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Severe Thread: 5/13-17

KevinH

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13z update hints at (some) possibility of later upgrade to ENH in the South today, if uncertainties can be removed:

Thunderstorms in the expanding warm sector, and along the front,
should pose a risk for a few tornadoes, along with large hail and
severe gusts, this afternoon into tonight. The main uncertainty at
this time is coverage, and by which preferred mechanism the greatest
lift will occur (related concepts). Warm-frontal passage and
diurnal heating will combine to destabilize the airmass inland from
southwest-northeast today, with upper 60s to mid 70s F surface
dewpoints. This will yield MLCAPE in the 2000-3500 J/kg range,
beneath 45-60 kt effective-shear magnitudes, once surface-based
parcels are attained. A more-focused area of severe potential may
develop within the lengthy corridor outlooked, particularly near the
inland-shifting baroclinic zone where low-level shear should be
maximized. However, mesoscale uncertainties are still too large to
introduce greater unconditional severe probabilities at this time.
This reminds me of what @JPWX said about this set up being sneaky. sigh

Time to turn on alerts.
 

wx_guy

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Observations from LIX show a cap in place at the moment, but the area in southern LA and MS and extreme SW AL shows an area of increased potential instability. If the cap can erode, this area (and areas further north and east as the day progresses) could have explosive thunderstorm development. Theta-E values surge dramatically across this area, with TEI on the observed LIX sounding of nearly 30. If the mitigating factors can be removed, this points to very strong instability as the day progresses.

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wx_guy

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The latest HRRR and the forecasted mesoanalysis over the next few hours point to the FL Panhandle and the bottom 3rd of AL and far SW GA as the zones of most dangerous weather potential, roughly coinciding with the SLGT risk area. The models are not very bullish on extending that risk any further north of that. But the storms/MCS that forms looks to be quite intense.


EDIT:

If there IS an ENH risk released, I'd expect a very small one around the Dothan, AL area, given the latest models and mesoanalysis. But I could definitely still see it staying SLGT.


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Clancy

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Interesting solution offered by CSU's ML system today, though so far this year I feel like it hasn't fared very well with day-of forecasts. Feel like HRRR is probably on the money with a localized but potentially potent situation for parts of AL and GA, especially as CAPE and kinematics reach their maximum late this afternoon (sounding from SE AL at 21Z).
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Clancy

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Watch forthcoming for the Panhandle, south AL and southern GA.
Mesoscale Discussion 0815
NWS Storm Prediction Center Norman OK
0231 PM CDT Fri May 17 2024

Areas affected...southern Alabama into southwest Georgia...and parts
of the Florida Panhandle

Concerning...Severe potential...Watch likely

Valid 171931Z - 172200Z

Probability of Watch Issuance...80 percent

SUMMARY...Storms will likely increase in coverage and intensity over
the next few hours, with damaging winds most likely. A brief tornado
or two will be possible.

DISCUSSION...Surface analysis shows a stationary front extending
roughly from far southern MS across the AL/FL border and toward the
GA/FL border. Observational trends currently indicate little change
in wind direction along the front, however, some northward
progression of the boundary is expected as southerly winds over 30
kt persist just off the surface.

The 18Z LIX sounding shows a very moist and unstable air mass with
PWAT over 2.00" and SBCAPE over 3000 J/kg. Moderate deep-layer
westerly flow exists as well with around 50 kt at 500 mb.

Latest radar trends show convection increasing along the stationary
front from southern MS into southern AL. Echo tops are gradually
increasing, and this trend should continue, especially with cells
near or just on the warm side of the boundary.

Given moderate westerly flow and favorable deep layer shear, a few
bowing structures are anticipated, producing damaging winds.
Enhanced shear along the east-west oriented front may support
storm-scale rotation with a brief tornado or two possible through
early evening as cells evolve east.

..Jewell/Hart.. 05/17/2024
 

wx_guy

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Watch forthcoming for the Panhandle, south AL and southern GA.

Yay! haha Wonder how the MCS will evolve. Storms will be moving into a very ripe environment. Hopefully no more 100 mph bow segments.
 

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Newest 20z update:

...20Z Update...
Greater (5%) tornado probabilities have been realigned/focused along
an outflow boundary extending from parts of southern AL into the FL
Panhandle/north FL and southern GA. Recent surface observations and
visible satellite imagery show heating is occurring on both sides of
this boundary. But, it has moved little over the past couple of
hours. Current expectations are for the overall severe threat to
increase through the rest of the afternoon along/near this boundary,
where low-level shear is maximized per recent VWPs from KEOX. A few
tornadoes may occur with any supercell or small bowing cluster that
spreads eastward through this evening, along with damaging winds and
large hail. See Mesoscale Discussion 815 for more details on the
short-term severe threat for this area.
 

wx_guy

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Here's the STORM-NET Point Forecast for Dothan AL, covering the next 3 hours. The last few times I've looked at these, I kinda thought the probabilities were inflated, so take this with a grain of salt. But generally speaking, it is showing environments conducive to severe winds and a possible tornado in the next 1-3 hours. Have to see how these evolve over time -- the STORM-NET did a fantastic job forecasting the severe winds in Houston yesterday. So we shall see.

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wx_guy

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Observed Tornado Warning now for Geneva County storm.

EDIT: Looks like a quick TDS, but gone now. May be cycling. We'll see.
 
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