Severe storms target the southeastern U.S.

Overall setup of this storm on Tuesday and Wednesday of this week (November 22-23, 2011).

An active weather week continues across the United States as a stationary front, which is stalled across Texas, Arkansas, and Kentucky, finally pushes east.  Record warmth ahead of the front with temperatures in the 70’s and 80’s and dewpoints in the 60’s will help fuel the thunderstorms that develop ahead of the cold front.  The biggest threats across the southeast will be strong winds, hail, and isolated tornadoes.  I expect a quasi-linear convective system (QLCS), or squall line, to develop and push east into Mississippi, Alabama, and Georgia Tuesday afternoon into Wednesday morning.

The Storm Prediction Center has issued a slight risk for severe thunderstorms across southeast Texas, Louisiana, Mississippi, Alabama, Georgia, Tennessee, and Kentucky.

The SPC has outlined the biggest threat for severe wind of 50 knots (60 miles per hour) within 25 miles of a point in areas from eastern Mississippi, most of north and central Alabama, and central Tennessee.  These same areas also have the best chance to see an isolated tornado this afternoon (5% probability).

This event will likely be similiar to last week’s event.  There will be enough instability and wind shear in the environment to sustain thunderstorms in the evening and overnight hours.  As the area of low pressure moves northeast, it will strengthen and form a QLCS.  The QLCS is just a squall line that can produce some bowing segments and widespread wind damage with gusts at or over 60 mph. The tornado threat looks minimal as the environment does not look prime for tornado development. QLCS are known to produce small spin-up tornadoes that are usually rain wrapped and are difficult to spot on radar.  People should take squall lines seriously because they can affect a larger area of property and cause a lot of damage pending on the strength of the system.

Storms overnight on Monday, November 21, 2011 in Oklahoma and Arkansas produced heavy rains and flash flooding. Image Credit: National Weather Service

As of now, this system is a huge rainmaker across Oklahoma and Arkansas. It has already produced at least tw0 inches of rain in many areas, and has triggered flash flood warnings.  Little Rock, Arkansas recorded  an impressive 5.98 inches of rain on November 21, 2011. The Hydrometeorological Prediction Center (HPC) shows a huge area east of the Mississippi River receiving at least an inch of rain.  (See image below) The stationary front has formed an area of low pressure across Texas and Arkansas that will eventually lift to the northeast.  It will continue to pump warm, humid air into the southeast United States.  Instability levels will remain highest south of Tennessee. Many areas south of Tennessee will see around CAPE of around 1,000 Joules/kilogram or less, which is enough instability to maintain strong storms.  The more unstable the environment, the better chances to see severe weather.  Since the area of low pressure will be moving further away from the southeast, helicity levels, or spin in the atmosphere, will be less.  It is worth noting that tornadoes are still possible across the southeast, as some storms could spin up a few small, brief tornadoes.  This event will be nothing like the outbreak of April 27, 2011.  A few schools in Alabama have already decided to release their students early before the event begins this afternoon. (You can make your own opinion as to whether or not that is a smart decision. If you like, comment about it below!)

Rainfall totals will exceed over half an inch in many areas. Image Credit: HPC

As you can see in the image above, heavy precipitation will also fall across the northwest United States in northern California, Oregon, and Washington.  Heavy rain and wind will affect this region, and the northern parts of Washington will experience winter storm warnings with snowfall accumulations of 3-12 inches with higher amounts in the mountains.

Looking ahead: The weather should be pleasant for most of the United States during Thanksgiving. The only nasty spots appear to be in the northwest as another system impacts Washington and Oregon.  The next storm system will develop across the central United States on Saturday and cause major weather changes across the eastern United States.

Bottom line: A squall line, or quasi-linear convective system (QLCS), will likely form ahead of the cold front in the southeast and push east.  Alabama will likely see this push into their area late in the evening hours.  Tornadoes are possible with this event, but the biggest threat will be strong winds of over 60 mph.  The system will produce heavy rain from Louisiana into New England with many areas receiving over an inch of rain.  Flash flooding is possible in some areas, and it is important for people to not drive in flooded areas. Remember- “turn around, don’t drown”.  Squall lines can produce widespread damage pending on the strength of the system.  If the system moves faster than what the projected models show, then stronger storms could develop across Alabama due to the daytime heating.  If the system is slower, then the storms will push in during the nighttime hours, which will limit the severity of the storms (lack of daytime heating).  Everyone across the southeast should monitor these storms as they develop and treat all warnings seriously.


November 22, 2011

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