March 16, 2019

Historic Storm Triggers Historic Flooding Across Nebraska

The record-setting storm that blew across the Plains states earlier this week led to a catastrophic flooding situation in Nebraska, unleashing the some of the worst flooding ever seen by many of the region's living residents Waterways across eastern Nebraska surged over their banks as a confluence of weather conditions led to a sudden surge of runoff into rivers and streams.

An intense low-pressure system developed over eastern Colorado and western Kansas earlier this week. The storm set record minimum air pressure readings in many communities across the region; storms of this strength usually don't form this far south or west. The storm brought blizzard conditions to Colorado, Nebraska, and the Dakotas—the Colorado Springs airport recorded a record-high 97 MPH wind gust—and a round of severe thunderstorms on the southern end of the system.

The eastern edge of the system didn't get much attention beyond the risk for severe thunderstorms. Strong winds circulating around the east side of the low brought in a surge of warmer temperatures from the south. Temperatures soared into the 60s through parts of Nebraska and Iowa, a sudden change after so many weeks of subfreezing temperatures. The situation led to an intense period of flooding across the regions rivers and streams.

Sen. Ben Sasse has been tweeting out pictures of the flooding and damage as he tours the flooding in his home state today. The images of the flooding are otherworldly in many places.

NWS Omaha, located in Valley, Nebraska, had to evacuate their office and relocate 155 miles away in Hastings, Nebraska, due to the rising waters. A helicopter tour later that day revealed that their office was completely surrounded by floodwaters.

Multiple communities, including Fremont, were completely isolated by the flooding due to high waters and severed bridges. Large chunks of ice flowing through floodwaters damaged and destroyed homes, businesses, and bridges.

Dozens of river gauges across Nebraska—as well as surrounding states—saw water levels reach major flood stage. The Missouri River in Omaha is expected to crest at 34.5 feet on Sunday, firmly within moderate flood stage and six feet below the all-time record of 40.2 feet.

A significant number of waterways reached their highest crests ever recorded. We saw a record crest along the Elkhorn River, not far from Norfolk, Nebraska...

...and a record crest along the Platte River near Ashland, Nebraska...

...and along the Platte River near Leshara, Nebraska...

...and along the Platte River at Louisville, Nebraska...

...and along the Big Sioux River near Hawarden, Iowa...

...and along the Big Blue River near Crete, Nebraska...

...and on, and on.

No single factor led to the flooding; each factor compounded on the next to created the perfect conditions for historic flooding.

The ground was still frozen from many weeks of subfreezing temperatures, which means that much of the rain that fell and snow that melted this week simply ran off into waterways.

The above chart shows temperature data for Omaha, Nebraska, from March 1 through March 16. Omaha spent most of March below freezing before this week's storm. The storm sent temperatures surging up to 60°F on March 13, accompanied by 1.37" of rain over the course of three days.

Daily snow depth measurements between March 8 and March 16, 2019. (NOAA/NOHSRC)

The sudden burst of warmth combined with the steady, heavy rainfall chewed away at the snowpack left on the ground from previous snowstorms. Omaha went from 7" of snow on March 8 to no snow on the ground by March 13.

Frozen ground combined with a sudden snowmelt due to warm temperatures and heavy rain all forced massive amounts of excess water into local rivers and streams. This sudden surge of water sent rivers to record crests.

This was a relatively well forecast event. It didn't come as a surprise as far as forecasts go. The National Weather Service warned of "major to historic flooding possible" across the region. I mentioned the threat in my post on the storm earlier this week. But when you're experiencing record crests, you have no frame of reference for what to expect because you've never experienced it before.

Excess water in Nebraska and Iowa will slowly filter downstream through next week. The National Weather Service expects major flooding along the Missouri River near St. Joseph, Missouri, by next Thursday. Water levels in the hardest-hit areas of Nebraska should slowly recede through next week. A threat for rain on Tuesday could lead to around half an inch of rainfall for the affected areas. Waters should recede enough that the forecast rainfall shouldn't add insult to injury.

[Top Image: March 16, 2019 (RAMMB/CIRA)]

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March 12, 2019

Unusually Intense Storm Set To Bring Every Type Of Weather To Central U.S. This Week

Would you like some weather? Here, have much weather!

A low-pressure system developing over the central Plains will grow into quite the storm over the next couple of days. Latest forecasts indicate that the low could bottom-out with a minimum central pressure around 974 mb, which could break all-time minimum low pressure records in some areas. Low-pressure systems don't typically get that deep until they're much farther north or east.

This storm is the result of a sharp trough moving across the Rockies and perfectly placed jet stream in the upper levels of the atmosphere. The combined lift from the trough and the jet stream will allow the low to develop fast and strengthen even faster. Models and forecasts both show the storm developing into quite the sight on satellite imagery, so expect tons of satellite shots to cross your social feeds over the next couple of days.

The center of the country will run the entire gamut of hazardous weather over the next couple of days, spanning from dangerous severe thunderstorms over in the south to a raging blizzard in Nebraska and the Dakotas. Heavy rain and quick snowmelt could also lead to significant flooding along some waterways in the Midwest.

Record Low Air Pressures?

Several weather stations in the southern Plains could come perilously close to their all-time record low air pressure readings when the center of this low-pressure system moves overhead. It's common for lows to form in this part of the country and strengthen as they move north and east toward the Great Lakes. But the depth of the trough crossing the Rockies today will allow the low to ramp up over southeastern Colorado and southwestern Kansas, areas not usually traversed by such a dynamic storm.


David Roth over at the WPC has a fantastic page showing maps of all the record high/low air pressure readings from across the country. The black map above shows the all-time record lows across the contiguous United States. The Gulf and Atlantic coasts stand out for their exceptionally low pressure records set during major hurricanes. (Add a '9' to the reading to get the air pressure—743, for instance, is 974.3 mb.)

Minimum air pressure records for much of the southern Plains are in the same range. The all-time record low pressure in Dodge City, Kansas, is 974.9 mb, and it's 973.6 mb in Hays, Kansas. Depending on the exact strength and track of the low, a few of these all-time low pressure records could fall.


Weather records like this are nice for weather geeks, but what practical effect do they have? This is going to be a sprawling, high-impact storm for the center of the country.


A strong low-pressure system wrapping up across hundreds of miles of flat terrain is a recipe for strong winds. High wind warnings and wind advisories are in effect for the next couple of days as the storm develops and moves through the region. The strongest winds are likely across the western Plains, where wind gusts up to 70 MPH could lead to structural damage, downed trees, and power outages.

Dakotas Blizzard

All of that wind will be a huge problem farther north. Cold air to the north of the center of low will allow precipitation to fall in the form of snow, and snow there will be.

The developing storm system will work with deep plumes of tropical moisture from both the Pacific Ocean and the Gulf of Mexico. All that extra moisture will allow this storm to produce a widespread swath of one to two feet of snow from Wyoming to North Dakota. Some cities will likely see two feet of snow by the end of the storm.

Snow will begin from southwest to northeast during the day on Wednesday and continue through Thursday night in northern areas.

If the snow isn't bad enough, those roaring winds will lead to white-out conditions for much of the snowstorm. Blizzard warnings are in effect from northeastern Colorado through central North Dakota in anticipation of a prolonged period of low- to no-visibility conditions during the heaviest snow and highest winds.

Severe Thunderstorms

Thunderstorms are a concern on the south side of the system. There's an enhanced risk for severe thunderstorms for the remainder of Tuesday across parts of Texas and New Mexico. At the time of this post's writing, several severe thunderstorms were ongoing in southeastern New Mexico. A tornado watch is in effect through Tuesday night for parts of western Texas and southeastern New Mexico, including Carlsbad, Midland, and Roswell.

The severe threat will advance east on Wednesday and Thursday as the cold front pushes into warm, unstable air flowing north from the Gulf of Mexico. Damaging wind gusts, large hail, and a couple of tornadoes are possible each day.

Heavy Rain and Flooding

Areas stuck between the snow to the north and storms to the south can expect a general period of cold, windy rain. The latest precipitation forecast from the Weather Prediction Center shows much of the country east of the Rockies seeing at least an inch of rain over the next week.

NWS river flooding forecast for the next seven days. (NWS)
The rain could be a particular problem in the Midwest where snowpack will rapidly melt in the heavy rain and sudden warm-up. River level forecasts from the National Weather Service show dozens of locations across the Midwest surging to major flood stage this week. More than 100 river gauges are forecast to see moderate or major flooding this week. The Platte River in Louisville, Nebraska, about halfway between Lincoln and Omaha, could come within a few tenths of an inch of breaking its all-time record high water level, a record set back in March 1960.

[Top Image: NOAA/WPC]

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March 9, 2019

There's an Enhanced Risk for Strong Tornadoes in the Mid-South on Saturday

An enhanced risk for severe thunderstorms exists in parts of the southeastern United States on Saturday, the third weekend in a row residents have to look out for an outbreak of dangerous thunderstorms. Today's risk includes the possibility for strong, long-track tornadoes, as well as the chance for damaging wind gusts and large hail.

The overnight update from the Storm Prediction Center shows an enhanced risk for severe thunderstorms stretching across the eastern Mississippi Valley from about Cairo, Illinois, down to northeastern Mississippi. The enhanced risk includes eastern Memphis and western Memphis. It's important not to focus solely on the bullseye in the forecast. Dangerous thunderstorms are possible across a large portion of the southern United States, stretching from Dallas/Fort Worth east through Birmingham and north through St. Louis and Louisville.

The enhanced risk is in effect because of the heightened risk for strong, long-lived tornadoes. If this sounds familiar, we went through this last weekend in Alabama/Georgia and the weekend before that over pretty much the same areas as today. That's not to say that Saturday will be a repeat of either of those events. But all of the ingredients exist for thunderstorms to begin rotating and possibly produce significant tornadoes, in addition to large hail and damaging winds.

The SPC's overnight update characterized this as a "complex forecast scenario" given all the moving parts in the atmosphere that could lead to (or hamper the development of) destructive thunderstorms during the day on Saturday. Stable air from morning showers/thunderstorms could rob certain areas of the instability storms need to thrive. Storms could also develop into squall lines and become predominately a straight-line wind threat. The greatest tornado risk will exist in discrete thunderstorms that develop independent of other lines or clusters, allowing them to fully engage with the instability and wind shear around them. The risk areas on the maps above will evolve through the day as forecasters get a better feel for the environment.

If you're in or near the affected areas, make sure you have a way to receive weather warnings the moment they're issued—most importantly, make sure wireless emergency alerts are activated on your phone—and ensure you have a plan in case you go under a warning. Avoid mobile homes and big box stores on a day like this. Put on closed-toe shoes if dangerous storms approach and keep a bicycle or motorcycle helmet in your safe place at home that you can put on if you need to take cover for a tornado warning.

(Apologies for snagging maps straight from the SPC rather than making them myself like I usually do. It's late and my mapping program is giving me a hard time.)

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March 4, 2019

Deadly Alabama Tornado Tests The Boundaries Of Advanced Tornado Warnings

A significant tornado outbreak in the southeastern United States on Sunday killed at least 23 people despite hours of advanced warning that dangerous storms were on the way. Most of the fatalities occurred in Lee County, Alabama, likely making that tornado the country's deadliest in nearly six years. Both the strength of the tornado and the types of homes in the storm's path are a stark reminder that advanced tornado warnings can only go so far in keeping people safe.

The Storm Prediction Center recorded 38 tornado reports as of 12:00 AM Central Time on Monday. Many of those tornado reports were likely duplicates from the same long-track tornadoes as they moved across Alabama and Georgia. The worst tornado moved through Lee County, Alabama, around 2:00 PM CST on Sunday afternoon. The tornado killed at least 23 people, with officials warning that the death toll could climb as rescuers comb through the debris.

It may not seem like it given the high death toll, but Sunday's tornado outbreak was a well-predicted and well-warned event. The SPC issued an enhanced risk for severe weather with its update on Saturday morning, giving residents a full day to prepare for the risk for dangerous thunderstorms.

Sunday began with an enhanced risk for tornadoes across the hardest-hit areas. The black hatching within that 10% risk zone indicated the risk for strong, long-lived tornadoes. The amount of wind shear and instability across the region was ideal for the development of supercells capable of producing strong tornadoes.

The SPC issued a tornado watch for the region at 11:40 AM CST, about two hours before the fatal tornadoes touched down. The storms were hauling toward the east at highway speeds, but meteorologists were able to keep up and issue alerts and warnings well ahead of their arrival.

The above radar image is enough to put a knot in your stomach when you know the environment is ripe for severe weather. If you're a frequent reader of my posts, you'll know that one of my go-to lines ahead of a severe weather day is "the severe threat will come in two rounds—first in discrete storms, then with the squall line that follows." Discrete storms are more likely to produce tornadoes and large hail, while the predominant threats with a squall line are damaging winds and occasional tornadoes.

Discrete thunderstorms are able to fully engage with the environment in which they develop. If they're far enough away from other storms, they can take advantage of all the instability and wind shear they need to attain the maximum strength the environment will allow. We saw that happen in abundance on Sunday. There were dozens of tornado warnings in effect at one point across the southeast, with many meteorologists remarking on Twitter that they can't remember how long it's been since we saw so many confirmed tornadoes occurring at once.

That discrete-before-the-squall scenario doesn't always materialize during a severe weather event. Oftentimes we'll see sloppy storm modes that result in widespread, blobby masses of thunderstorms that are rough, sure, but not what they could've been given the amount of wind shear and instability.

That's what happened up near Memphis during last week's severe weather threat. All of the ingredients were's just that only a couple of storms were able to take advantage of it. (It turns out that the rare storm that could fully engage produced an EF-3 tornado in Columbus, Mississippi.)

A mesoscale discussion—basically a localized heads-up from the SPC—issued a full hour before the lethal tornado in Lee County, Alabama, actually pinpointed the affected areas as having the maximum risk for a strong tornado as the storms moved through. "Given the ample buoyancy and intense shear profile in place, it appears tornadogenesis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring."

The NWS office in Birmingham issued a tornado warning for Lee County at 1:58 PM CST, continuing the warning polygon from the storm's previous track. The tornado was already on the ground by the time it entered Lee County's tornado warning. Radar imagery shows how quickly the storm wrapped up and produced an intense tornado. Forecasters soon upgraded the warning to a rare tornado emergency once it was clear that a destructive tornado was underway.

If you're familiar with radar imagery, you know this image. We've seen it too many times. This is a supercell like you'd see in a textbook. The tornado is the pendant at the end of the hook, right at the intersection of where the inflow of unstable air wraps in to meet the rear-flank downdraft pumping around the back of the system. The dark purple circle within the hook isn't just rain and hail—it's debris swirling around in the tornado. That's pieces of homes and trees and vehicles being picked up by the radar.

The long-track tornado appears to have followed the entire length of Lee County, continuing for a while across the Georgia border. The storm missed several dense population centers, including Auburn and Opelika to its north and Phenix City to its south.

Unfortunately, the storm missing population centers didn't keep the death toll down. A cursory look at satellite imagery along the tornado's path—confirmed by a scientist who study tornadoes in the southeast—shows that many of the homes damaged or destroyed in the storm were likely mobile homes. There are dozens of them on satellite imagery, some directly beneath the debris signature in the different radar images.

Meteorologists and other scientists are always working to lengthen the lead time ahead of a tornado. The longer ahead of time someone has a warning, the longer they have to get into a safe place and brace for impact. The SPC warned of the threat hours ahead of time and the NWS issued a tornado warning before the storm arrived. Local news channels showed the debris swirling around in the air on radar imagery as the tornado moved from town to town.

From beginning to end, this was a well-warned event.

We always focus on making sure as many people as possible hear a tornado warning the moment it's issued. But all of the advanced warning in the world can only go so far in preparing people for one of nature's strongest forces. Sunday's tornado was at least an EF-3 and probably stronger than that. The storm was moving at 60 MPH. Many of the residences in its path appear to have been mobile or modular homes. A human being simply cannot survive that kind of storm without being underground or deep within a much stronger structure. Many people in weaker homes out in the country simply have nowhere to go to ride out a storm like that. A tornado warning is only as good as your ability to act on it.

[Maps: me | Supercell Radar Images: Gibson Ridge]

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February 26, 2019

AccuWeather, Man, What Are You Doing?

AccuWeather published a story today predicting that tornado activity this year will focus on springtime twisters in Tornado Alley and that the United States will see 1,075 tornadoes in the year 2019, coming in six percent below average. Oh, word?

The statement offers no qualification. They don't show their work. There's some text about what they expect this year and what's normal and how tornadoes get ratings, but there's nothing more about that extremely specific number that says "this is an estimate" or "here's the margin of error" or "we arrived at this number by taking Phil the Groundhog hostage and pumping him full of M&Ms until he was wired enough to shake a crystal ball and see through the time-space continuum."

Nope. Just a matter-of-fact statement: "AccuWeather predicts there will be 1,075 tornadoes in 2019, which is nine percent more than the 987 tornadoes in 2018."

Okie dokie.

The Pennsylvania-based weather company has a history of making maverick-adjacent moves when it comes to long-term weather prediction. Predicting the exact number of tornadoes that will form this year—with no further explanation, no margin of error, just presented as if decreed by an oracle from on high—is the latest in a series of vanguard-ish stances taken by AccuWeather in recent years.

The company started releasing 45-day weather forecasts earlier this decade, stretching them out to 90 days after a couple of years. The company says that these forecasts are really trends to help you get a general idea of the weather in a few months, but they're...uh, not accurate...and, even as they're justified as "just trends," they're certainly presented as a detailed weather forecast to their target audience.

For example, AccuWeather's forecast for my town for Sunday, May 26— 90 days from today—shows a high of 74°F and a low of 54°F with a 40% chance of thunderstorms. Their forecast for May 26 issued on February 26 predicts an east wind of 7 MPH with gusts to 15 MPH. They say that the Sunday three months from today will see rain for three hours that amounts to two-tenths of an inch.

Trend? No. And the tornado forecast they issued today isn't a trend, either. They know that people are going to read this PR stunt literally. Whether or not it's accurate doesn't even begin to approach the fact that it's not scientifically justifiable to predict an exact number of tornadoes for the year or tell us what the weather's going to be like with mind-bending specificity 2,160 hours from now.

A specific number of tornadoes doesn't really help people much, anyway. What's 1,075 tornadoes in the grand scheme of things? How many tornadoes constitutes an outbreak? How many tornadoes does it take to destroy your house? You could make basically the same argument when it comes to hurricane season predictions, but at least those involve ranges and some general vagueness.

I'm sure that some news organization or weather blog is going to reach out to AccuWeather for comment. They're going to try to get an answer for how AccuWeather arrived at this extremely specific number. Whoever speaks with them will likely talk in circles and avoid answering the question. It's proprietary. It's a trend. Take it seriously, not literally. But clarification after the fact doesn't matter. The statement is out there. The number is out there. That's what they chose to present to the world.

AccuWeather predicts 1,075 tornadoes this year. And yet, somehow, they couldn't predict the reaction to last month's marketing stunt putting down National Weather Service meteorologists to boost their own products during the lengthy government shutdown that caused many of those hard-working government scientists to drain their bank accounts to zero to stay afloat.

I should point out that there are a lot of fantastic scientists who work for that organization and I consider many of them good friends. Stuff like this, though? This is coming from the top. And as long as the top-level culture gets a rush whenever the weather community condemns their latest marketing stunt, this is the kind of stuff we're going to have to deal with.

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NOAA Delays Release of Updated GFS Weather Model Because It Likes Snow Too Much

NOAA announced on Tuesday that the agency will delay the release of the updated GFS weather model because they accidentally gave it the personality of a husky.

The update's release, which had been scheduled for the middle of March, was already pushed back once due to the government shutdown in January. NOAA's press release covers the issues scientists still have to resolve before it goes operational:

-The snow depth and the water equivalent of snow depth at the surface have unrealistically large values when precipitation occurs in environments with low-level temperature profiles close to freezing. Techniques that use either of these variables for deriving snowfall will exhibit excessive snowfall values.

-The model forecasts exhibit a cold bias in the lower atmosphere that became more prominent after late September 2018.
In other words, certain situations can cause the model to exaggerate cold and snowy weather like a Twitter account that inexplicably has a five-figure following.

The GFS model, often referred to as "the American model" in weather forecasts, is one of two major global weather models—the other being the much-vaunted European model—that meteorologists use to guide their forecasting process.

GFS-FV3 utilizes a new dynamical core (Finite Volume Cubed-Sphere) that gives the model improved resolution and accuracy. The updated GFS model can, for instance, visualize thunderstorms in a way previous versions couldn't.

The operational readiness of the updated GFS model has been a hot topic of debate for meteorologists on social media. The Washington Post's Capital Weather Gang covered the issue just a couple of weeks ago when some meteorologists began to voice concern that a buggy model was being rushed into service.

The scientists working on this (and all) of NOAA's weather models are hard at work resolving the issue, and they'll probably have it fixed within the next couple of months. NOAA's press release notes that the delay is indefinite, giving the team time and space to work on a fix.

You can still view experimental runs of the model on pretty much any weather modelling website, including Tropical Tidbits, which is the (fantastic) site I usually use anytime I need a model graphic.

[Top Image: Tropical Tidbits]

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February 24, 2019

Power Outages Likely as Intense Winds Blow Across Much of the Eastern U.S. on Sunday

An intense windstorm will rake much of the eastern United States on Sunday as a powerful low-pressure system tracks across the Great Lakes. The storm system in question is part of the same upper-level trough that brought rare winter weather to low elevations in southern California and the desert southwest on Thursday and spawned dangerous severe thunderstorms in the southeast on Saturday. Wind gusts topping 70 MPH in some places could lead to widespread power outages and tree damage.

The most striking feature on weather maps going into Sunday was the giant splotch of wind advisories and warnings covering all or part of more than 20 states. The criteria for a wind advisory or a high wind warning differs from region to region, but the gist is that winds will get high enough on Sunday to knock down trees and power lines, especially where the ground is saturated from recent heavy rains.

Blizzard warnings supplant some of the wind advisories to the west of where the alerts on this map stop, including in the northwestern part of Michigan's Lower Peninsula, as well as parts of Iowa, Minnesota, and Wisconsin.

Source: Tropical Tidbits

Intense wind gusts are likely on Sunday due to a low-pressure system explosively intensifying as it moves northeast across the Great Lakes. The low will undergo what’s known as bombogenesis, a process that’s often referred to as “bombing-out” or (annoyingly) a “bomb cyclone” in news coverage.

Bombogenesis occurs when the minimum central pressure in a cyclone drops by 24 millibars or more in 24 hours. The low had a minimum pressure of 994 mb as it moved over eastern Iowa at 10:00 PM EST on Saturday. By Sunday afternoon, the low will be near the Great Lakes with a minimum pressure of about 975 mb, which represents a 26 mb drop in 24 hours. 

An intense low-pressure system that’s rapidly intensifying is prime territory for intense wind gusts. The strongest wind gusts will occur near the Great Lakes where winds could top 70 MPH at times. The windstorm could be particularly severe in western New York, where a long fetch over Lake Erie will allow winds to gust to 75 MPH near the coast.

Not only will the winds cause damage, but this storm could result in a significant seiche on Lakes Erie and Ontario, potentially leading to major coastal flooding on the eastern shores of the two bodies of water.

Here's what NWS Buffalo had to say about the coastal flooding potential on Lake Erie in their forecast discussion published at 1:17 AM EST on Sunday:



A seiche occurs when wind causes the water in a lake to slosh back and forth across the entire body of water. Think of a child sliding back and forth in a bathtub—gradually causing larger waves on either end of the tub that eventually splash over the edge and make a mess—just occurring on a much larger (and slower) scale. A wintertime seiche on an ice-covered lake is particularly dangerous because chunks of ice could cause damage to structures along the coast, in addition to the ice chunks causing additional flooding on top of what's already occurring.

Make sure you've got batteries and actual flashlights—not just the flashlight feature on your cell phone—as well as some non-perishable, ready-to-eat foods just in case the power goes out for an extended period of time on Sunday. You can keep track of power outages over at PowerOutages.US, a great resource for real-time power outage information across the country.

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February 23, 2019

A 'Significant Tornado and Wind Damage Event' Is Possible in the South on Saturday

The first significant severe weather outbreak of 2019 is likely to unfold on Saturday across parts of southern United States. An approaching storm system will allow for a dangerous environment in which thunderstorms can grow severe in a hurry. The Storm Prediction Center characterized the threat for Saturday as a "significant tornado and wind damage event" in their Friday night forecast. The severe weather threat will develop during the late morning hours and continue through nightfall.

The same upper-level trough that brought unusual snow to southern California and the desert southwest on Thursday will serve as the catalyst for the severe weather we'll see on Saturday. A surface low-pressure system was already developing in northeastern New Mexico on Friday night. This low will continue to strengthen as it moves across the southern Plains and into the Midwest through Saturday evening.

A combination of warm air, muggy dew points, and an advancing cold front will trigger the development of strong thunderstorms across the Deep South on Saturday afternoon. The storms will likely turn severe as they interact with robust wind shear passing over the region.

Friday's late-night forecast from the Storm Prediction Center paints a moderate risk for severe thunderstorms across parts of Mississippi, Alabama, and Tennessee, which is a 4 out of 5 on an ascending scale measuring the threat for severe weather across a region. An enhanced risk for severe thunderstorms—a 3 out of 5 on the scale—stretches from northeastern Louisiana to central Tennessee, including Memphis, Nashville and Jackson, Mississippi.

The moderate and enhanced risks are in place due to the threat for strong, long-lived tornadoes and damaging straight-line wind gusts in excess of 70 MPH. The threat for these two significant hazards isn't confined to the areas shaded orange and red on the map, but it's where the SPC expects the environment to be most conducive for the most dangerous thunderstorms to develop.

Storms are likely to fire-up across the western risk areas in the late morning hours on Saturday, continuing to grow in strength and extent as they move east through the afternoon and evening hours. Many areas in the eastern part of the risk areas could deal with their severe threat after nightfall, which adds an extra element of danger for folks who insist on looking outside (which, let's face it, is pretty much everyone) before diving for the basement or interior bathroom.

It's tempting to focus solely on the bullseye in a severe weather forecast, but it's important to remember that severe thunderstorms are also possible in the marginal (dark green) and slight (yellow) risk areas. Everyone between Little Rock, Baton Rouge, Birmingham, and Evansville should stay alert tomorrow and prepare to take quick action if a warning is issued.

Damaging straight-line wind gusts are most likely in storms that form into squall lines, while the threat for strong tornadoes and large hail are more likely in individual storms that are able to fully interact with the environment without contamination from nearby storms choking off their supply of unstable air.

There's also a risk for flash flooding across the affected areas. Flooding adds insult to injury when there's a threat for major severe weather. Parts of Mississippi and Alabama have seen 8-10 inches of rain in the last week. Many of these same areas will see even more heavy rain with the storms on Saturday. It's a good idea to stay off the roads (and avoid storm chasing!) to avoid running into flooded roadways when you might have to outrun dangerous thunderstorms.

Safety Tips

Here are some tips to remember on Saturday, especially if you're visiting this region and you're not accustomed to preparing for severe thunderstorms.

Remember that a tornado watch means that conditions are favorable for the development of tornadoes over the next couple of hours. A tornado warning means that a thunderstorm is capable of producing a tornado based on strong rotation detected by Doppler radar, or that someone spotted a tornado in progress. A watch means to watch for storms. A warning means you need to act immediately.

Tornado warnings are represented on most weather maps and radar imagery as a red polygon. The threat for the tornado is greatest within that polygon, but areas just outside of a warning polygon should keep an eye on the storm. Polygons are drawn based on the speed and motion of a thunderstorm. Storms can change direction and speed up or slow down without much notice.

Make sure you have a way to receive severe weather warnings the moment they're issued. Enable the emergency alerts function on your cell phone. Wireless Emergency Alerts are free push notifications sent out by the National Weather Service using cell phone towers to triangulate your position and determine if you're in a tornado or flash flood warning.

Do not rely on tornado sirens for tornado warnings. These aging systems are unreliable and they're only designed to be heard outdoors. People have died in tornadoes because they relied on tornado sirens that they never heard. If you don't have a cell phone—or if you live in an area with an unreliable signal—listen to television, radio, or NOAA weather radio for severe weather updates. Local news stations will run wall-to-wall coverage if tornado warnings are in effect, and many radio stations in the south will stream those severe weather newscasts.

The best place to ride out a tornado is in an underground basement or dedicated tornado shelter. If you can't get underground, your best bet is an interior room on the lowest level of the building. The goal is to put as many walls between you and flying debris as possible. An interior bathroom works really well since there are pipes in the walls and (usually) a bathtub to crouch in.

Wear closed-toe shoes like sneakers or boots if you're under the threat for tornadoes. You don't want to get caught walking through broken glass and wooden splinters barefooted or in flimsy flip-flops.

Wear bicycle or motorcycle helmets if you have to take shelter during a tornado warning. You might look like an idiot, but the fashion police are taking cover, too, and the helmet could save you from head injuries if the worst happens.

Since we're getting into severe weather season, it's a good idea to make the Storm Prediction Center's website a part of your daily routine if you live in a part of the country prone to severe thunderstorms. The SPC's forecasts are tremendous and they can keep you informed of a potential severe weather outbreak days in advance.

(I updated this post at 2:00 AM EST to mention the threat for flash flooding across the areas expecting dangerous thunderstorms.)

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