| Above: Multiple swaths of heavy rain—much of it associated with severe weather—will douse large parts of the central United States from Tuesday, May 7, to Sunday, May 12, as depicted in this 5-day precipitation forecast from the NOAA/NWS Weather Prediction Center. Legend at upper left is in inches. Image credit: NOAA/NWS/WPC. |
Ingredients were coalescing on Tuesday for a multi-day bout of severe weather expected to gradually shift from the Southern Plains on Tuesday into the Lower Mississippi Valley by Thursday. All types of severe weather are possible, with tornadoes a particular threat on Tuesday afternoon and evening.
As of 11:30 AM CDT, the NOAA/NWS Storm Prediction Center (SPC) had placed the central and eastern Texas Panhandle and far western Oklahoma under a moderate risk of severe weather, the second highest of SPC’s five risk categories. Thunderstorms were already firing across the High Plains from Texas to South Dakota at midday Tuesday, with additional heavy storms extending east across Kansas.
The most focused area for severe weather—including the potential for supercells with strong tornadoes, hailstones bigger than 2” in diameter, and wind gusts of 74 mph or more—will be in the moderate risk area, ahead of a dry line across west Texas and south of a stationary front extending northeast into Kansas. Moist, unstable air is surging into this pocket from the Gulf of Mexico, and spokes of upper-level energy will rotate across the region, helping to trigger severe storms. We’re nearing the climatological peak of spring tornado season for Texas and Oklahoma, and this setup has many of the classic hallmarks of a regional severe outbreak.
Surprising fact of the day
— Greg Diamond (@gdimeweather) May 7, 2019
Today is the first time the city of Amarillo, TX has been in a Moderate Risk since May 18, 2010 pic.twitter.com/H52YtMxRD7
If anything is a bit unusual, it’s the westward extent of Tuesday’s threat area. Amarillo, Texas, is no stranger to severe weather, but the most intense supercells often develop just east of the metro area of about 300,000 before heading toward Oklahoma. Amarillo's most destructive and deadly tornado on record occurred on May 15, 1949, when an F4 twister killed 6, injured 70, and demolished some 200 homes.
Supercell storms could persist for several hours across the eastern Texas Panhandle and perhaps into far western Oklahoma until the activity morphs into a squall line. Very high surface winds will then become a key threat as this complex races across Kansas, Oklahoma, and northern Texas late Tuesday into early Wednesday. Large hail will still be possible, and tornadoes (mostly short-lived) could be embedded within the squall line.
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| Figure 1. Probabilities of tornadoes (left), severe wind (center), and severe hail (right) from late morning Tuesday, May 7, 2019, to 7 am CDT Wednesday. Probabilities denote the odds of each event happening within 25 miles of any particular point on the map. Severe wind is defined as gusts to at least 50 knots (57 mph), and severe hail is at least 1” in diameter. The crosshatching indicates where there is a risk of significant tornadoes (EF2 or stronger), significant severe wind (gusts to at least 65 knots or 74 mph), or significant severe hail (at least 2” in diameter). Image credit: NOAA/NWS/WPC. |
A powerful upper-level trough of low pressure in the Southwest U.S. will be slow to move eastward, prolonging the severe weather threat into Wednesday for parts of the Southern Plains. By Wednesday afternoon, the overnight squall line will likely have pushed the most unstable air into eastern Texas, Arkansas, and northern Louisiana, so the highest odds of severe weather will translate into these areas. Tornadoes may again be possible, along with severe winds and hail.
By Thursday, the weakening frontal zone will have pushed into the lower Mississippi Valley. Severe storms packing high winds will be possible from southeast Arkansas and Louisiana into Mississippi, but upper-level winds and surface features are not expected to be as as supportive of widespread, intense severe weather as they will be on Tuesday and Wednesday.
It's already been quite an active year for U.S. tornadoes. According to weather.com's Linda Lam, the preliminary total for April of 277 tornado reports from SPC is higher than the final total for any April outside of 2011, when the catastrophic Super Outbreak ravaged the Southeast. However, Lam notes, "There is no strong correlation between a busy early spring and an unusually active May or June."
Active tornado year thus far in the United States -- 410 confirmed. Currently running above normal when compared to Doppler Era average (1990-2018) through May 5. pic.twitter.com/j53DJRJrfU
— Steve Bowen (@SteveBowenWx) May 6, 2019
Another round of serious flood threats
Saturated soils across many parts of the central U.S. have laid the groundwork for a spring of recurrent flood risk. Multiple bouts of “training” thunderstorms will cruise across the southeastern half of Kansas, where the NOAA/NWS Weather Prediction Center (WPC) has issued a moderate risk of excessive rainfall (enough to cause flash flooding) for Tuesday afternoon into Wednesday morning. “Areal averaged rainfall of 2-3" appears likely, with pockets of 3-5" probable,” warned WPC. “Given the saturated ground across much of this region, this will likely be enough to cause another round of widespread flash flooding...some of which could be significant in nature.”
Another moderate risk for excessive rain is in place from Wednesday into early Thursday for the Arklatex region, where the surging squall line is expected to stall out by midday Thursday before new thunderstorms fire on Wednesday night across roughly the same area. Multiple models are suggesting that 2” or more of rain early Wednesday could be followed by localized amounts of 5”or more on Wednesday.
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| Figure 2. Marissa Whitman, 20, wades in about 3 feet of floodwater from the swelling Mississippi River in East Foley, Missouri, while guiding a boat carrying her boyfriend Brendan Cameron and his mother, Tory Cameron, to their home along Pet Street on Sunday, May 5, 2019. "I just need to see if the water reached inside," said Tory. The family had to evacuate Saturday when the water rose suddenly. Image credit: Laurie Skrivan/St. Louis Post-Dispatch via AP. |
Meanwhile, a major spring flood crest is working its way down the Mississippi. Preliminary data show that an local all-time record crest was set on the Mississippi on May 2 at Rock Island, Illinois, with 22.70’ beating the 22.63’ recorded during the Great Midwest Flood of 1993. Another of the Quad Cities—nearby Davenport, Iowa—saw parts of its downtown under water after a levee failed along the Mississippi on Tuesday, April 30.
At St. Louis, the crest of 41.33’ on Monday, May 6, was the seventh highest in records that go back to at least 1785. Four of the top ten crests in the last 234 years at St. Louis have occurred since 2013. Because of the high water levels and fast-moving currents, the U.S. Coast Guard closed a five-mile-long section of the Mississippi near St. Louis to boat and barge traffic on Friday.
