A significant severe weather event will affect Alabama tomorrow morning. A large upper-level storm system and associated cold front are currently out in the Plains, and will approach Alabama overnight. Southerly flow has already brought warm, humid air in here, and the southerly winds will increase tonight. Dewpoints are in the 60s in central Alabama now, and in the lower 70s along the Gulf Coast and out in the Gulf.
The blue circle on the following map shows where the air that will be here tomorrow is coming from. And buoys indicate dewpoints around 72 degrees out there. The air will dry out somewhat as it comes inland, but we could see dewpoints near 65 by tomorrow morning, very high for this time of year.
The wind shear with the approaching system is incredible. Check out this plot of wind speed vs. height tomorrow morning (from the NAM model).
Winds at the ground will average 20 to 30 mph, with gusts as high as 50 mph at higher elevations (Red Mountain, Shades Mountain, Oak Mountain, Mt Cheaha, etc.) So, some minor wind damage and power outages are possible before the storms even get here. Note that the winds at 1,000 feet are almost 50 mph, and above 3,000 feet they go over 70 mph. Any thunderstom downdraft can transport some of these winds down to the surface. The winds also change direction with height, meaning there will be spin in the atmosphere and some storms could rotate. Isolated tornadoes are possible.
Still, upper level temperatures will not be that cold, so instability is the limiting factor with this system. This will not be like April 27, 2011, when CAPE values are 3,000. However, with dewpoints to our south near 70 and southerly flow, we could see CAPE 1,000, and that would be enough for severe thunderstorms. The WRF model shows CAPE values of 750 to 1,000 over west Alabama tomorrow morning as the main squall line of storms comes through. Other models have lower CAPE, but we are leaning toward the WRF because of what we are seeing on current maps. There comes a time when models are less important and current data is what you have to look at!
The bottom line is that we expect a line of severe thunderstorms to move through Alabama tomorrow morning. It could enter northwest Alabama as early as 5 am or so, will probably move through Birmingham around 8 or 9 am, then into east Alabama by 11 am. The main threat still appears to be damaging straight-line winds of 70 mph in some locations. However, if the really humid air to the south makes it up here, tornadoes could occur also. If the air gets unstable, it is possible that a few supercells could develop ahead of the main line, affecting western Alabama before the main squall line.
Go ahead and have a weather plan before you go to bed tonight. Have a source of weather information with batteries, and a flashlight. Keep your NOAA Weather Radio on alert. Also, decide where you will go if a tornado warning is issued. Small, interior rooms on the lowest floor are best. This will come through BHM around rush hour, so evaluate the weather before you leave home and if the storms are getting close, it will be best to wait until they pass through, because you do not want to be driving in heavy rain and winds strong enough to push trees over on cars. Watch Fox 6 news at 5 pm for additional information from JP Dice.
Dr. Tim Coleman
Fox 6 Severe Weather Expert
Twitter _ timbhm