Weather Whys: The other side of heat? Ridge-riding thunderstorms

In brief: Today’s post takes advantage of a quiet tropics to explain what ridge riding thunderstorms are, as you may be hearing a bit about those in addition to heat next week across a swath of the Central and Eastern U.S.

There’s nothing to report in the Atlantic tropics, so today we’ll talk about a topic that will become more relevant in the coming peak summer heat: Ridge-riding thunderstorms.

What are they?

Well, the name tells you everything. They are thunderstorms that develop and ride the periphery of high pressure in the upper atmosphere. These tend to occur most often in the summer months. We sometimes refer to this as “ring of fire” thunderstorms or “ridge rollers.”

(University of Wisconsin CIMSS)

How and where do they form?

Like any thunderstorm, you need rising air, instability, and moisture. All of those are usually present around the periphery of high pressure ridges, or what you often hear referred to as “heat domes” or “death ridges.” High pressure itself indicates sinking, hotter air, so underneath the ridge, it’s dry. But on the outskirts? Not so much. In fact, instability is typically pretty potent in those areas, as you’re straddling the border between very hot, dry air and cooler air. This is why the storms usually initiate on the top (poleward) side of the ridge, not on the southern periphery. Northern areas are also closer to the jet stream, offering some extra punch. Areas on the southern periphery of the ridge can see storms too but the overall instability, temperature contrast, and proximity from the jet stream work against them.

Are they usually stronger than typical summer thunderstorms?

They can be, yes. There’s a pretty decent connection between intense heat waves in the Central U.S. and derechoes, or the strongest of these strong clusters of thunderstorms that produce widespread, significant damaging wind. Not all heat waves will produce derechoes. Not at all. But many historic derechoes have occurred in heat waves. There are numerous exceptions, of course. But there is a connection. The reason is that as these thunderstorms develop with daytime heating, somewhere on the ridge periphery, depending on upper level disturbances, etc., they often quickly organize into “clusters” of thunderstorms. Those clusters can often then develop into a more organized complex of thunderstorms, known as a mesoscale convective system, or MCS. In most cases, these complexes of storms will produce scattered severe weather and torrential rain. But periodically, we can get some that overachieve. Those are the ones that usually go on to acquire derecho status.

Dry air from heat domes or high pressure ridges in close proximity to these storms can often infiltrate the storms aloft. That matters because while dry air in general inhibits thunderstorm development, a layer of dry air in the mid-levels of the atmosphere can turn an ordinary cluster of storms into a potent one.

What part of the country sees these most?

Usually in summer, this type of pattern is most likely to occur with a Plains or Midwest ridge of high pressure, and the thunderstorms will tend to follow the periphery across the Upper Midwest and Great Lakes, periodically into Upstate New York, New England, or parts of the Mid Atlantic. Occasionally, you can get strong to severe storms that plow across the Plains, much like we saw this past weekend in Oklahoma and Arkansas.

Favored areas for occasional derecho events (more often MCS’s). (NOAA)

Why do they often occur at night?

Well, as noted above, the initiation of these storms often occurs during the late afternoon and evening, as is typical for summertime storms. However, as they organize and cluster, they will keep going through the night, often fueled by internal dynamics and a low level jet stream (strong winds around 5,000 feet up) that tend to help them maintain their intensity or at times intensify further. These storms transition from surface based, which means heating at the surface and instability develop the thunderstorms to “elevated,” which occurs when you lose most of the instability available as temperatures cool. Precipitation varies diurnally, especially in the Plains, where the maximum average precip actually occurs at night, not during the afternoon. Frankly, it often gets difficult to sustain these complexes of thunderstorms after sunrise, as you lose the nighttime low level jet, the cloud cover and leading cool air ahead of the storms further stabilizes the atmosphere, and you see cloud tops begin to warm up (stronger storms have colder air at their tops).

What are the hazards with these types of storms?

Again, as noted above, some (not all or even many, but some) have become historic derecho events, like the 1995 Adirondack blowdown or the June 2012 Mid-Atlantic derecho. Most others never get remembered in history but can cause locally significant damage. But all threats tend to exist with these storms: Wind, hail, tornadoes. Straight-line winds are far and away the biggest threat (in addition to prolific lightning). But hail and tornadoes can and occasionally do occur, especially as the storms initiate early on. Additionally, heavy rainfall is likely with these systems, and depending on the ground conditions ahead of the event, they can cause flash flooding.

So, next week?

Yes, we may get a good example of ridge rolling thunderstorms next week. If we look at the operational European model, you can see a couple of things happening. First, the ridge will prominently expand. We’ve already seen messaging from the National Weather Service on the heat potential next week.

(NWS Weather Prediction Center)

Second, if you follow the Euro here, you’ll see that ridge expand but also a lot of yellow and orange color riding along top of it. What we’re looking at is 500 mb vorticity. Each time one of these passes by, it’s indicating a potential “seed” for thunderstorm complex development basically.

The European operational model for next week showing a big heat ridge and also thunderstorm development risk. (Tropical Tidbits)

There are at least 2 or 3 of them showing up in the model between Sunday and Saturday next week. So I would anticipate we see at least a couple days of some severe weather risk across the Northern Plains, Upper Midwest, or Great Lakes. The Storm Prediction Center discussion this morning for day 5 and onward says as much:

Severe weather will be possible across portions of the Northern
Plains into the Upper Midwest Sunday and into next week. However,
predictability remains too low to highlight areas in this timeframe.
The presence of strong to very strong instability across a large
region with multiple shortwave troughs emerging from the primary
trough centered over the West will surely support severe weather
potential most days. However, probabilities for specific areas
cannot be added until the upper-level pattern becomes more clear. – SPC Wednesday AM discussion

So the point of today’s post was to educate you a bit on why you see this happen in summer but also do so in a timely manner because you’ll probably see it in action in a few days!

Tropics quiet, as we review last week’s Gulf Coast flooding

In brief: Today’s post focuses primarily on recapping the Gulf Coast flooding from last week, with a look at why Arthur was a contributor but not the only cause of the flooding. We also argue that had Arthur not technically been a tropical storm, the outcome would likely have still been similar.

Tropics

Nothing to do. It’s quiet out there, and it looks to stay that way over the next 7 to 10 days. The Atlantic, as it often is this time of year, is caked in dust.

Widespread Saharan dust litters the Atlantic, Caribbean, and Gulf. (University of Wisconsin)

So at this time, there’s nothing expected. We still expect a below average season overall, but it’s fairly common for the end of June and front half of July to struggle with much tropical activity. So this is not abnormal in the grand scheme of things and doesn’t indicate anything of note heading out beyond the forecast horizon.

Recapping rain

The Gulf Coast is a tough place to live sometimes. There’s a lot of socioeconomic history involved there, but from a weather perspective almost every weather event seems to be magnified.

(HopeForce on X)

Avoyelles Parish, Louisiana is the most recent victim of magnified weather, as last week’s rains from the remnants of Arthur and a water-fueled jet stream pattern dumped 2 feet of rain on a good chunk of the Parish. This caused extensive flooding, dozens of rescues, and another chapter in a long, storied flood history in Louisiana. For those looking to contribute to relief, Building a Better Avoyelles is a local charity helping with flood relief now. The United Way of Central Louisiana is also another local organization that can help. The usual suspects are also fine organizations as well, but I wanted to focus on local support here. Feel free to leave other sources in the comments.

Avoyelles Parish residents can’t really recall anything this bad there in memory. The flooding was not limited to just there, however. Southern Mississippi also took a big hit, with areas north of Gulfport and Biloxi seeing as much as 17 to 18 inches of rain.

72-hour rainfall ending on Friday morning. (NOAA NSSL)

How rough have the last 7 to 10 or so days been in the South? Rough.

Rainfall from the prior 7 days ending at 8 AM Sunday. Avoyelles Parish and southern Mississippi stand out, but 10″+ totals also occurred elsewhere too. (Pivotal Weather)

Plaucheville, LA ended up with 31.51 inches over the course of last week. Perkinston, MS had 21.53 inches of rain. Red Level, AL saw 12.60 inches. A CoCoRaHS station at Cottonport, LA saw over 37 inches of rain as well. Wiggins, MS was 22.84 inches.

The CoCoRaHS report from near Cottonport, LA showing 29″ of rain on June 18 and over 37 inches for the week. (CoCoRaHS)

Beyond damage and totals, it’s important to remember that while Tropical Storm Arthur played a role in this, we would have had a substantial rain and flooding event still, even without Arthur. We had been talking about this for several days before the event itself. You can see this show up when we look at the European model’s precipitable water analysis image from last Thursday morning.

The precipitable water normalized anomaly map for Thursday, June 18th. (Tropical Tidbits)

What is this showing? Well, for one, Arthur shows up progressing through Louisiana and Mississippi. But beyond that, you have Caribbean moisture and Pacific moisture converging across the Gulf. Remember, Pacific Ocean water temperatures are outlandishly warm, and not just because of El Niño either. What’s coming out of the Pacific is record warm.

Eastern North Pacific water temperatures are at record levels. (University of Arizona)

Keep in mind that even if Arthur never technically became a tropical storm, it was indicative of a vigorous disturbance that may have also been influenced by Pacific Tropical Storm Cristina, which weakened about week before Arthur formed. I would argue that Arthur would have been about the same intensity and rain-laden had it never even made it over the water.

So it raises a valid question that my friend and colleague Michael Lowry lobbed out there, which is whether this may have confused the issue somewhat. Messaging a tropical storm and messaging a flood are occasionally one in the same, as tropical storms can and often do produce flooding. But did the idea that Arthur developed and made landfall in Texas so quickly maybe suggest that areas farther east were off the hook? The NHC’s job is to follow the science with respect to naming storms. If it’s a tropical storm, then it’s a tropical storm, and they should not ignore that just because it may alter perception in some places. (We can argue whether Arthur was indeed a tropical storm or something more hybrid, and we’ll see what the postseason analysis says) But in this case, it had been broadcast for days in advance that serious flooding potential was coming to the Gulf Coast. So no one should have been caught off guard, per se. (Though seeing nearly 30 inches in a day in Avoyelles Parish is tough to really argue was “expected.”)

At any rate, the takeaway here: Focus on the threats, not the technicalities. Just because it looks like a duck and quacks like a duck doesn’t mean you won’t get a goose or a swan too.

Odd duck Arthur’s remnants still a major flooding threat for the Gulf Coast

In brief: PTC 1 became Arthur yesterday and offered some erratic behavior around Texas before dissipating. The moisture associated with Arthur remains a serious flooding threat for the Deep South and Gulf Coast.

Let’s start with the present and future and then walk back to yesterday.

First off, Arthur is a remnant low now, but the moisture funnel remains directed at the central Gulf Coast. In those areas, there are numerous flood watches and warnings.

Tropical rains are falling across the South with numerous flood warnings posted. Even a few severe storms are possible. (WeatherFront)

The Weather Prediction Center pulled the trigger on a rare high risk for flooding in the South. This includes Pensacola, Mobile, Biloxi, Gulfport, and Hattiesburg, MS.

A high risk for flash flooding is posted along the central Gulf Coast in Mississippi, Alabama, and the western Florida Panhandle. (NOAA)

The good news is that some of this has already fallen. The bad news is that additional rains will cause flooding, somewhat locally significant flooding problems. For instance, rain totals around New Orleans ranged from 1 to 3 inches in the city proper to over 7 inches at the airport in Kenner.

Rain totals since about 9 PM Wednesday have piled up in the New Orleans area. (NOAA)

Closer to the high risk itself, we’ve seen anywhere from 6 to 10 inches of rain in southern Mississippi away from the immediate coast. Hancock County has been especially hard hit by rainfall.

Rain totals have been quite high in southern Mississippi, particularly in Hancock County. (NOAA NSSL)

Through the rest of today, modeling varies on exactly how much rain to expect, but southern Alabama may be in the crosshairs for the heavier rain next, with areas near and north of Mobile at highest risk. Additional heavy rain is possible across interior southern Mississippi as well. Locally heavy rain may also occur north of Pensacola and into parts of Georgia.

Forecast additional rain totals from the RRFS model through midnight. (Pivotal Weather)

The risk of heavier rainfall continues into tomorrow, with the higher rain risks shifting eastward. Alabama remains under a moderate risk for flooding tomorrow, and that may be because any rain will cause flooding after today’s saturation.

Juneteenth will risk some flooding in southern Alabama. (NOAA WPC)

The total rainfall from this event will diminish as you go east, dropping back to 2 to 4 inches in Georgia and 1 to 2 inches in South Carolina.

Total additional rainfall through Sunday morning. (Pivotal Weather)

And with that, we’ll be able to close the book on Arthur.

Roundtable on Arthur

Let’s talk for a moment about Tropical Storm Arthur. We’ve seen some interesting cases of middling tropical storms on the Texas coast in the last 15 years or so. There was Tropical Storm Don in 2011 that absolutely imploded upon approach to the coast. Tropical Storm Bill was a big rainmaker that held its own as it tracked into Oklahoma with saturated soils and the “brown ocean effect” possibly helping aid the heavy rains up there. Tropical Storm Imelda held tropical storm intensity for less than 12 hours, yet caused $5 billion in damage from catastrophic flooding.

Arthur was odd in its own right. First off, maximum sustained winds peaked at about 45 mph. Arthur formed right on the coast of Texas, quickly made landfall, and then tried to reform again near the coast before coming up short. All along, the bulk of the rain and wind was well to the north and east of Arthur’s center.

A satellite loop from Wednesday showing Arthur’s evolution. (College of DuPage)

You can see in the satellite loop how Arthur quickly devolved into what we’d call a “naked swirl,” where the center of the storm is exposed without thunderstorms atop it, often a harbinger of a weakening system. By the time the center of the storm hit Galveston, you needed to travel to near Lafayette, LA to encounter any real rain from Arthur. Wind shear played a huge role in capping Arthur.

Northwesterly wind shear of 15 to 30 kts helped displace Arthur’s thunderstorms to the east, which only increased the farther away from the storm. (University of Wisconsin)

Despite this mess and the general slop that Arthur was, there’s no question the winds verified at tropical storm strength. Maximum wind gusts on Wednesday were anywhere from 40 to 65 mph on the Texas coast between Freeport and High Island. Offshore winds gusted over 80 mph on a (presumably) 100 foot oil platform. Even some 30 to 40 mph gusts occurred in Houston proper.

Maximum wind gusts from Wednesday on the Texas and southwest Louisiana coasts. (NOAA)

But this was not by any means a typical behavior for a tropical storm. Most of these winds were way out in front of the center. In fact, some of the stronger gusts occurred with clearing skies, so either the drier air punching in behind Arthur’s thunderstorms helped mix down these stronger winds that had been sitting mostly aloft or something else was at play.

Either way, this behaved a lot more like I would have expected to see in later fall (Hurricane Delta in 2020 comes to mind, which mixed down very strong winds in southeast Texas as the storm transitioned to an extratropical feature). I will be curious to see if in the postseason report, the NHC reclassifies Arthur as subtropical or notes what exactly happened here. Again, none of it was what you would have necessarily expected, so if you were thinking Arthur was an odd duck, you are correct.

No other major concerns right now. At this point we’ll plan to return Monday unless the situation changes. Enjoy the Juneteenth holiday weekend for those celebrating and honoring the day (An unsolicited plug for Conroe, TX native Annette Gordon-Reed’s “On Juneteenth,” which explains history and why this day really matters.)

Flooding rains in the South, with an unusually potent severe weather outbreak possible in the Midwest today

In brief: We continue to watch Potential Tropical Cyclone 1, which now seems unlikely to become Tropical Storm Arthur. It will, however, deliver areas of flash flooding to the Deep South and Gulf Coast. Additionally, we’re watching an abnormally strong severe weather outbreak risk today in the Midwest.

Potential Tropical Cyclone One is struggling to organize this morning for a couple reasons. As we noted yesterday, the center is displaced far west of the most thunderstorm activity, so the system just isn’t exactly healthy to begin with. Secondly, the center of PTC 1 is really just hugging the coast at this point. If the storm can’t get over water, then it can’t really have a chance to become a tropical storm.

When you look in a textbook, this is not the image you will see for “tropical storm.” (WeatherFront)

Whatever the case, this remains a rainstorm above all else, and it will likely drift inland even if it ends up over water after a short while.

(NOAA/NHC)

Heavy rains do continue to fall along the Texas coast, where we’ve seen as much as nearly 9 inches of rain so far today near Lake Jackson and Surfside Beach. The overall trend today will be for the rain in Texas to gradually subside by midday or so, tapering back to more scattered showers. For more details specific to the Houston area, please check Space City Weather.

Rain totals peaked south of Houston and southwest of Galveston overnight. (NOAA)

As the day goes on, the heavier rain will likely refocus toward coastal Louisiana. We should then see heavier rain begin to expand into interior portions of Mississippi and Alabama. The moderate risk (level 3 of 4) for heavy rainfall expanded to that area today. That continues over southern Mississippi, Alabama, and Georgia tomorrow, as well as perhaps the Florida Panhandle.

A moderate risk for flash flooding exists in Acadiana into southern Mississippi. (NOAA)

In general, rain totals seem to have backed off a little since yesterday, or at least they’re more focused on Alabama than Mississippi and Louisiana now. Still, upwards of 3 to 6 inches of additional rain, with isolated higher amounts are likely from about Baton Rouge through Auburn. Flood watches have been extended all the way to the Atlanta area now as this heavy rain expands. Those may also be expanded into parts of South Carolina eventually as well, as the system actually tracks pretty much due east across the South. About 1 to 4 inches of rain is possible in portions of north Georgia and South Carolina, especially inland.

Rain forecast from NWS (Pivotal Weather)

Bottom line, the bullseye for flooding risks is in southern Mississippi and Alabama at this time, while more isolated flooding risks exist around those areas.

Once PTC 1 is out of the way, things should begin to calm down and heat up in the Deep South and Texas.

Potent Midwest severe weather outbreak

One other note while we have you this morning. There is also flash flooding risk today in the Chicago area, as well as in parts of northwestern Indiana. We could see as much as 2 to 4 inches of rain from storms today.

Heavy rain and flash flooding risk have led to a moderate risk (3/4) for Chicagoland and northwest Indiana. (NOAA)

Additionally, there is likely going to be a rather potent severe weather outbreak today across a chunk of the Midwest. A moderate risk (level 4 of 5) is in place for areas just south of Chicago, north of St. Louis, and northwest of Indianapolis. All modes of severe weather are possible today from large hail to large, long-tracked tornadoes to powerful straight-line winds.

A broad enhanced and moderate risk for severe storms is in place today between Toledo, OH and just east of KC. (NOAA)

Surrounding the moderate risk is a large enhanced risk (level 3 of 5) that includes those cities noted above and extends from just outside Kansas City through Toledo. Basically, folks in these areas should ensure maximum awareness today around any warnings that are issued, and it’s probably a good day to take those warnings a little extra seriously given the potential that exists. There are already some potent storms impacting places like Peoria, IL this morning with wind gusts of 60 to 80 mph reported northwest of there back into Iowa.