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7:08PM CST Update: I've posted the liveblog diary for this outbreak. I'm going to keep it going as long as I can.

5:35PM CST Update: A PDS Tornado Watch has been issued for the area that the SPC put in the bullseye this morning. The watch is in effect until Midnight CST. PDS means "Particularly Dangerous Situation," which is enhanced wording added to a tornado watch to indicate the potential for large, destructive, long-lived and long-track tornadoes. Storms are rapidly popping up across central Arkansas right now, and they should start to rotate as they move into the watch area over the next few hours.

I'll post a Severe Weather Liveblog when the tornado warnings start going up.

The dynamics are coming together for quite the January tornado outbreak across a good chunk of the south-central US and Ohio Valley, with the worst weather expected to occur across a large swath of real estate from southern IN/IL through central Mississippi.

It looks like it could shape up to be a major tornado outbreak, with large, strong, long-track tornadoes forming in the strongest storms. In addition to the dangerous tornado risk, storms have the potential to produce damaging winds in excess of 80 MPH and large hail.

This tornado outbreak is expected to occur late this afternoon into tonight, so much of the severe weather will take place overnight tonight into tomorrow morning. A tornado outbreak is dangerous any time of the day, but it's especially dangerous at night when you can't see a tornado and most people are sleeping, oblivious to the warnings. Do not rely on tornado sirens to hear warnings. Subscribe to a severe weather text alert service, get a weather radio, or keep your local news on all night to stay up-to-date on what's going on.

Graphic overview of the severe weather outbreak tonight, from NWS Memphis.

Jump the fold for more discussion.

Severe Weather Risks for Sunday, January 22 2012

Categorical Severe Weather Threat:

The green shading indicates the risk for general thunderstorms, not expected to be severe.

The yellow shading indicates a slight risk for severe thunderstorms. This includes many major cities including (but not limited to) Indianapolis IN, Cincinnati OH, Atlanta GA, Mobile AL, New Orleans LA, Little Rock AR, and St Louis MO.

The red shading indicates a moderate risk for severe thunderstorms. Moderate risks are generally rare, so folks in this area need to take today's weather very seriously. The moderate risk area covers eastern Arkansas, northern/central Mississippi, northwestern Alabama (including Birmingham and Tuscaloosa), western/central Tennessee (including Memphis, Jackson, and Nashville), western Kentucky (including Bowling Green), and the southern tips of Illinois and Indiana.  

Tornado Threat:

This product shows the threat for tornadoes within 25 miles of a point located in the shaded areas on the map. For example, if you're in the 15% risk area, there is a 15% chance (relatively high) that there will be a strong tornado within 25 miles of you. As the legend shows, the warmer the colors, the higher the risk. The hatched area indicates the chance for significant/strong tornadoes.

Damaging Wind Threat:

As with the tornado risk, this chart indicates the chance of seeing damaging winds within 25 miles of a point. The warmer colors indicate higher risk, and the hatched area indicates the chance of seeing > 75 MPH winds in thunderstorms that form.

Large Hail Threat:

The risk for large hail is mostly confined to where the strongest supercell thunderstorms are expected to form, right around the Mississippi River and the Memphis area.

The Setup

A deepening low pressure center is moving quite briskly over Kansas right now, and is expected to reach Missouri by tonight. The low is deepening under a strengthening jet stream, which is expected to reach 100 knots by the time it enters the Mississippi River region later tonight. This jet streak will provide enhanced lift to help build the storms, as well as provide the upper-level shear necessary to create supercell thunderstorms capable of maintaining intense, long-track tornadoes like the ones forecast to form tonight.

GFS model's 250mb jet stream forecast at 9PM CST. Click to enlarge.

Thanks to winds starting to flow into the deepening low, a strong low level jet (a jet of strong winds a few thousand feet above the surface) is developing over the northern Gulf Coast and will serve to pump warm, moist air right into the region where the Storm Prediction Center expects the worst weather to occur.

GFS model's 850mb wind forecast at 9PM CST. (The yellow shading indicates precipitable water at the 850mb level). Click to enlarge.

As the low makes its way into this region of instability, a cold front extending off the low is expected to provide enough lift to fire off some nasty thunderstorms. As these storms build taller into the atmosphere, they will eventually tap the winds from the jet stream and turn into supercells. The intense shear between the jet stream at 250 millibars (generally up around 32-35k feet above the surface) and LLJ (around 4-5k feet above the surface) will intensify the supercells, and create the conditions necessary to create bad tornadoes.

Diagram of the structure of supercell thunderstorms, from the NSSL.

Storms will begin to develop later this afternoon (around supper time...4-5 PM CST), and will continue to grow in number and intensity throughout the night. The worst weather is expected between 9PM tonight and 8AM tomorrow morning.

If you live in the threatened areas, keep an eye on the weather today and throughout the night. This has all the makings to be a dangerous tornado outbreak. Here are some ways you can keep track of the bad weather:

National Weather Service Main Page
National Weather Service -- Memphis TN
National Weather Service -- Birmingham AL
National Weather Service -- Huntsville AL
National Weather Service -- Jackson MS
National Weather Service -- Little Rock AR
National Weather Service -- Paducah KY
National Weather Service -- Nashville TN
National Weather Service -- Louisville KY
National Weather Service -- Mobile AL

Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages

Wunderground's Detailed Radar (click the + nearest to you to see your local radar)

NOAA Weather Models

Severe Weather Liveblog on DailyKos

Me (weatherdude) on Facebook and Twitter
James Spann (great BHM-based meteorologist) on Facebook and Twitter
Jim Cantore on Twitter
Brad Panovich (great CLT-based meteorologist) on Facebook and Twitter

WeatherCall, a for-purchase subscription service for landline and mobile severe weather alerts.

Central Alabama Severe Weather Text Alerts (free, from NBC)
Northern Alabama Severe Weather Text Alerts (free, from NBC)
Memphis TN Email Alerts (free, from ABC)

If you know of any other text alerts from local news stations in the area threatened by today's severe weather, please let me know in the comments so I can add it to the above list.

11:05 AM PT: For those interested, the TOR:CON index from TWC's Dr. Greg Forbes is 6/10 for the threatened areas

12:24 PM PT: From James Spann on Facebook...

He's right. Outdoor warning sirens are notoriously unreliable and if they sound in time, you may not hear them until it's too late.

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