Some of the higher-resolution forecast models have come in and the story has changed a little in what we will be expecting for the potential of severe weather on Saturday. The latest extended HRRR (12z run) goes all the way through 7 am on Saturday morning, while the 12z NAM 3 km goes through 7 pm Saturday evening. Here is what they are showing…
HRRR
This model run really only goes through the first round of storms that may bring a threat of severe weather to Central Alabama. This shows a potent line of storms moving into the western parts of the area around the 3-4 am window and quickly progressing eastward across the area and will be pushing out of the area by 1 pm. If this model lines up with what happens on Saturday morning, this will be more of a damaging wind threat with the chance of a couple brief spin-up tornadoes.
There will be plenty of veering with height with this set up as there will be a low-level jet over the area with wind speeds around 50-70 knots, and helicity will be rather high as well, but instability will be at zero or very low at this time. Therefore, the tornado threat will be very low and will be more of the brief spin-up variety. Once again, the main threat with this setup will be from damaging wind gusts up to 60 mph.
We do see in the last couple of frames that another cluster of storms will be moving across the Mississippi River by 8-10 am. We’ll have to see if these actually materialize and move into the area during the late morning through the early evening hours. Instability will be on the rise and therefore strong to severe storms may be possible if these make it into the area, with damaging winds and large hail being the main threats. The low-level jet will have moved out of the area, and that will remove any tornado threat with the second round.
NAM
This model run is different from what the HRRR is showing, but has some of the same ingredients as well. We look to start off with some shower activity over much of the area starting before midnight on Friday night with the heavier rain and thunderstorm activity moving into the western parts of the area around or just after midnight. The activity forms into more of a line of storms by the 3-4 am time frame on Saturday morning, then breaks into more scattered storms as we get closer to sunrise with a shield of rain and storms over the northern half of the area.
Similar with the HRRR, the NAM is showing plenty of helicity and shear as there is a low-level jet over the area with winds in excess of 50-70 knots. Once again, instability is not found with these storms, so the main threat will be from damaging winds. We cannot rule out a brief spin-up tornado just with the amount of shear and helicity that will be available.
For the second round of storms, it looks like the first round will leave a boundary behind that storms will form along starting around or just after midday and move southeastward across the area through the afternoon and into the early evening hours, before exiting by 7-9 pm Saturday night. Instability will be high during this time, but the low-level jet will be over Georgia by this time which will remove nearly all the tornado threat from the area. Damaging winds and large hail will be the threat with this round of storms.
So, with the combination of these models, here is what I am thinking for the threat of strong to severe storms across the area. The primary threat of severe storms (first round of storms) will be from 3 am to 11 am (west to east) with the potential of damaging winds up to 60 mph and a brief spin-up tornado or two. The secondary threat (second round of storms) will be from 12 pm to 8 pm (northwest to southeast) with the potential of damaging winds and large hail.
The secondary threat of severe storms will be conditional according to how much instability can form across the area after the first round moves through, along with how much dry air gets pulled into the area.
The Storm Prediction Center has placed much of the southern half of the area in a level 2/5 Slight Risk for severe storms from 7 am Saturday morning to 7 am Sunday morning for locations along and south of a line from Gainesville (Sumter Co.) to Vestavia Hills (Jefferson Co.) to Weaver (Calhoun Co.). A level 1/5 Marginal Risk has been issued for locations north of that to as far north as a line from Vernon (Lamar Co.) to Cullman (Cullman Co.) to New Moon (Cherokee Co.).
We also note that the SPC has included a level 1/5 Marginal Risk for locations south and west of a line from a few miles north of Panola (Sumter Co.) to Clanton (Chilton Co.) to Tuskegee (Macon Co.) to Baltic (Pike Co.) on their Day 2 Severe Weather Outlook valid from 7 am Friday to 7 am Saturday.
There is still a good bit of time before this event arrives, so the forecast could change for the better or for the worse. Stay tuned as updates will continue through the rest of the week and through Saturday.
Central Alabama Weather 