Twin States Weather Blog

Midnight Musings

* Forgive me for being late (and lengthy) with this post - I have really been doing a detailed evaluation of the 0z (evening) computer model data. Here are my findings:

PROGRESSION: A closed upper low will become an open trough as it swings southeast from Kansas towards the lower Mississippi Valley. A surface low will also track southeast, along roughly the same track as the upper trough. If that's correct, the surface low would be weakening a bit as it moves eastward. However, I believe the upper trough will likely be a bit farther north, which would keep the surface low fairly intense.

This low will bring a surface front through late Friday, which will interact with a very "primed" atmosphere. Instability values are forecast to be in excess of 1500 J/kg on both the GFS and NAM, with cold air aloft.

Showers and thunderstorms will first fire across northern Mississippi, with activity developing southward through late Friday and into Friday evening. The extent of this southward development is a key issue - I believe the thunderstorms will develop as far south as Highway 84. The best area for severe weather seems to be along and north of U.S. Highway 80 - including but not limited to Philadelphia, Carthage, Newton, Meridian, Scooba, De Kalb, Livingston, York, Eutaw, Demopolis, and Aliceville. The time of maximum concern seems to be from 3pm Friday to midnight Friday night. That's a flexible time frame that could be adjusted later.

MODEL OUTPUT: First, a quick note on severe weather forecasting: it's hard. Also, it's very important to know that severe weather parameters are great forecast tools, but should never solely constitute a forecast. That being said, let's evaluate some of the model-derived forecast parameters:

The NAM is much more troubling than the GFS. It brings our CAPE well above 2000 J/kg, with intense shear. The Energy Helicity Index incorporates both of these, and the NAM forecasts the EHI to approach 5 by Friday evening. Anything over 2 is favorable for a significant severe weather event. Also, the hodograph, a graph of wind speed and direction at various heights in the atmosphere, is forecast by the NAM to be very favorable for supercells and tornadoes.

The GFS is less impressive with the instability and dynamics than the NAM, but still gives us reason for concern. It shows CAPE values over 1500 J/kg, with an EHI approaching 3. Again, 2 is a rough estimation of when the atmosphere is supportive of supercells and tornadoes.


QUESTION MARKS: There is some uncertainty about if our atmosphere will remain "capped" or not. The GFS computer model says that at least a small inversion will be present in the lowest levels, while the NAM indicates otherwise. The GFS has been more accurate than the NAM lately, but I'm leaning towards the NAM solution. The reasoning for that is the favorable surface low track and plentiful warm, moist air. Dewpoints will be in the middle to upper 60's, which is quite favorable for severe weather.


THINGS TO WATCH: First, we'll have to carefully watch the intensity of the surface low as it pulls through the southern Plains and into the Ohio Valley. The stronger the surface low, the higher our chance of severe weather.

Secondly, we must watch for sunshine on Friday. This will serve to destabilize the atmosphere even more, and could also create some differential heating boundaries. These boundaries are created along the line between places that have cloudy skies and those who have sunny skies. The sunny area gets warmer, and a temperature boundary is created. This boundary can serve to enhance the environment for severe thunderstorms.


* The bottom line is that severe weather seems likely Friday afternoon and Friday night. Please arrange to stay close to a good source of weather information Friday. The best way to get severe weather information is a NOAA Weather Radio, which can be purchased at any consumer electronics store. If you live in a mobile home, please stay aware of the latest weather info. In fact, Friday night might be a good night to spend with a friend or relative who lives in a more substantial structure. Remember, the vast majority of tornado fatalities in Mississippi and Alabama occur in mobile homes.

* Scroll down to check out the forecast Significant Tornado Parameter...

posted by WTOK Weather Staff @ 12:38 AM