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Severe storms could produce baseball-sized hail, tornadoes, torrential rain

Severe weather headed towards southern Plains

Just days after the biggest severe storm outbreak of the season so far, another major thunderstorm threat is on the horizon. A vigorous, slow-moving system will impact areas from Texas to the Eastern Seaboard from Wednesday through Saturday.

The National Oceanic and Atmospheric Administration (NOAA) Storm Prediction Center in Oklahoma has issued alerts for the south-central U.S. for Wednesday and Gulf Coast for Thursday.

This next spate of severe weather is forecast to be almost as widespread as last weekend's damaging storms. The final tally of that outbreak included nearly 60 tornado reports and over 550 reports of wind damage.

The main focus of severe storms Wednesday will range from central Texas north through Oklahoma to southern Kansas. A strong upper-level low pressure system, with cold air pooling aloft, will emerge from the deep Southwest. Thunderstorms will ignite on the "dry line," a sharp division between dry desert air and moist Gulf of Mexico air, as well as a strong cold front.  As these storms tap the cold pool aloft, very large, baseball-sized hail is possible.

The storm system will intensify Thursday as it moves eastward along the Gulf Coast. Upper level winds will strengthen, enabling more energetic "supercell" thunderstorms. As a result, there is a significant threat of tornadoes and damaging winds in Louisiana and Mississippi Thursday and Thursday night.

Since the beginning of April, rainfall has been two to four times above normal along parts of the Gulf Coast. Wet weather in this area is typical of El Niño conditions, which result from the warming of sea surface temperatures in the equatorial Pacific. El Niño invigorates the subtropical jet stream, making for an active southern storm track from Texas to Florida. The effect this season is being boosted by a prominent "warm blob" feature west of the Baja California peninsula in Mexico, where water temperatures are an impressive 5 degrees above normal.

This strong subtropical feed combined with Gulf of Mexico moisture will produce torrential rains from the Gulf Coast Thursday northward into the Northeast on Friday. Rain amounts and localized flooding will be enhanced as the storm system grinds to a halt late week.

The reason for this slowing is fascinating. It starts 4,000 away in Scandinavia, where a mountain of warm air, known as a blocking high, will set up shop. The river of air that crosses the Atlantic, also known as the the jet stream, will be forced to slow down and pile up behind the blocking high. You can think of it as a weather traffic jam. When this happens, a normally fast and flat west-to-east flow undulates, buckles and amplifies, causing big storms to develop and stall out. That is exactly what will happen along the U.S. East Coast Friday into Saturday.

Heavy rain bands may get stuck over eastern Pennsylvania south to the Carolinas. Just above the surface, a strong low-level jet stream will build overhead with 80 mph winds. If strong thunderstorms manage to tap into that low-level jet stream and bring that wind down to the surface, pockets of damaging wind gusts will result. With this kind of energetic pattern even a few isolated tornadoes are possible in those same areas.

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