Twin States Weather Blog

Early Morning Update

* Still watching all of the data come in...The 6z NAM is rolling out, and it shows thunderstorms developing over northern Mississippi later this afternoon, then forming into a line and sinking southeastward.

* I get the idea that many locations along and south of I-20 will stay dry. Conversely, northern parts of our viewing area will probably see some thunderstorms, some of which will be severe.

* Here are some severe weather parameters off of the 0z NAM. These values are valid for 6 pm Sunday:

CAPE: 1660
Storm Relative Helicity: 177
Energy Helicity Index (EHI): 1.7 (combines CAPE and Helicity)
LI -6.4

I don't want to just throw these values around without some explanation.

CAPE is a measure of instability, or the tendency of air to rise. Rapidly rising air is the process behind thunderstorm development. Values above 1000 are generally unstable and favorable for thunderstorms.

Storm Relative Helicity (SRH) measures the vertical wind shear in the atmosphere. When the wind direction shifts in a clockwise fashion as we go up in the atmosphere, this is a profile favorable for enhanced thunderstorm development. For example, if the winds at the surface are from the southeast, and the winds 5,000 feet above the ground are from the southwest - we have shifted clockwise with height. This is favorable for severe weather. Values over 150 are characteristic of thunderstorm environments.

EHI combines CAPE and Storm Relative Helicity. By looking at both instability and wind shear, this value provides a thorough assessment of severe thunderstorm potential. Values over 1 are favorable for severe weather.

LI stands for "Lifted Index". It takes into account how warm the surface is and how cold the temperatures are about 15,000 feet off the ground. Negative numbers are unstable, the lower the number, the more unstable.

These are some of the parameters we look at to determine the severe weather threat on a given day. I must caution you that these are not hard, fast guidelines. Instead, we use these parameters along with a detailed analysis of the atmosphere to try and pinpoint the severe weather potential.

* Maybe it's just late, but I have to wonder why the SPC's Day 1 "Slight Risk" doesn't extend further north. I believe the storms will pop initially across the northern part of Mississippi, and they will be severe there. It wouldn't surprise me to see the "Slight Risk" shifted northward early tomorrow morning. Just food for thought.

* Time to hit the hay!

posted by WTOK Weather Staff @ 2:27 AM