Posts Tagged ‘east texas’

Severe Weather Threat Tonight into Thursday: Analysis and Timeline

Published on Wednesday, 20 February 2013 14:34

David Reimer

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NOTE: Graphics have Wednesday instead of Thursday. If a graphic says Wednesday, it is for Thursday. Sorry for the hiccup! 

Good afternoon!

You wouldn’t think we would be talking about severe weather tonight with the weather we have going on right now. Temperatures across North Texas are in the 40s with cloudy skies and light rain. We even had a period of sleet this morning!  The atmosphere over the state is going to undergo a rapid transition tonight as our next storm system influences the state. While the lowest two thousand feet of the atmosphere will likely remain cool and stable, warm air above that will make the rest of the atmosphere unstable. This is referred to as mixed-layer instability and will result in elevated thunderstorms tonight.

Are you wondering what in the world are elevated thunderstorms? I’ll tell you!

Thunderstorms that are elevated develop above an inversion layer, known as a cap. This cap prevents surface parcels from towering upward into thunderstorms. It’s a thin layer of stable air in the lower atmosphere. For tonight’s case, air above this lid is going to be unstable. Tonight, thunderstorms will form on top of the cap, hence the term elevated thunderstorms. Since these storms will be in an unstable air-mass, they could become strong to severe with large hail and gusty winds. The tornado threat will be minimal with these elevated storms since they are rooted above the stable air in the lower atmosphere.

This is the type of setup we expect across much of Texas tonight. The one exception could be part of South-Central Texas. More on that in a bit. Lets talk about the time-frame for our storms tonight.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday at 12 AM.

The next few graphics come from a high-resolution version of the North American Weather Model, known as the 4 KM NAM. The NAM is just like any other weather model and thus don’t expect the radar later tonight to look exactly like the graphics you’re about to see. This first graphic is for midnight tonight (12 AM Thursday) and shows what the NAM believes the radar will look like at that time. It is showing moderate precipitation occurring over the Texas Panhandle and the northern parts of West Texas. Some of that could be frozen precipitation.

Further to the east, the light blues indicate what I expect to be low clouds, fog, and probably occasional mist and light rain. It is not going to be a pretty night. The green blotches over North Texas are indicative of isolated showers and thunderstorms.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday at 3 AM.

By 3 AM Thursday, we expect a line of thunderstorms to be developing generally from Vernon down to near Abilene and San Angelo. Keep in mind that the timing may be off by an hour or two, but our thoughts remain the same. By this point, elevated instability values and wind shear will support the possibility of this line being strong to severe with large hail and gusty winds. Isolated thunderstorms may begin forming ahead of this line over parts of Central and North Texas. These storms would also have a threat for large hail.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday at 6 AM.

By 6 AM, the NAM has a line of strong to severe thunderstorms extending from Henrietta south to Breckenridge to San Saba. This line will likely be producing large hail and gusty winds. Isolated to scattered thunderstorms ahead of this line across North and Central Texas, will also be capable of producing large hail. Lots of cloud to ground lightning could also occur with these cells.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday at 9 AM.

The line of thunderstorms will approach Interstate 35 between 7 and 9 AM. The NAM has this line running from Sherman, Dallas, Waco, to just west of Austin and San Antonio by 9 AM. Still capable of producing large hail and damaging winds, this line will make a mess of the Wednesday morning commute. Additional thunderstorms will have formed ahead of this line across Northeast and East Texas. There are some indications that a second line of storms may form 25 to 50 miles ahead of the original line by this time. Should that occur, the new line would likely begin to weaken the original line. This is something we’ll watch for in the  morning, but isn’t something we can predict with accuracy until it begins to happen.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday at 12 PM.

By lunch time on Thursday, our line of thunderstorms will be moving into East Texas and likely building southward towards the Gulf Coast. We still expect it to be strong to severe with the possibility of large hail and damaging winds. Cells ahead of the line will be capable of producing large hail.

Here is the severe weather outlook for tonight through sunrise Wednesday. The severe weather threat is expected to develop sometime after midnight. Don’t pay much attention to where the risk ends to the east as the severe weather risk is highlighted in tomorrow’s outlook. The main issue we will be watching for tonight with discrete thunderstorms and the initial development of the squall line will be large hail. While most hail will be between dime and quarter size, the strongest storms could produce hail up to golfball size. Not all storms will produce hail. The threat for damaging straight-line winds could increase as the squall line pushes east and becomes better organized.

There is a narrow zone where we will be monitoring for the chance of surface-based thunderstorms. Along and south of the warm front, the cap will be weaker. In this area, we will have to watch for any discrete thunderstorms that develop and can become organized. With a weakened cap and strong wind shear, if any storms were to become organized and surface based, they would likely have a tornado risk. At this juncture, the threat for tornadoes is low, but not zero. We’ll be watching this possibility closely this evening and will update the risk if it appears that surface based storms are more likely to develop.

NOTE: Graphics have Wednesday instead of Thursday. This is for Thursday.

Here is the severe weather outlook for later on Thursday. As you can see, the risk continues for all of East and Southeast Texas. The risks will continue to be large hail and damaging winds, although we cannot rule out a brief tornado. Within the red shaded zone, there is a higher concern about the threat of discrete supercells capable of producing tornadoes. Most of this threat is east of Texas, but we will have to watch parts of Southeast Texas closely.

I won’t spend too much time talking about rainfall totals right now. With the line of storms expected to be moving eastward quickly, rainfall amounts won’t be too high. Generally speaking, we will average between 0.25 inches of rain upwards to 0.75 inches of rain. Where some training of cells occurs, rainfall totals could top out over one inch.

We’ll be here all night providing updates as needed. Make sure you have a way of receiving weather warnings tonight.