The overall weather pattern will be very active across the central and eastern U.S. over the next few days, as an upper-level low pressure area and associated cold front move slowly eastward. Significant severe weather is expected today and tomorrow to our north and west. It looks like the most widespread severe weather may occur tomorrow from Monroe to St. Louis, with the system weakening a bit as it moves into Alabama, as upper level support moves farther north. However, there is some disagreement among the models on this, and either way we will face a severe weather threat this Friday. The question is how big of a deal it will be.
The NAM shows dewpoints Friday in the mid 60s…enough for the air to be unstable, with CAPE near 1000 J/kg. However, the GFS shows dewpoints reaching the lower 70s in BHM Friday afternoon, with CAPE near 3000 J/kg as far north as BHM. A few extra degrees of dewpoint, especially once they get that high, makes a big difference! (I’ll post a more thorough discussion of CAPE sometime tonight or tomorrow).
The strongest wind shear should remain north and west of Alabama (hopefully, with CAPES like that). But, the GFS also shows the upper-level jet a little bit farther south, with a little more wind shear over Alabama Friday afternoon (0-1 km helicity 150-250 m2/s2, as opposed to only 100-150 on the NAM).
The bottom line for now…expect severe storms on Friday. If the NAM is right, we’ll probably see a squall line move through, with straight-line winds, hail, and only isolated tornadoes, much like last Friday. If the GFS happens to be right, several tornadoes could occur in Alabama on Friday. It’s something we’ll have to watch very closely.
As James has been mentioning, some cold air comes in behind this system. We could easily stay in the 40s on Sunday, with low clouds. The latest GFS is keeping enough moisture in here at low-levels to keep most areas from going below freezing from BHM south, but with lows in the 30s Sunday night and Monday night, frost could form. And, that’s still 4 days away.
