Wow... it has been an active several days for tornadoes this past weekend in October (see my recent post about the EF2 tornado in Florida from T.S. Nestor).
As anyone watching news the last couple days knows, Sunday evening (10/20/19) saw several tornadoes in the Dallas, Texas area after dark, including one EF3 that struck the North Dallas-Richardson corridor around 9 pm CDT (top image above). Another of EF1 intensity hit the Rowlett, Texas area a little later from the same supercell storm (middle and bottom images above, note the very impressive power flash!) .
**** Update 10/24/19: Additional tornadoes have been surveyed by NWS Dallas/Ft. Worth in recent days, including a short-track EF2 tornado in Garland just before the Rowlett tornado mentioned above, and an EF1 in Rockwall after the Rowlett tornado. These tornadoes were from the same supercell that produced the North Dallas and Rowlett tornadoes. ****
No one wants to see damaging tornadoes in a metro area, especially after dark, but it is great news that there were no injuries or deaths reported in the Dallas area, largely due to a tornado watch and good warnings by NWS.
I've had several people ask me, "Isn't that odd for this time of year?" Not really. Based on statistics over the past 25 years, Texas sees an average of around 9 tornadoes in October each year, and 58 tornadoes occur on average nationally in October. So it does happen with the right meteorological settings.
Sunday evening's surface map (below, 7:00 pm CDT) showed a dryline west of Dallas with moist air (dew points upper 60's to near 70 deg F) that had moved back into north Texas on south winds during the day, with the deepest moisture from the Red River southward:
Tornado parameters from the SPC mesoanalysis at 7:00 and 8:00 pm CDT (below, enhanced energy-helicity index or EEHI, and the effective-layer significant tornado parameter or STP) suggested that combinations of instability and wind shear were quite supportive of tornadoes over the Dallas area as storms were developing rapidly and becoming supercells to the west:
A forecast of instability and wind shear from the RAP model sounding at Dallas a couple hours before the tornado shows excellent combinations of CAPE (instability), low-level wind shear (0-1 km storm-relative helicity or SRH), and deep-layer wind shear (0-6 km shear) were in place, with not much convective inhibition (CIN) in the environment. These factors were all supportive of significant supercell tornadoes if discrete storms developed:
It is worth noting that the High-Resolution Rapid Refresh (HRRR) model from earlier on 10/20/19 (see below) was not successful in forecasting convective storms and supercells over the Dallas area that evening, although it did forecast storms in Oklahoma, and over west central Texas:
So, even with our often impressive automated model guidance of convective storms these days, the human forecast element is still greatly needed!
In my post this past weekend about the Lakeland, Florida tornado, I pointed out from radar images how that supercell behaved somewhat like a typical Plains tornadic supercell with one tornadic mesocyclone occluding and dissipating, while a new one formed to its east-southeast. That same evolution was seen Sunday evening with the North Dallas and Rowlett tornadic mesocyclones, as is evident on the reflectivity and storm-relative velocity images below:
Looking at the larger synoptic picture, the NAM model 500 mb forecast for that evening showed a very large and strong midlevel trough (dashed red line below) moving through the Central Plains, with a typical "branching" jet pattern ahead of the trough. This area of dynamic forcing overspreading the returning low-level moisture through the Plains was where the bulk of severe weather occurred Sunday evening and Sunday night:
One final note... The north Dallas tornado touched down 15-20 miles east-northeast of AT&T Stadium where the Dallas Cowboys were playing at the time of the tornado. Although the soon-to-be tornadic North Dallas supercell stayed well north of the stadium, it is nevertheless very fortunate that the EF3 tornado did not directly impact the thousands of people at the game!
- Jon Davies 10/22/19