect.

Heat is a compounding hazard. Enduring a successive period of extremely warm days and nights prevents the indoor temperature (and, back east, the humidity) from rebounding to a livable level. The longer a heat wave lasts, the more unlivable it becomes indoors. That's why so many people fall ill or die during long heat waves in low-income communities or climates where air conditioning isn't a standard in homes and businesses.

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June 7, 2019

A Rare Tornado Hit A Small Town In Northern Canada, One Of Only 4 Recorded So Far North

A tornado struck a tiny town in northern Canada’s vast wilderness on June 2. The tornado damaged homes and businesses in Fort Smith, Northwest Territories, leaving residents shocked by the storm that just hit them. Tornadoes are rare at such a high latitude, and it’s even more rare that the tornado managed to hit such an isolated community.

Environment Canada confirmed that an EF-1 tornado touched down in Fort Smith on the afternoon of June 2, causing some structural damage and bringing down trees and power lines. Photos obtained and posted by CBC North show significant tree damage, crushed vehicles, and what appears to be a metal shed that was tossed and smashed in someone’s yard.

Residents in this part of the country have no reliable way to know a tornado is coming unless they see it for themselves. Environment Canada only has 31 weather radar sites set up across the country, centered on population centers near the southern border and in parts of the tornado-prone Prairie provinces. The nearest weather radar to Fort Smith is more than 350 miles away—that’s like using the radar at Washington’s Dulles Airport to see a storm over Providence, Rhode Island. This leaves folks up north to rely on satellite imagery or old-fashioned sky watching to stay ahead of an approaching thunderstorm.

Folks in Fort Smith probably never thought they'd see a tornado there. Tornadoes are extremely rare this far north. This is reportedly only the fourth tornado on record to strike Northwest Territories. It’s possible there are more tornadoes than we realize in interior and far-northern Canada, but communities are so few and far between that it takes a direct strike like we saw in Fort Smith for a tornado confirmation.

Tornado data maintained by Environment Canada shows more than 1,800 confirmed tornadoes across the country between 1980 and 2009, mostly focused around populated areas where people are actually around to witness tornadoes. Most tornadoes in Canada are relatively weak, though some tornadoes on the Prairies and in southern Ontario have been quite strong. The strongest tornado in Canadian history was an EF-5 that hit Elie, Manitoba, on June 22, 2007.

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June 5, 2019

Tropical Downpours Could Lead To Flash Flooding In Parts Of The Southeast This Week

Heavy downpours could lead to localized flash flooding across parts of the southeastern United States this week as a deep plume of tropical moisture spreads over the region. Forecasts on Tuesday night called for a widespread drenching across the southeast through early next week, with some areas potentially seeing more than five inches of rain by the end of the period.

Last week, we started watching a tropical disturbance in the Bay of Campeche for signs of tropical development. The National Hurricane Center had given the system a 60 percent chance of developing into a tropical system at its beefiest, but the disturbance was never able to take root and grow. The disturbance ran out of room to develop on Tuesday as it approached eastern Mexico.

Source: Tropical Tidbits

Nothing ever really goes away in the weather, of course. Even though the soon-to-be-erstwhile disturbance is no longer a thing of interest on weather maps, the remnant moisture from the system will continue spreading across the southeast. The above chart from Tuesday night's run of the GFS model shows precipitable water (PWAT) values through the weekend.

Precipitable water is a great way to visualize how much moisture shower and thunderstorms can work with. PWAT tells us how much rain would fall if you could wring all the moisture out of that part of the atmosphere. Higher PWAT values indicate a greater potential for showers and thunderstorms to produce heavy downpours that could lead to flooding. A PWAT value over 2.00" is considered delightfully soupy and tropical, a ripe environment for drenching rains.

As a result of all that evaporated paradise moving over land, it won't be hard for a hefty thunderstorm to put down a quick inch or two of rain if it sits over one spot for too long. It's important to note that not everybody covered under, say, the five-inch rainfall contour in the Weather Prediction Center's forecast will definitely see five inches of rain. Storms are hit-or-miss during the summer. Most everyone will see rain, but some could see a whole lot more than others.

Stay alert for flash flood watches and warnings over the next couple of days. It's always wise to memorize or program multiple safe routes to get home, to work, or wherever you need to go, just in case your normal route is covered in water and you need to turn around. It only takes a few inches of moving water to pick up a vehicle and carry it away, and it's impossible to tell how much water is covering a roadway—or if the road is even still there under the water.

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