Tornado Watch Expected for Northwest Alabama by Early Afternoon
The Storm Prediction Center just issued a heads up notice in the form of one of their Mesoscale Discussions that a tornado watch would be required by early afternoon for parts of Northwest Alabama back through northern Mississippi into southeastern Arkansas and northern Louisiana.
A cold front is approaching the Mississippi River this afternoon over eastern Arkansas and southeastern Missouri. You can see that in the right hand panel on the graphic above, along with the visible satellite and radar composite. On the satellite, you notice some breaks in the clouds over northwestern Alabama back into northern and Central Mississippi. This has allowed temperatures to warm into the 70s across all of Mississippi over into the Tennessee Valley, where it is 73F at Muscle Shoals and 70F at Huntsville.
The surface low is over southeastern Minnesota. It had gotten quite strong, dropping to 988 millibars. It has since started weakening, but it has already pulled lots of moisture northward. Dewpoints are now in the lower 60s across Central Alabama, edging toward the middle 60s over Central Mississippi.
The clouds are helping to keep things in check to a degree, and there is a strong capping inversion over Alabama. That is also keeping a lid on things. But the breaks in the clouds are troublesome, as they will allow things to heat up and instabilities to increase from Louisiana and Arkansas through Mississippi and Tennessee into northern Alabama. Sunshine is bad folks.
The NWS in Birmingham just launched a special balloon at noon and we will soon be getting data back from that about temperatures and moisture and winds aloft. Other NWS stations across the South are doing the same thing. This will help us to understand the situation better.
Thunderstorms are developing ahead of the cold front as it pushes southeastward this afternoon and evening. You can see them forming south of Little Rock. They will form into a long line of storms that will push through the area this evening.
But storms are forming well ahead of the front over northern Mississippi around Tupelo and back near Oxford. These have the best chance of becoming severe and producing potential tornadoes. A tornado watch is already in effect for areas to our north, and that additional tornado watch is likely by early afternoon into Northwest Alabama. Instabilities are over 1,000 j/kg in that area, which is sufficient for strong updrafts in storms. The wind fields are also strong over the area still because of the low to the north.
The significant tornado parameter is still high over northeastern Mississippi and northwestern Alabama, although it has come down a bit in the past couple of hours. It is over 1, which is the general threshold for significant tornadoes, west of a line from roughly southern Sumter to eastern Walker to western Madison Counties. The biggest threat for tornadoes will be northwest of this line through late afternoon. Hopefully, the low level shear parameters will diminish as we get into the early evening hours and the threat will shift to more of a damaging wind threat. There still will be the possibility of an embedded tornado in the eventual squall line late this afternoon and tonight as it pushes into Alabama. It should arrive in Northwest ALabama around 5 p.m. and I-20 after 10 p.m.
It should be weakening by around 9 p.m. or so.
Stay close to a reliable source of weather watches and warnings throughout the afternoon and evening and check back here for frequent updates and additional severe weather information.
Here is the text of the MCD:
MESOSCALE DISCUSSION 0184
NWS STORM PREDICTION CENTER NORMAN OK
1146 AM CST WED FEB 29 2012
AREAS AFFECTED…SERN AR…NRN LA…NRN MS…NWRN AL
CONCERNING…SEVERE POTENTIAL…WATCH LIKELY
VALID 291746Z – 291915Z
STORMS ARE EXPECTED TO EVENTUALLY DEVELOP ALONG THE COLD FRONT AND
SWEEP EWD WITH WIDESPREAD DAMAGING WINDS AND A FEW TORNADOES. A
WATCH IS LIKELY TO BE ISSUED BY 19-20Z.
SURFACE ANALYSIS SHOWS THE COLD FRONT JUST W OF SHREVEPORT LA INTO
NERN AR AT 17Z. ALTHOUGH EXTENSIVE LOW CLOUDS EXIST OVER MUCH OF THE
AREA…THERE ARE POCKETS OF HEATING ESPECIALLY OVER CNTRL LA INTO
CNTRL MS. GRADUAL DESTABILIZATION AND LIFT ALONG THE FRONT SHOULD
RESULT IN A LINE OF SEVERE THUNDERSTORMS THIS AFTERNOON.
AMPLE DEEP LAYER FLOW EXISTS FOR FAST STORM MOTIONS AND SEVERE
WINDS…BUT LOW LEVEL FLOW IMMEDIATELY NEAR THE FRONT WILL BE
VEERED…WHICH MAY LIMIT TORNADO POTENTIAL SOMEWHAT…ESPECIALLY IF
MODE IS LINEAR. HOWEVER…BETTER LOW LEVEL SHEAR/BACKED FLOW WILL
REMAIN OVER ERN MS INTO NRN AL.
OBJECTIVE ANALYSIS AND AREA SOUNDINGS SHOW A PRONOUNCED CAPPING
INVERSION NEAR 700 MB…WHICH WILL LIMIT WARM SECTOR DEVELOPMENT
AWAY FROM THE FRONT. HOWEVER…NRN PORTIONS OF THE AREA…FROM NRN
MS INTO NRN AL WILL HAVE THE HIGHEST RELATIVE THREAT OF THIS TYPE OF
DEVELOPMENT. IF THIS SHOULD OCCUR…THE TORNADO THREAT WOULD BE MORE
SUBSTANTIAL.
..JEWELL.. 02/29/2012
Category: Alabama's Weather, Severe Weather