A widespread outbreak of severe thunderstorms, including an elevated threat of strong tornadoes, is getting underway across the central and southern United States on Thursday. The areas at the greatest risk of severe weather stretch from Arkansas to southern Illinois, southward to northern Louisiana. States at the highest risk of seeing tornadoes also include parts of Kentucky and Mississippi, according to the Storm Prediction Center (SPC), which is part of the National Oceanic and Atmospheric Administration (NOAA).

The SPC has placed all of Arkansas in a "moderate risk" area for severe thunderstorms and tornadoes, which indicates a high confidence for severe weather. Counting areas in "slight risk" zones, NOAA said that nearly 52 million people are at risk of seeing severe weather through Thursday night.

The greatest threat for strong tornadoes and very large, damaging hail will last through this evening across Missouri and Arkansas, the SPC said in a press release. Later Thursday night, the storms may congeal into a series of lines or storm complexes, which would raise the risk for damaging straight line winds and reduce the tornado risk slightly.

Image: Weatherbell.com

Flood watches were also issued for the areas at risk of seeing strong thunderstorms on Thursday and Thursday night, since the thunderstorms are bringing heavy rainfall.

Bill Bunting, chief of operations at SPC, told Mashable that so far this year, the U.S. has seen just one-third of the typical number of tornadoes expected through the beginning of April. “This is really the first regional severe weather event this spring,” he said. The cold winter played a major role in suppressing tornado formation during much of the winter months, since the configuration of the jet stream blocked warm, humid air from moving north out of the Gulf of Mexico.

Severe Weather Briefing for April 3,2014 http://t.co/ITxRJDSeWF

— NWS Memphis (@NWSMemphis) April 3, 2014

Thursday's storms are the result of a collision of air masses along with strong disturbances at higher levels of the atmosphere. This time, warm and humid air is being drawn northward from the Gulf of Mexico, Bunting says, and this air is meeting a colder air mass behind a cold front sweeping into the central U.S. from the north and west. In Minnesota, on the other side of the front, more than a foot of snow is forecast to fall through Friday. Across the severe weather threat zone, temperatures are warm near the surface but cold aloft, which encourages air to rise and form clouds and precipitation. In addition, an upper level weather system is moving east from the Rockies, and this is providing the necessary wind shear to create long-lived, rotating thunderstorms.

1200 UTC surface analysis. #arwx pic.twitter.com/71bgbLhLrZ

— NWS Little Rock (@NWSLittleRock) April 3, 2014

Wind shear occurs when wind changes speed or direction with height, and it is a critical ingredient for forming tornadoes.

Bunting says the greatest threat for strong tornadoes is from interstate 70 southward into Missouri, all of Arkansas, and portions of Kentucky, Tennessee, and Mississippi. At least one tornado in Missouri had already been confirmed as of Thursday afternoon.

The current watch summary as of 2pm CDT this afternoon. All currently valid watches are tornado watches and... http://t.co/FJpjKFd67M

— NWS SPC (@NWSSPC) April 3, 2014

Moderate risk days are relatively rare, with the U.S. averaging five such days during a typical April. Tornado season ramps up from now through early summer, and this year follows an unusually quiet season last year. Due to a lack of skill, given that tornadoes are such a localized phenomena, meteorologists do not yet issue seasonal tornado outlooks, so it is unclear what this tornado season has in store for the U.S.

One potentially significant wrinkle in ensuring public safety during this storm event is a series of outages affecting NOAA's websites. As of 3:40 p.m. ET, as the agency was issuing tornado watches and warnings, the SPC's website was down, as was the map on Weather.gov showing the national view of watches and warnings. These outages are not the first time that NOAA's websites have had hiccups during severe weather days.

As of 3:45 pm ET, tornado watches were in effect from southern Illinois to Texas, indicating the potential for tornadoes in those areas.

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