Update April 28, 12:15 a.m. ET: Seven people were killed in central Arkansas on Sunday when a massive tornado ripped across the state. Local media are reporting extensive damage in the towns of Mayflower and Vilonia from a large, violent tornado that witnesses say was half a mile wide. Elsewhere, two people were killed when a tornado struck the northeastern Oklahoma town of Quapaw. Mashable has the latest here.
We've got major damage in mayflower. #arwx @KATVBarry @KATVNews @KATV_Weather pic.twitter.com/AxG5FpRLoe— Jesse (@WhistlePig11) April 28, 2014
7:54pm CT: Debris now moving higher up into the tornado and meso. http://t.co/8HfKnvOPyw Ughh #arwx— Brad Panovich (@wxbrad) April 28, 2014
A huge swath of the central United States, stretching from Nebraska to northern Louisiana, is on high alert through Sunday night for severe thunderstorms, including the possibility of multiple strong tornadoes, large hail and damaging straight-line winds.
While it is unclear whether a worst-case scenario -- a multi-day tornado outbreak featuring violent, long-lasting tornadoes -- will occur, forecasters are urging residents in affected states to stay tuned to the latest forecasts, and prepare to take tornado precautions if necessary.
The Storm Prediction Center (SPC), which is the tornado forecasting branch of the National Weather Service, has issued a string of tornado watches, with the greatest risk of long-lasting, strong tornadoes focusing on Arkansas, eastern Oklahoma, northeastern Texas and northern Louisiana.
It is in these locations that the necessary ingredients for powerful tornadoes to form are most likely to come together, something that has not happened so far this spring. This year to date has seen one of the quietest tornado seasons on record, which forecasters worry may have lulled residents in tornado-prone states into a sense of complacency.
Cities keeping close tabs on the threatening weather through the early morning hours on Monday include North Platte, Neb., Des Moines, Iowa, Springfield, Ill., Joplin, Missouri, St. Louis, Miss., Little Rock, Ark., and Shreveport, La. The severe weather is likely to break out again on Monday, only a bit further to the east.
According to the SPC, through Sunday night, there is a "moderate risk" of severe thunderstorms, including strong tornadoes at the higher end of the Enhanced Fujita Tornado Damage Scale (EF-2 or greater). At 4 p.m. ET on Sunday, SPC forecasters upgraded a small area in southern Arkansas to a "High Risk" of seeing severe thunderstorms, including the possibility of violent tornadoes. Forecasters also issued a rare "particularly dangerous situation" tornado watch for the same area.
To put the risk level into context, there are, on average, only a handful of "High Risk" severe thunderstorm days in the U.S. each year, as this is the peak alert level on the Weather Service's scale. This is the first "High Risk" day in the U.S. in 2014. The last one was in Nov. 2013, and last one in Arkansas was in May 2011.
Even with the "High Risk" label, it is uncertain as to whether a widespread outbreak of violent tornadoes is going to occur.
For example, across Arkansas, there is an ample supply of low-level moisture to supply storms with energy, along with significant amounts of wind shear, which is when winds vary in speed or direction with height, or both. Wind shear is a critical ingredient in tornado-formation.
If we are going to see strong tornadoes this area where they will likely happen. New watch http://t.co/Nv38oFWdxN pic.twitter.com/fXv9PyxZtC— Bill Karins (@BillKarins) April 27, 2014
However, a third key ingredient, an unstable atmosphere that forces warm air to rise, cool and condense into towering thunderstorms, is on the lower side of the scale in the areas that have the greatest wind shear. To increase instability, the surface air temperatures need to increase across Arkansas, and SPC forecasters expect this to happen throughout Sunday afternoon and into early evening, as breaks in the clouds increase, and more sunshine heats the surface.
If instability grows considerably, the atmosphere will be like a tinderbox, ready to blow up with severe thunderstorms when lit with the slightest fuse.
On a larger scale, these storms are being triggered by a huge swirl in the atmosphere, known as an upper-level low-pressure area, which is now over the Plains, along with a strong low-pressure area at the surface. The counterclockwise circulation around these weather features are pulling moisture from the Gulf of Mexico northward like a giant vacuum cleaner. With all this moisture around, and slow-moving storms forming, this weather system also poses a flash-flooding risk.
The entire weather pattern across North America will be temporarily blocked, or stuck, into the first week of May. An unusually sprawling and strong area of high pressure across southern Canada will prevent the storminess in the Mississippi River Valley from moving east and exiting the East Coast, keeping much of the U.S. cooler and wetter than average for this time of year, and stretching the flood and severe thunderstorm threat for multiple days.
While this storm outbreak comes in an unusually quiet season, it is located close to the 30-year average location for the highest tornado risk on April 27, according to data from 1982 to 2010. Some well-known tornadoes have occurred on this day, including the 2011 Tuscaloosa tornado, which was a large EF-4 long-track tornado that killed 64.
Here is a list of some key Twitter accounts to follow for storm updates.
@afreedma: Mashable's senior climate reporter is tweeting storm warning and other info as this event unfolds.
@SevereWarnings: This account tweets all National Weather Service tornado warnings as they are issued.
@TwcBreaking: Breaking news feed from The Weather Channel, includes warning information and updates on storm damage reports.
@JessJHamilton: Weather Channel producer who tweets valuable insight into the meteorology behind the ongoing storm.
@NWSLittleRock: National Weather Service forecast office with responsibility for much of Arkansas.
@NWSTulsa: National Weather Service forecast office with responsibility for eastern Oklahoma and western Arkansas.
@NWSSPC: Storm Prediction Center, which is the National Weather Service center responsible for issued tornado outlooks and watches.