WEATHER WEDNESDAY: Understanding this weekend’s severe weather threat

Weather Wednesday

The CBS 42 Storm Team has issued a WEATHER ALERT for the potential for severe weather on Saturday. While the exact timing and threats will unfold as we inch closer, here’s an in depth look at what the Storm Team is looking at for this event. While this may get technical at times, keep reading because the goal is to explain these complex meteorological processes in an easy to understand manner.

WATCH: Meteorologist Sarah Cantey provides the latest on this weekend’s weather

SETUP: A deep upper level trough will dig through Oklahoma at the end of the week and pass through the Deep South Saturday evening. The low pressure system will pass to our north, likely through the Ohio Valley.

While this may sound complicated, think of this as a spinning top (turning counter clockwise). Winds ahead of the top (the low) are streaming around the circulation really fast from the south. Behind the area of low pressure, winds are circling around really fast from the north. 

You can imagine how this impacts our area with the warm, moist air over the Gulf of Mexico being pulled up across Central Alabama and then a much colder air mass racing in behind the warm air as the low moves to the east. Where the two air masses meet, that’s the cold front (the blue line on a weather map). So, as the cold front approaches the state and eventually moves through Central Alabama, this will be our window for severe weather. 

Some of the ingredients we look at are highlighted below: 

INSTABILITY (CAPE or Convective Available Potential Energy): What really helps to enhance the amount of instability (and the potential for severe weather) is having colder air higher up in the atmosphere and warmer air at the surface. With strong winds out of the south, it does look like the warm air will definitely be in place. It’s going to be WARM and MUGGY when you wake up on Saturday morning. But, as the cold front approaches, it does look like the temperature difference higher up in the atmosphere is not high enough to support more impressive instability. TIMING is going to be everything with this setup. If the cold front approaches earlier in the day, this will limit the amount of instability (i.e. severe weather potential will be lower). However, a later timing will be more favorable for severe thunderstorms. Think of how much the sun helps to warm things up in the afternoon (warming= more instability).

This image shows one model (the GFS) forecast for CAPE. This model is showing limited instability values.

SHEAR: When we talk about wind shear, we are looking at a change of speed or direction with the winds as you go higher up in the atmosphere from the surface. If the winds are changing direction quickly with height, this leads to rotation (i.e. potential for tornadoes). Think of a pinwheel- When you blow on the top of the pinwheel, the “wind” is faster at the top of the pinwheel than the bottom and it begins to spin. If winds are much faster above the surface (high speed shear), damaging winds are a concern because these strong winds can be pulled down to the surface. This situation (with strong speed shear) appears to be likely for Saturday. This means we are likely going to see a line of storms setup with really strong, damaging wind potential, along the leading edge. 

Directional shear refers to a turning of the winds with height. So, if winds are blowing out of the southeast at the surface and then out of the west when you move higher up in the atmosphere, this too will lead to rotation. Right now, as it pertains to Saturday, the wind profile does look like it could lead to rotating thunderstorms. This is why there is a tornado threat in the forecast. However, how much “spin” in the atmosphere is still a bit uncertain at this time.

TIMING: We will have a few showers around the area early on Saturday. This will be ahead of any severe weather threat. The main line should move into West Alabama on Saturday morning, possibly as early as 7-8 a.m. The line should approach the I-65 corridor between late morning to midday and then push east throughout the afternoon. We will be done with the severe weather threat by the evening. 

It should be noted, the timing WILL change as we get closer to Saturday. So, for now, this is just a rough estimate on the timing. 

THREATS: While there is still uncertainty with the forecast (as mentioned above with questions about timing and the amount of instability, etc.), this setup suggests the potential for all modes of severe weather. Tornadoes and damaging winds appear to be the main concern. However, since we’ve gotten so much rainfall recently, localized flooding will be possible.

HOW TO PREPARE: We are trying to give you enough advanced warning of the potential for severe weather in the hopes that this will give you ample time to make arrangements and spread the word. Hopefully, no one will be caught off guard. There is no reason to panic. With a little bit of preparation and plan, you can ensure your family’s safety. 

  1. If you live in manufactured housing: Riding out the storm in a mobile/manufactured house is NEVER a safe option. Have a plan of where you will go, whether it is a storm shelter or a neighbor’s home with well-built construction. Contact your local county EMA office if you are unsure about the location of a storm shelter near you. When should you seek shelter? The answer is NOT to wait until a tornado warning. This may not give you enough time! When a TORNADO WATCH is issued that’s when you should be putting your plan into action. A WATCH means that ingredients are favorable for tornadoes, where as a WARNING is an imminent threat and alert that a tornado is potentially moving your direction.
  2. Know your location on a map: Make sure you can point out your location on a map and that you know what county you are in. Throughout Saturday, make sure you know where the current warnings are. You can do this by watching CBS 42, going to CBS42.com or checking our social media pages. It’s important to know where the current warnings are so that you can see what is heading your way. This will give you more advanced notice than simply waiting on a tornado warning to be issued.
  3. Have a way to warn you: Have your NOAA weather radio ready to go. Have your cellphone charged. Download the CBS 42 Storm Team Weather App and turn the location services on to get the latest warnings for your area. 

Copyright 2020 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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