Gusty south to southeasterly winds have brought a return of the summer-like dewpoint and humidity values across a majority of Texas this afternoon. Those in the Borderland and the northwestern Panhandle remain on the drier side of the dewpoint spectrum, but everyone else is seeing those values rise. A dewpoint value of 60 begins to make it uncomfortable outside, 65+ is summer humidity, and a dewpoint above 70 is comparable to taking a shower outside – with sweat. Southerly winds will continue quite noticeably tonight helping to pump in high moisture values from the Gulf of Mexico northward not only through Texas but into Oklahoma and Kansas. Unseasonably high moisture values for late October will be one of the contributing factors for tomorrow’s severe weather threat.
One look at the upper-level maps for tomorrow explain the chance of thunderstorms. A positive-tilt shortwave will move east tonight and on Saturday across the southwestern United States. That shortwave – combined with a jet streak even higher up – will provide plenty of ‘lift’ for thunderstorm development near a dryline and cold front tomorrow evening. Upper-level winds combined with increasing low-level winds will provide favorable wind shear for thunderstorms to become organized and possibly severe.
The Storm Prediction Center maintains a level 3 out of 5 risk for severe weather across portions of Northwest Texas and Texoma for late Saturday afternoon through the evening hours. The standard level 2 out of 5 risk includes a majority of North Texas, the eastern Big Country, and Northwest Texas. Finally, the ‘marginal’ level 1 risk zone includes the remainder of the Big Country, the eastern Concho Valley, the Hill Country, the northeastern Edwards Plateau, Central Texas, the northwestern half of the Brazos Valley, and East Texas. This outlook technically runs to 6 AM Sunday morning. Don’t be surprised to see level 1 severe probabilities expanded southeastward after 6 AM Sunday in a future outlook.
There is some question on how far south thunderstorms will initially develop tomorrow afternoon. A strong cap will be in place, but if we can get enough upper-level lift combined with the afternoon heat, we should be able to get a few severe thunderstorms to develop along the dryline in Northwest Texas to Southwest Oklahoma after 4 PM. These initial storms will probably be discrete supercells capable of producing very large hail up to the size of baseballs, localized wind gusts over 60 MPH, and an isolated tornado threat. Storms will tend to move east/northeast. By the early evening hours, a cold front will be moving southeast across western and central Oklahoma. A nearly solid line of severe storms will be in progress along that cold front as it moves east/southeast across Oklahoma.
If the cap is indeed able to be overcome in the Big Country late tomorrow afternoon it is possible we may have a second line of thunderstorms moving east toward western North Texas by the mid-evening hours. Some of those storms may be severe with large hail initially, and a damaging wind threat if they grow upscale into a line segment. The best combination of wind shear and instability looks to be across Oklahoma and along/north of Interstate 20 in North Texas. Those locations will be where we’ll be watching for the most severe weather reports tomorrow evening as a squall line moves east/south with the cold front. South of Interstate 20 conditions won’t be as favorable for widespread severe storms, but strong to locally severe wind gusts and hail will be possible. The squall line will continue to move east/southeast tomorrow night and early Sunday into Central Texas through Northeast Texas with gusty winds, but weaker compared to its strength farther north.
A non-zero (low) risk exists for a few discrete thunderstorms ahead of the squall line tomorrow evening. Any discrete storm that develops ahead of that squall line tomorrow night in the Big Country, Texoma, or North Texas will be watched closely for signs of rotation. Strong low-level wind shear will be present, so any discrete storms may pose a tornado threat tomorrow night. This is not a high likelihood at this time but will be watched for just in case. In addition, a few brief tornadoes within the squall line itself can’t be ruled out due to the strong low-level wind shear. If we do have brief squall line tornadoes tomorrow night they’d likely be co-located with straight-line winds over 70-80 MPH, so straight-line wind or tornadic wind really wouldn’t matter too much – they both do damage and should be taken seriously.
I want to remind readers that some timing changes are probably as we get closer to tomorrow afternoon and evening. The speed of the cold front, timing of the upper-level lift, and any sort of localized factors not foreseeable more than a few hours in advance can lead to forecast changes. That’s just a matter of how the weather works. Once the cold front passes through a given location the threat for severe weather will come to an end. Gusty north/northwest winds will usher in much drier and cooler air, just like last weekend. While it is a look out into voodoo land, it does look like we will have a fairly strong cold front toward next Friday. That one may have some pretty cold air with it and would bring the coldest air so far this fall.