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.

NOAA/WPC


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.

NWS


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.

Wind

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 there...it'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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