Seasonably warm weather has returned for now. Rain and storms by Thursday. Colder again late week.
We are working through yet another chilly overnight during these predawn hours of Monday morning. Temperatures range from the mid 30s to lower 40s across the Tennessee Valley. Despite the milky appearance on satellite data, which is actually the infrared satellite sensing the cold temperatures, skies are clear early this morning. That will allow temperatures to drop a few more degrees before daybreak.
Skies will stay clear on this Monday, and that will allow temperatures to warm quite nicely by afternoon, with temperatures very near or just over 70 degrees for almost all of us. Temperatures drop quickly after sunset, and we will already be back into the mid 50s by 6:00 or 7:00 this evening, but overnight lows will be several degrees milder than the last few nights, only bottoming out in the low to mid 40s by daybreak on Tuesday.
A weak frontal boundary passes by to the north on Tuesday, acting to increase clouds a little bit, but we will remain dry across the area. Wednesday will also be another partly cloudy, dry, and warm day with high pressure off to the east and a southerly wind beginning to crank up with time ahead of the next storm system. Temperatures on Tuesday and Wednesday look to get into the lower 70s, and I can't rule out temperatures near the mid 70s in a few isolated north Alabama communities. The main storm system ejects out toward the area, sending a strong cold front toward the area on Thursday. This significantly increases the chance of showers and storms Thursday into Thursday morning, with showers ending as we head into Friday morning.
We've been watching the storm system scheduled for Thursday for well over a week now to try to determine if there would be a severe weather risk. This is the heart of the fall tornado season across our area, and we have to be vigilant of every storm system. For days, because of this cold air mass in place over the area this weekend, it has looked like moisture supportive of a severe weather threat would not get a chance to return northward ahead of this cold front approaching our area Thursday. However, the last few runs of all the medium-range models we use are suggesting that may not be the case. We are starting to see increasing signs that lower 60 dewpoints (in the 60-62 range) may be able to make it as far north as southern middle Tennessee by the midday through early evening of Thursday. When combined with the really cold air in the upper-levels, this may provide enough instability for not only thunderstorms, but possibly strong thunderstorms. Forecast soundings now show CAPE values (instability) ranging from the 300-500 j/kg range across southern TN to the 400-800+ range across north Alabama during the afternoon on Thursday. That isn't overly high, but it is definitely high enough to support a severe weather risk IF other factors are favorable.
Another thing that gets our attention with this system is that the upper-level trough swing through in a negative tilt. That means that the trough axis tilts from northwest to southeast, and it means that the storm system is more dynamic. There is more "muscle" with the storm system, and more upward motion. The wind profiles are also very strong on Thursday with this system. While the best wind energy and shear (spin) will be over the Ohio Valley in association with the surface low pressure up over the Great Lakes, it is still at a supportive level for concern all the way down into central Mississippi and central Alabama. That wind shear is what would allow storm updrafts to be more organized and longer-lived, and what would give them the potential to rotate.
We still want to watch a few more data runs for consistency in the idea of the adequate moisture coming northward before we start officially talking about severe weather potential for Thursday. Right now, we are not going to officially call for a severe weather threat on Thursday in our forecast products, but that may change if the idea of the 60+ dewpoints coming north into the area remains consistent. Because of this, it is too soon to be talking about timing of any risk, exact areas affected, or the magnitude of any risk. This is simply an early tap on your shoulder, asking you to pay attention to forecast updates as we head through the next few days, as we may have to beef up wording for the Thursday system if current trends continue.
Behind the Thursday system, colder air rushes back into the area for Friday and the weekend. Afternoon highs are back in the 50s on Friday and in the 40s on Thursday. Morning lows by the weekend are back into the low to middle 30s, and it's very possible we may have to drop some of these morning lows toward the 20s as we assess new data. If you're a fan of the warmer weather, enjoy it the next few days while it is here!