Archive for March 15th, 2012

Details on Friday’s Severe Weather Risk in TX/OK

Published on Thursday, 15 March 2012 16:55

David Reimer

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If you’re not a weather nerd, just read this section here. This graphic shows the latest severe weather outlook for Friday Afternoon and Evening. As you can see, the Storm Prediction Center has issued a standard risk of severe weather for the Texas Caprock, Northwest Texas, and Western Oklahoma. The risk generally extends along and east of Interstate 27 from Lubbock and Amarillo eastward into Oklahoma to the Interstate 44. The southern edge of the risk approaches Interstate 20. The primary timeframe will be 4 PM to 12 AM on Friday. Large hail will be likely in the initial discrete supercells, but the damaging wind threat will increase as storms grow upscale into a large complex.

If you’re interested in the meteorological aspect of this setup, grab some coffee and get comfy, it’s geek time!

[caption id="attachment_6696" align="aligncenter" width="600" caption="18Z NAM: Dewpoint values at 7 PM Friday"][/caption]

These graphics will be coming from the 18Z (1 PM CDT) North American Model. This specific graphic shows surface dewpoint values at 7 PM on Friday. We have a very large warm sector in place with a moisture graphic looking more like something you would see in May verses Mid-March. Needless to say, much of the Central and Eastern United States continues with unseasonably high temperatures and moisture levels. Whenever you see something like this in March and April, you usually know you’re going to get something bad. Friday is not going to be the bad day, however Sunday and Monday are looking very dangerous in terms of significant severe weather, including tornadoes. This threat extends all the way from South Texas north into Kansas and may be the most significant severe weather threat that Central and North Texas have faced in a long time. That said, I’ll post about that event in a later blog post. Lets concentrate on Friday…

The 60°F isotherm line will be roughly from Brownwood, Texas northeastward to about Dallas at 7 PM according to the NAM. To the northwest, into the Texas Panhandle and West Texas, dewpoint values are between 53°F and 57°F. As we are going to be dealing with a typical spring setup with lower shear and higher instability amounts, we have to start being more picky about dewpoint values. Generally speaking and depending on surface temperatures, 55°F dewpoint values will support organized convection (fancy term for storms). While I won’t show you the graphic, surface temperatures will likely be around 80°F tomorrow afternoon. The difference between the temperature and dewpoint help determine what the cloud base height will end up being. The lower the spread, the lower the cloud bases. Generally speaking, I like to see values around 15 to 17 degrees apart to support surface based storms with tornado potential (assuming the ingredients are in place to support tornadoes.) With surface temperatures near 80°F and dewpoint values near 55°F, storms on Friday will likely be high based. That means instead of being very close to the ground, their cloud bases will be several thousand feet above the ground. While this doesn’t really mean any difference in terms of the severe hail or damaging wind potential, it does lessen the tornado threat considerably.

[caption id="attachment_6697" align="aligncenter" width="600" caption="18Z NAM: 500 millibar winds at 7 PM Friday"][/caption]

This graphic shows mid-level winds at 500 millibars, or 18,500 feet above sea level, at 7 PM on Friday. Unlike the major tornado outbreak in the Southeast United States on March 2 where we had over 100 knots of wind at 500 millibars, we have a much weaker upper level disturbance for tomorrow’s setup. Notice that we even barely register on the color scale. Basically speaking, I like to see 35 to 40 knots of wind at 500 millibars to support longer lived thunderstorms. We’re right on the edge of having enough wind shear. Practically speaking, wind shear will be fairly marginal tomorrow. If this was a average March setup with low instability amounts, tomorrow would likely amount to nothing. As you’re about to see however, instability amounts are going to be quite impressive on Friday.

[caption id="attachment_6698" align="aligncenter" width="600" caption="18Z NAM: Surface Based Convective Available Potential Energy (Instability) at 7 PM Friday"][/caption]

This graphic shows surface based instability at 7 PM on Friday. Generally speaking, anything over 1500 J/Kg gets my attention in March. Instability amounts on Friday are going to be closer to what you would usually see in April. Instability will be unseasonably high tomorrow across all of Texas and the Southern Plains. The reason we’re not going to see widespread severe weather over a larger area, outside of the standard severe weather risk, is that wind shear will be very weak and a cap will limit the overall extent of thunderstorm activity. Where the marginal wind shear meets the unstable atmosphere is where tomorrow’s potential for severe weather, including supercells, will take place. Right now, that is in Northwest Texas into Western Oklahoma. A cap will keep thunderstorms from developing until upper level energy arrives in West Texas. Current guidance suggests that storms will likely fire between 3 and 5 PM in the form of supercells. After a few hours, they will merge into a larger complex of thunderstorms that will move eastward into Western Oklahoma and North Texas. There is some uncertainty how far east this complex of thunderstorms will make it overnight, but the severe weather threat will gradually diminish by midnight.

The overall marginal wind shear and high cloud bases (Lifted Condensation Levels) will keep the tornado threat low on Friday. However, the high instability amounts will favor large hail. We may see very large hail, larger then golfballs to baseballs, in the strongest supercells. As the storms begin to merge into a complex of storms, the hail threat will remain but a damaging wind threat may also develop as the storms form a cold pool.

We will be chasing on Friday with our new Verizon Mifi (WOOT!) and will have live streaming video at www.TexasStormChasers.com/live – We’ll have a short update later tonight on Friday‘s potential along with a discussion on the potential major severe weather threat on Sunday and Monday.