As always before diving into the details with any severe weather setup I want to start by highlighted the latest severe weather outlook from the Storm Prediction Center. Compared to the outlook I shared early this morning there have not been many changes. An enhanced risk of severe weather is in place across the eastern Texas Panhandle and portions of Northwest Texas for the late afternoon and evening hours on Wednesday. Perryton, Pampa, Shamrock, Clarendon, Childress, Quanah, and Vernon are in the enhanced risk zone. Much of Western Oklahoma is also in the enhanced risk zone. Surrounding the enhanced risk zone is the area where severe thunderstorms will be possible but will be more isolated (possibly only one or two storms). This zone encompasses the eastern half of the Panhandle, parts of West Texas, and West-Central Texas into part of Northwest Texas. Borger, Plainview, Lubbock, Lamesa, Midland, Snyder, Abilene, and Wichita Falls are included in this risk zone.
The enhanced risk zone is where confidence is highest that several severe thunderstorms are likely to occur on Wednesday. Initial thunderstorms are expected to develop in the 4 PM to 6 PM timeframe and will likely be in the form of discrete supercells. Atmosphere ingredients are favorable for severe weather with destructive hail possibly exceeding the size of baseballs in the strongest storms. Localized damaging winds and a tornado are also possible. Further south in the ‘possible’ risk zone is where confidence is lower in thunderstorms developing. I still believe we’ll likely see at least one or two supercells develop with an associated risk of golf ball to baseball size hail and localized gusty winds. Wind shear and instability values would support a tornado threat on Wednesday BUT moisture is going to be a problem thus keeping cloud bases high and reducing the tornado threat. I’ll have more on the moisture issue in my ‘nerd’ discussion in a bit.
By the early evening hours storms will likely begin congealing into one or two clusters of storms. These clusters will move east and pose a threat of damaging winds over 60 MPH along with a marginal hail threat. Similar to the last few nights any cluster that develops could remain until the early morning hours on Thursday as it moves east towards the eastern edge of the “possible” risk zone of severe weather. The good news is if we can get a cluster of storms to develop in Northwest Texas hopefully we can get some more rain to our most drought-stricken communities. If you’re not a weather nerd or short on time that’s the gist of the forecast.
Weather Nerd/My Personal Chase Thoughts
This graphic is a model representation from the 18 Zulu (1 PM CDT) run of the high resolution North American Model (NAM) showing surface dewpoint temperatures at 7 PM on Wednesday. The first and most obvious thing to note is the dryline which is located across the eastern Texas Panhandle into West-Central Texas. To the west of the dryline very dry air will be in place along with gusty west winds promoting critical fire danger. To the east of the dryline is where moisture will be streaming northward from the Gulf of Mexico (technically South/Southeast Texas based on the current moisture trajectory). Dewpoints should be in the lower to mid 50s by early afternoon with values continuing to increase through the late afternoon. By about 5 to 6 PM we expect dewpoint values to range from 54°F to 58°F along and ahead of the dryline. 60 degree dewpoint values will be spreading north/northwest across the Red River further east towards Vernon and Wichita Falls into Southern Oklahoma from Texas.
You might be asking yourself why I just spent so much time describing a five degree difference in dewpoints across several hundred miles. Well in terms of forecasting surface-based severe weather, cloud heights, and the threat of tornadoes the dewpoint is a key-factor. The base of a cloud or the point in which water vapor will condense into a cloud is determined in part by the surface temperature and surface dewpoint temperature difference. For example: A surface temperature of 81°F with a surface dewpoint of 72°F would likely yield a lifted condensation level (LCL) and cloud base height of about 500-750 meters above the ground which is prime-time territory for tornadoes (assuming you have the ingredients in place to support tornadoes).
Being a little selfish and forecasting for my chase-target area near Interstate 40 and Shamrock, TX in the Panhandle I find that the 18Z NAM has a surface temperature in in the 86°F-88°F with a surface dewpoint at about 57°F. Generally speaking you want your temperature/dewpoint spread to remain less than 15 to 20 degrees for cloud bases to be low enough for tornado potential (again assuming you have all the other ingredients in place). The forecast temperature/dewpoint difference for tomorrow in the eastern Texas panhandle and Northwest Texas looks to be around 30° which will mean cloud bases are around 1500-2000 meters above the ground. In most circumstances that makes the tornado threat minimal because the cloud bases are simply too high off the ground to produce tornadoes. That said those storms may not produce tornadoes but they are in fact some of the most impressive structure-wise and can produce giant hail.
There’s a little catch about tomorrow though: The eastern Texas Panhandle is a bit higher in elevation then other parts of Texas and thus sometimes lower dewpoint values are able to be compensated somewhat by the higher terrain. In Wednesday’s case wind shear and instability will both be favorable for rotating supercells. If we end up with a storm that can remain discrete and supercellular during the 7:30 PM to 9:30 PM time-frame there is a small possibility the lowering surface temperatures may be enough to cause cloud bases to lower to the point where a isolated tornado may occur. That’s very speculative at this point and more related to my chase experience verses forecasting. The tornado threat tomorrow is marginal because of the low dewpoint values and high cloud bases, but we’ve seen rare setups produce a tornado or two even with higher cloud bases. It would require the mesoscale environment (aka the environment in close proximity to the storm) to be almost perfect to produce a tornado. the likelihood of that happening is low, but not zero. My goal for tomorrow is to capture a photogenic supercell producing very large hail (hopefully not on my windshield). I’ll be keeping an eye out for any signs of local features that could enhance the storm around sunset since cloud bases will begin lowering somewhat as surface temperatures begin falling after the prime-heating of the day. Based on weather model data from this morning and afternoon I’d have to say my target (where I’ll head first and go from there) is Shamrock, TX in the eastern Panhandle. If the dryline sets up further west and storms fire further west than obviously I’d have to go west on I-40 and adjust from there. My target storm should be crossing into Oklahoma by around 7 PM and move into a slightly better environment moisture-wise with dewpoint values in the 58-60 degree range. Combined with strong wind shear that might be enough to get at least some low level rotation. We’ll see what happens but in terms of seeing a supercell tomorrow I’d say the chances are good for me.
Disclaimer: The above discussion is related to my personal chasing thoughts for Wednesday and not the overall forecast.