UPDATE - 12:40pm Saturday
A Tornado Watch has been issued for the Memphis metro until 8pm. SPC says that within the Tornado Watch area, there is a 50% chance of 2 or more tornadoes, 40% chance of an EF-2+ tornado, and a 60% chance of 10 or more high wind events. Map (showing radar at issuance time) below.
UPDATE - 11:00am Saturday
The Storm Prediction Center has issued updated severe weather risk maps as of 10:30am. The new overview map (below) has lowered the Moderate Risk to a Slight Risk for a portion of west TN (north of I-40 from Memphis to Jackson). This is due to the presence of showers and more extensive cloud cover over this area, which is limiting instability.
What's instability? Think of a pot of water on a stove. The higher you turn up the heat, the warmer the water becomes. Turn down the heat a touch and the water temp decreases. Instability is the heat from the stove and the water is the atmosphere. In a primed atmosphere like we have today, it doesn't take a lot of heat (instability) to fire storms. A little less instability lowers the risk just a bit.
Even though instability is a bit lower, wind shear is still high so any storms that fire CAN still produce tornadoes. The chance of strong tornadoes still exists along and east of the Mississippi River in the latest SPC update. Damaging wind (up to 75 mph) is the primary threat. Timing has not changed from our original post below. We expect a Tornado Watch to be issued in the next hour or two at most. DO NOT LET YOU GUARD DOWN BECAUSE OF A MINOR CHANGE IN THE RISK AREA.
Originally issued 8:25am Saturday:
The Storm Prediction Center, this morning, has expanded the Moderate Risk area - the most likely area to see severe storms - well to the northeast (see above), but the entire metro is still included. The details of the storm system, including it's threats and timing, remain nearly the same as we posted in our blog on the topic late yesterday afternoon.
Below are the probability maps for today, showing the chance of tornadoes, severe wind (58 mph+), and large hail (1" or larger) within 25 miles of you. Click the graphic for a better view. According to SPC - the worldwide leaders in severe weather prediction - we have a 15% chance of a tornado (a few possibly strong), a 45% chance of severe wind (with potential for very damaging gusts of 75 mph+ in storms), and only a very small chance of hail.
Given the high probabilities of severe weather, SPC has issued a "Public Severe Weather Outlook" (below) to draw attention to a potential outbreak of severe storms.
PUBLIC SEVERE WEATHER OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0237 AM CST SAT DEC 21 2013
...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF THE LOWER MISSISSIPPI
AND TENNESSEE VALLEYS TODAY THROUGH TONIGHT...
The NWS Storm Prediction Center in Norman, Oklahoma is forecasting
the development of a few strong tornadoes and swaths of damaging
wind over parts of the lower Mississippi and Tennessee Valleys today
through tonight.
The areas most likely to experience this activity include:
Northwest Alabama
Central and Eastern Arkansas
Western Kentucky
Much of Louisiana
Far Southeast Missouri
Western and Northern Mississippi
Western and Middle Tennessee
Surrounding the greatest risk area, severe storms are also possible
from eastern Texas and the central Gulf Coast region into the Ohio
Valley.
A potent jet stream disturbance now over west Texas will track
northeast into Red River Valley later today, before accelerating
northeast into the mid-Mississippi Valley tonight and early Sunday.
Associated with this disturbance, an area of low pressure now
forming over eastern Texas will become better organized as it moves
northeast into southern Arkansas this afternoon, northwest Tennessee
this evening, and the lower Ohio Valley early Sunday.
Ahead of the low, a broad flow of unseasonably warm and humid air
will surge north from the western Gulf of Mexico into the lower
Mississippi, the lower Tennessee, and the Ohio Valleys through early
Sunday.
The increasing moisture and strong wind field that will accompany
the jet stream impulse will create an environment favorable for
bands of rotating thunderstorms capable of both potentially strong
tornadoes and damaging wind over a broad swath extending from
eastern Texas and southern Louisiana northeastward into the lower
Mississippi and lower Tennessee Valleys.
Although the severe weather threat is expected to be greatest this
afternoon through early tonight over parts of Louisiana, Arkansas,
Mississippi, and Tennessee, a more conditional risk for tornadoes
and damaging winds will persist into early Sunday from the central
Gulf Coast northeast into the mid-Ohio Valley.
State and local emergency managers are monitoring this developing
situation. Those in the threatened area are urged to review severe
weather safety rules and to listen to radio, television, and NOAA
Weather Radio for possible watches, warnings, and statements later
today.
..Corfidi.. 12/21/2013
Today's scenario is not one to be taken lightly. The best chance for severe storms is after 2-3pm and the threat ends before midnight. Tornado Watches will be issued. If your safe place is not ready for you, get it ready! You don't want to be cleaning it out when storms moving 60-70 mph are headed your way. We can't guarantee you'll need it, but it's better to be safe than sorry.The most likely severe weather scenario is a squall line, or perhaps a couple, containing very strong wind, however the tornado threat is not to be disregarded. If a warning is issued, seek shelter immediately. Storms will be moving fast and wind of 60-70 mph or higher will pick up objects and can break the window you're staring out - even without a tornado.
We can't stress enough the need for multiple ways to receive severe weather warnings and paying attention to them today. NOAA Weather Radio, local TV/radio, and smartphone apps are all good ways of receiving info. We strongly suggest getting StormWatch+ activated in your MemphisWeather.net mobile apps this morning and set your location(s) that you want warnings for. Then, don't silence your phone. it could be the best $8 you spend this Christmas season. Links to download are below.
Follows us on social media throughout the day for the latest (links below). We'll strive to keep you informed and safe, but it's up to you to know what to do if severe weather happens, and then to DO IT, if and when it becomes necessary. Stay safe.
Erik Proseus
MWN Meteorologist
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