A Close Look at Thursday’s Stormy Weather
10:20 PM Update
Some of the evening guidance (the RPM) is indicating an enhancement in the severe weather threat for Thursday, and that same guidance is showing a slower line of storms. The slower it moves, the more time we have to destabilize. Other guidance still has us on the borderline between a very small risk of severe storms and just a windy, rain event.
We have been talking about stormy weather coming in on Thursday for several days now, and we are starting to see some of the details a little more clearly. Weather forecasts for very specific times and conditions are a lot like trying to figure out what make and model an automobile is from a mile away on a flat highway. You know it’s there, but you just can’t see a lot of the specific characteristics until it gets closer.
Two of the main things to watch for with a potential severe weather event are dynamics and thermodynamics. Dynamics are the mechanical processes creating wind (horizontal and vertical) in the atmosphere such as a cold front, warm front, or wind shear. Thermodynamics refer to the fuel – the warm, humid air that storms feed on.
Sometimes these storm systems have the dynamics of a jet engine with lamp oil for fuel – that jet won’t fly. The dynamics generally need to be in balance with the thermodynamics, so let’s investigate this system in the modeling and see what we find:
- Dynamics are very strong. The upper air winds are forecast to be very strong in the approaching system; that will bring some strong dynamic lift assisting in thunderstorm development. The change in wind speed and direction with height is also noteworthy; one factor we are watching is the helicity. Helicity is a measure of the wind shear (speed and direction) throughout a certain depth (1 km or 3 km) of the atmosphere.The helicity forecast over North Alabama at 10 AM on Thursday is in the 350-450 m2/s2 range. That is concerning; the higher than number gets over 300, the more likely it is that storms will be capable of intense rotation. It’s not a guarantee of a tornado, but it definitely assists in the production of them. The key word there is “assists;” it takes more than wind shear to bring tornadoes.
- Thermodynamics in storms are often best measured by CAPE (Convective Available Potential Energy). This particual set up is not a “high CAPE” environment. Dewpoint is a great measure of the quality of the low-level storm “fuel,” and in the case of Thursday, it looks like those dewpoints will struggle to reach the lower 60s in North Alabama and Southern Tennessee.
We have had a few documented cases where tornadoes occurred in environments where dewpoints only warmed into the 50s; however, it is rare. The last time it occurred in Alabama was the Five Points Tornado in Huntsville in January 2010. That kind of set-up produced CAPE values in excess of 1000 J/kg (high enough for storms); this storm system does not look the same. CAPE values are forecast to just bump up into the 100-200 J/kg range. That is very, very low.
There is also the issue of the best dynamics running away from the limited thermodynamics we expect. If that happens, this will just be a line of brief heavy downpours with some gusty winds. We’ll cross our fingers and hope for the best, but we are always on the ready if the worst should appear.
The Bottom Line:
- So here’s what you can expect on Thursday morning: a squall line moving out of North Mississippi will come into The Shoals around or just before sunrise. The atmosphere will be a little more unstable in Marion, Franklin, Winston, Cullman, Lawrence, Morgan, Colbert and Lauderdale Counties, so we will be watching that part of the Tennessee Valley closely for any signs of severe weather.
- At this point, since the dynamics and thermodynamics are not in good balance, this looks like a classic case of “a near miss” with severe storms. That being said, a “near miss” is still close enough that we want you to stick with us as this thing comes through. It only takes ONE severe storm to make a big impact.
- The risk of tornadoes is NOT zero, but it is very low. The most significant factor could end up being wind gusts to 40 MPH ahead of the storms on Thursday morning and some isolated gusts above 60 MPH within the storms.
- You do not need to spend the next 36 hours worrying about a major severe weather threat. This is an event that needs your attention, but all you have to do is stay informed and do the things we preach all the time: have your NOAA weather radio on and programmed (or have your smart phone apps active), have a plan to take shelter if necessary, and check back with us for the latest forecasts. We cannot pinpoint a single community this far in advance, so I can’t answer specific questions about when the worst storm will hit a particular town. That’s what the warning process is for…we let you know as soon as we are certain there is a threat for a specific area.