Gusty thunderstorms, flooding to threaten southern US this week

As a multi-day severe weather outbreak was hammering areas from Texas to the Carolinas, leaving a trail of damage last week, AccuWeather meteorologists were already monitoring the potential for flooding, and possible additional rounds of severe weather in some of the same areas this week.

After a below-average number of tornadoes touched down in the first couple of months in 2021, conditions are coming together for a potentially active start to spring, which officially began the morning of Saturday, March 20.

The parent storm that is forecast to trigger flooding downpours and potentially severe thunderstorms this week is first producing accumulating snow across Colorado through Monday.

"A strong jet stream that is forecast to dip from the Rockies into the central U.S. through the middle of the week certainly means that at least some risk for severe weather returns to the South Central and Southeast states," AccuWeather Chief On-Air Meteorologist Bernie Rayno said.

The dip in the jet stream will allow the cold air from the storm in the Rockies to clash with the surge of warm air holding across the eastern half of the country.

This collision is likely to produce some thunderstorms across northern and central Texas, Oklahoma and even western parts of Arkansas on Monday afternoon and evening.

These thunderstorms are likely to produce drenching thunderstorms and travel disruptions, but also the potential for severe weather, including damaging wind gusts, hail and isolated tornadoes.

People who live in Dallas, Houston, Oklahoma City and Little Rock should all pay close attention to the forecast in the coming days as downpours could drench the area along with chances for more tumultuous conditions.

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By Tuesday, the storm will lift northward into the Midwest, bringing the potential for snow in the northern Plains. Meanwhile, in the South, another round of rain and thunderstorms will hit the lower Mississippi River Valley.

Behind this storm, another storm will be moving through the Rockies, producing snow showers on Tuesday. On Wednesday, this storm will move into eastern Texas and Louisiana, bringing yet another round of rain and perhaps the threat for more severe weather.

"Copious amounts of Gulf moisture will flow northward into the South, which could make travel difficult at times along I-10 and other roadways later Tuesday through Thursday," said AccuWeather Senior Meteorologist Dan Pydynowski.

The continuous flow of Gulf moisture could lead to training thunderstorms in some areas, increasing the threat for flooding from eastern Texas and Louisiana through Mississippi, Alabama and the western Florida Panhandle to Georgia, Tennessee and the southern Appalachians of North and South Carolina. Downpours can extend northward into the Ohio Valley.

"Heavy rain could cause standing water on roadways, and downpours could greatly reduce visibility at times for travelers," Pydynowski warned.

The city of New Orleans could be particularly susceptible if repeated downpours hit the area.

In addition to the flooding threat, it is possible that the severe weather threat may extend as far west as eastern Texas, to as far east as the Carolinas during the middle to latter part of the week.

Communities that were struck by fierce winds, large hail, flash flooding and tornadoes, some rated as EF2 in hard-hit states like Alabama, last week may once again face risks from damaging thunderstorms.

At this time, however, AccuWeather meteorologists believe the threat for widespread severe weather will be less than that from the past week.

"The coverage and intensity of the severe weather this week may hinge on the overall strength of the storms and the amount of cloud cover in the zone where thunderstorms are forecast to erupt," AccuWeather Senior Meteorologist Brett Anderson said.

For example, an extensive shield of clouds may limit daytime heating and could reduce the number and strength of thunderstorms that develop. Cloud cover was believed to be one suppressing factor to severe weather on Wednesday across the South, even though more than two dozen tornadoes touched down across the region. Wednesday was the most active day amid a three-day severe weather outbreak, but despite more than 300 incidents of severe weather and close calls for some individuals, there were no fatalities reported during the multi-day outbreak.

"Should sunshine be out in force, thunderstorms will have a better chance of developing and turning severe," Anderson said.

Despite the recent uptick in damaging storms across the U.S., severe weather has been off to a slow start in 2021. The number of tornadoes is behind the average pace for this point in the season, based on data from the National Weather Service Storm Prediction Center.

Only 92 preliminary tornadoes have been tallied by the SPC as of March 18 -- or only 53% of the 16-year average (2005 to 2020) for the year to date. The average for this early point in the season is 173 tornado reports.

However, one change that has recently taken place in the Gulf of Mexico may be an indication that the trend of below-normal severe weather incidents is about to reverse in the U.S.

Major outbreaks of Arctic air in February not only wreaked havoc as the frigid air plunged all the way to the Gulf Coast of the U.S., leaving some communities to deal with the aftermath even weeks after the coldest weather, but it also helped to cool offshore waters. Gulf of Mexico water temperatures dipped to or below average levels in the northern and western parts of the basin. Chilly water in the Gulf can cool the air above, limiting or preventing thunderstorm development over the U.S.

This map shows Gulf of Mexico actual surface water temperatures in Celsius based on satellite data. (NOAA/GOES)

However, Gulf water temperatures had fully recovered by early last week and were generally climbing several degrees above average for the middle to latter part of March. Sea-surface temperatures ranged from the lower to middle 60s along the upper and western part of the coast to the lower 80s just west of Cuba on Tuesday.

When warmer water is present in the Gulf of Mexico, storm systems moving across the United States can more easily pull in warm and humid air that can swell across the South Central states up from the Gulf. Both warmth and humidity are necessary ingredients to fuel the development of severe thunderstorms.

Since Gulf of Mexico water temperatures have climbed to or above average levels in areas from the Plains to the Southeast states, the tornado trend could swing toward average levels, putting more Americans at risk for damaging thunderstorms in the coming weeks.

The time of year and other weather elements could also play a role in the uptick in severe weather. The period from late March through May typically brings a substantial increase in the number of severe weather and tornado incidents across the nation due to strengthening sunshine, warmer weather as well as strong weather systems and a potent jet stream leftover from winter.

AccuWeather will continue to monitor the potential for severe thunderstorms and other threats to daily activities, lives and property in the coming days.

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