From Illinois & Indiana south to the Gulf Coast, severe weather will be likely on Friday. Below is what the Storm Prediction Center is saying on this outbreak.
Day 3 convective outlook
National Weather Service Storm Prediction Center Norman OK
0228 am CDT Wednesday Apr 13 2011
Valid 151200z – 161200z
There is a slight risk of severe thunderstorms across parts of the lower to middle-Mississippi valley, central Gulf Coast…Tennessee Valley and Ohio Valley…
Lower to middle-Mississippi valley/central Gulf Coast/Tennessee Valley/Ohio Valley: A strong negatively-tiled upper-level low is forecast to move from the Central Plains eastward into the Ozarks on Friday. The exit region of a well-developed 80 to 100 knots middle-level jet is forecast to spread across the central Gulf Coast states during the day creating strong deep layer shear profiles favorable for severe storms. The GFS, European model (ecmwf) and NAM begin the day 3 period with an mesoscale convective system located in the middle-Mississippi valley driving this feature eastward into western Kentucky and western Tennessee by midday. South across the warm sector, the models develop moderate instability across much of the central Gulf Coast states and initiate numerous thunderstorms from middle Tennessee southwestward into north central Alabama, Mississippi and southeast Louisiana during the afternoon. The development of a severe mesoscale convective system seems probable across the central and southern parts of the slight risk area Friday afternoon and evening.
Forecast soundings in central MS and western Alabama at 21z on Friday show impressive wind shear profiles with 50 to 60 knots of 0-6 km shear and 0-3 km storm relative helicities around 400 m2/s2. This environment should support supercell development with a threat for strong tornadoes possible. However, the magnitude of the tornado threat will be conditional upon storm Mode. If a squall-line were to develop instead of more discrete convection, then wind damage could be the greater threat. The upper-level system is quite impressive with a well-developed low-level jet in place across the east central states. The GFS and NAM solutions focus the low-level jet further north in Tennessee and Kentucky by late afternoon while the European model (ecmwf) shows a strong component of the jet in the Tennessee Valley and central Gulf Coast states. If the European model (ecmwf) solution is correct and moisture return ends up being stronger than forecast, then a tornado outbreak could occur in parts of western to middle Tennessee, Mississippi and Alabama Friday afternoon. At this point, the potential for an outbreak remains conditional and differences in the model solutions raise uncertainties concerning the exact scenario.
Further north across the Ohio Valley, severe storms will also be possible Friday afternoon as the upper-level low approaches from the west. In spite of weaker instability, a strong wind field is forecast with cold temperatures aloft. This would result in a potential for hail and wind damage with the more organized convective clusters. A tornado threat will also be possible in the Ohio Valley due to the strong low-level shear forecast but this potential should be dependent upon moisture return.