Tuesday, October 22, 2019

October 20, 2019 Dallas, Texas tornadoes after dark - no deaths or injuries!

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:

The northernmost supercell on the 7:00 and 8:00 pm SPC images above produced the damaging EF3 tornado in north Dallas around 9:00 pm CDT, followed by the Rowlett tornado just after 9:30 pm CDT.  In fact, the warm sector setting east of the dryline over north Texas was so supportive of tornadoes that no boundaries were needed to help produce tornadoes, unlike last Friday evening in Florida.

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

Saturday, October 19, 2019

October 18, 2019: A strong Florida supercell tornado with Tropical Storm Nestor

It's been awhile since I posted about a recent tornado case, so I pulled together some graphics about the meteorological setting with last evening's EF2 tornado after dark (see images above) near Lakeland in west-central Florida east of Tampa.  This tornado was associated with a tropical storm (T.S. Nestor, centered well out in the Gulf of Mexico at the time).  But instead of the supercell being embedded within an outer band of storms as with many tropical systems, it was discrete and occurred near an east-west stationary front, behaving in some ways more like a Plains supercell storm.

The stationary front is shown on the surface map below at about 0300 UTC (11:00 pm EDT), about the time of tornado development near Lakeland:

There was not a tornado watch in effect at the time, probably because the environment most supportive of significant tornadoes appeared to be out in the Gulf of Mexico closer to the center of Nestor, as indicated on the 0300 UTC (11:00 pm EDT) SPC mesoanalysis graphic below using the effective-layer significant tornado parameter (STP):

However, low-level wind shear (0-1 km storm-relative helicity or SRH) and low-level CAPE (0-3 km MLCAPE) on the SPC mesoanalysis at 11:00 pm EDT (below) were notably co-located together over west-central Florida near the stationary front:

This was in the same area where a supercell storm (shown in later graphics down below) was moving north-northeast near and across the stationary front.  The combination of these two low-level parameters near the ground probably facilitated low-level stretching and tilting of environmental vorticity into the storm's updraft to produce strong low-level rotation, even though total instability and numerical shear/instbility combinations did not appear especially large.  The stationary front likely provided additional low-level shear to add to the background environment as the supercall moved across the front.

A 3-hr forecast sounding from the RAP model at Lakeland valid at 0300 UTC (11:00 pm EDT) also suggests that the environment was supportive of supercell tornadoes:

Notice above that the red CAPE area extended only up to around 30,000 ft MSL (300 mb), with the "fattest" area of CAPE located near 10,000 ft MSL (700 mb), rather low in the sounding relative to the ground.  This is similar to many "cold-core" low-topped supercell  environments that support tornadoes in the Plains, suggesting significant upward air acceleration in low-levels, resulting in strong vertical stretching of low-level vorticity near the ground, even though total CAPE does not appear unusually large.  

So, the vertical distribution of CAPE near the stationary front was probably important in this case.  Also, deep-layer shear (0-6 km shear) near 30 kts on the sounding above, while not overly impressive, was just enough to support supercells and tornadoes.

This discrete supercell associated with a tropical system also behaved more like a Plains supercell on radar.  The Tampa radar reflectivity and storm-relative velocity images zoomed in below show the original tornadic mesocyclone northwest of Lakeland occluding and dissipating (drifting toward the back side of the storm while wrapping in rain-cooled air), while a new mesocyclone formed to its east and southeast, similar to the evolution of many Plains supercells:

Thankfully, even though the EF2 tornado was on the ground after dark for 9 miles and was nearly a third of a mile wide at times, there were no injuries due to timely warnings from NWS Tampa.

- Jon Davies  10/19/19