Monday, April 23, 2018

Stunning Images from Ft. Walton Beach tornado -- April 22, 2018

It has not been a particularly active tornado year so far in 2018, largely due to repeated intrusions of cold air over the eastern United States.  But Sunday, April 22nd (yesterday) yielded some striking images from tornadoes in the western Florida panhandle and southern Alabama.

This was especially true in Ft. Walton Beach, Florida, where a supercell tornado came onshore over Okaloosa Island just after 4 p.m. CDT (2100 UTC), and then crossed into Fort Walton Beach proper.  Preliminary indications are that the tornado was only EF1 in intensity, and there were no major injuries reported, but many people caught the tornado on video, and it was visually impressive.

The sequence below (looking west) shows the tornado crossing from Okaloosa Island over into Fort Walton Beach, with debris visible from a roof being torn off a building.  Confirming the storm's supercell structure, a rear flank downdraft (RFD -- area of sinking air on the south side of the tornado; see last couple images in the sequence) is visible as the tornado passes just east of a tall condo building and onto the mainland:
A closer video shot from this tall condo building looking south at the tornado also shows the same large piece of roof debris moving from east to west across the front of the tornado and splashing into the water:

It's always interesting how different a tornado can look depending on which side a photographer is located.  An earlier video after the tornado had moved into Okaloosa Island as a waterspout from the Gulf of Mexico shows how "white" it appeared from behind, illuminated by light from the RFD against the darker background to the north.  Some smaller vortices (suction vortices) are also visible within the tornado:
Other tornadoes occurred on Sunday, including an EF0 near Foley, Alabama (west of Fort Walton Beach) where 3 people were injured in overturned RVs, another EF1 in Escambia and Crenshaw Counties of southern Alabama, and an EF0 near Fort Rucker, Alabama.

The synoptic weather setting (see image below with insets) showed a midlevel trough/disturbance moving eastward in the southern jet stream.  The afternoon tornadoes occurred ahead of this trough in an area where there was branching/spreading of the jet winds (see white arrows).  The surface map (first inset) at mid-afternoon also showed a warm front across the western Florida panhandle.  In previous posts this year, I've mentioned how tornadoes like boundaries due to the increased wind shear along them, and in the case of warm fronts, increasing warmth, moisture, and resulting instability.  This case was no exception (see yellow arrow on inset):

Although CAPE, a measure of instability, was not particularly large in this case (around 1200 J/kg; not shown), there was plenty of low-level wind shear or storm-relative helicity (SRH, greater than 300 m2/s2; not shown).  Combinations of CAPE and SRH included in such parameters as the significant tornado parameter (STP) and the energy-helicity index (EHI) help forecasters in predicting where storms may rotate in low-levels and tornadoes may be possible.  

In this case, an "enhanced' version of the EHI that I've been working on highlighted the area along the western Florida panhandle at mid-afternoon near the warm front, and STP from the Storm Prediction Center mesoanalysis page did the same:

This "enhanced" EHI (which also factors in deep-layer shear and low-level CAPE) needs some work on my part to improve it, as the values often tend to be too large compared to the STP (see above).  But both parameters definitely suggested an environment supportive of supercell tornadoes over the western Florida panhandle and southern Alabama yesterday afternoon.

- Jon Davies 4/23/18  

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