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


Stephen said...

Thanks for the write up, Jon. ! (from Cape Canaveral). Had been watching this situation closely (as notable on facebook posts)

Jon Davies said...

Thanks, Stephen, for the comment. A very interesting case combining tropical and extra-tropical factors!