The SPC has upgraded the severe weather risk for Monday for the Tennessee valley region of North Alabama into Tennessee, Kentucky, northern Georgia and points north into West Virginia. Nearly all of the rest of Alabama is in the standard slight risk category for tomorrow.
Severe weather is expected to break out late this afternoon and tonight over Arkansas into Missouri and southern Illinois. This activity will congeal into a squall line overnight, but the possibility is there that we could still see discrete cells ahead of that main line.
The line of storms should reach Northwest Alabama sometime around sunrise. There could be storms ahead of this line down into counties like Marion and Walker up into Lawrence and Limestone Counties. The line will push southeastward to near I-59 by late morning and to I-85 by late afternoon.
This timing could be in our favor, as surface temperatures will not be especially warm, in the middle 60s around daybreak. But dewpoints will be in the lower to middle 60s by then, which is plenty sufficient for severe thunderstorms. Skies will be mostly cloudy, but there could be some breaks in the clouds ahead of the line, and if that happens, temperatures will get into the middle and upper 70s by lunch time.
Instability values are expected to be in the 500-750 j/kg2 range, which is not high, but it is high enough to fuel thunderstorm updrafts.
Winds at about 5,000 feet will not be as strong as the last event, but will still be over 60 mph. So,winds changing with height will not be a problem. This is necessary for organized storms to form.
We look at the low levels of the atmosphere for signs that there could be a tornado threat. The GFS hints that we could see a secondary surface low over northeastern Arkansas by early morning. The proximity of this surface low could help back our surface winds around to the southeast, enhancing the spin in the atmosphere and raising helicity values. This could enhance the tornado threat, particularly on any discrete storms that form ahead of the main line.
Upper winds will be strong enough to produce damaging winds along the line and embedded tornadoes will likely be a problem. Numerous severe thunderstorm and tornado warnings are expected.
After the main line moves through, the atmosphere could recharge enough for a secondary round of storms around mid to late afternoon over North and Central Alabama, but I think it would be less likely to be severe.
Winds will increase out of the south this evening, averaging 10-20 mph with higher gusts. These windy conditions will continue into Monday, averaging 10-20 mph with occasional gusts to 30 mph. Showers and a couple of thunderstorms are possible over the northern half of the state tonight. but the best chances for severe weather should wait until early morning for Northwest Alabama, late morning for the I-59 corridor and later for points to the southeast.
SO BOTTOM LINE…
Timing…showers and storms possible overnight, with severe storms breaking out from the Shoals down into Marion and Lamar Counties as early as 5 a.m. progressing southeastward to the I-59 corridor by 8-9 a.m. and points east a little later. The main line should arrive in Northwest Alabama by midmorning and the I-59 corridor by late morning. There could be some redevelopment of storms by midafternoon behind the main line, with an isolated severe threat.
Intensity…all modes of severe weather are possible, including tornadoes. It appears the highest threat of tornadoes will be over the Tennessee Valley early, but the threat is real across the rest of Central Alabama during the morning. All areas have a threat of damaging winds and hail. Areas south and east of Birmingham may see an enhanced threat of severe weather by early afternoon as instabilities increase. The good news is that the best wind fields will be lifting out to the north by then.
TAKE STOCK NOW…
Review your severe weather safety plan for your family and/or business. Know how you will receive up to the minute weather warnings. Think about where you will be. Know what you will do if warnings are issued for your location. That is the time that you have to be in safe shelter.