Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171135

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 AM CDT Tue May 17 2022

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 354 AM CDT Tue May 17 2022

The return to an active Spring pattern will begin later today.
The flow aloft will become quasi-zonal today as the eastern U.S.
upper trof moves eastward resulting in flattening of the overall
flow from the Plains into the MS Valley. A series of low-amplitude
short wave trofs will then traverse the middle MS Valley within the
quasi-zonal flow beginning later today and continuing periodically
through Thursday. These disturbances will largely help regulate
thunderstorm chances.

Most of today looks dry with increasing clouds ahead of the first of
these short waves. Retreating high pressure today will result in
return south-southwest flow and gradual moisture return along with
weak WAA. Temperatures today will be seasonably warm. We could see a
few showers and perhaps a thunderstorm from mid-late afternoon
across parts of central MO in association with the initial short
wave currently centered across western KS/OK.

The main show however will begin tonight. While we could see a few
showers or storms across parts of central and eastern MO this
evening, the main show is expected overnight. An MCS is expected to
evolve this evening across the east-central Plains in response to a
very unstable air mass and lift via a strengthening southwesterly
LLJ and the next migratory short wave. The southwesterly LLJ will
also transport deeper moisture back into the CWA, and when combined
with steep mid level lapse rates, will yield MUCAPE of 1000-1500
J/KG back into parts of central MO later tonight. The instability
and lift attendant with the veering LLJ will support the east-
southeastward movement of the MCS into the CWA overnight. While
there should be a weakening trend of the MCS with time, the
instability and deep layer shear of 30-40 kts will support the
potential for a few severe storms mainly in central MO. I think the
primary severe threat will be large hail, but 500-1000 J/KG of
MLCAPE is progged into central MO, and this would support at least
some threat of damaging winds with linear structures within the MCS.
The other threat with the MCS overnight is heavy rain. PWs of 1.5+"
along with the potential for some training would support the
potential for locally heavy rain across parts of central MO.

The weakening MCS is expected to be centered from eastern MO into
southwest IL at daybreak on Wednesday, with the MCS continuing to
move east and exit the CWA by midday attendant with veering of
the LLJ and migration of the short wave. I am not sure if we will
see any additional storm development on Wed. afternoon in the wake
of the morning MCS. The east-west front will be settling
southward and the air mass along and south of it is expected to be
very unstable with MLCAPE of 3000+ J/KG. There is a lack of
appreciable forcing with no apparent short wave, and just weak
convergence along the front. This however may be sufficient to
initiate isolated storms. The degree of instability and deep layer
shear of 40 kts would suggest any storms that can initiate would
become severe, likely supercells, capable of large hail.


.LONG TERM...  (Wednesday Night through Monday)
Issued at 354 AM CDT Tue May 17 2022

The latest guidance suggests that Thursday may be a predominately
warm day and largely void of any storms. Backing of the flow
aloft in response to upstream upper trof amplification along with
falling surface pressure is expected to result in the northward
retreat of the front and strengthening low level WAA. While the
air mass will be very unstable it also looks a bit capped and
there seems to be a lack of any larger scale feature to help
reduce the CIN. It looks like a very warm day with highs well into
the 80s.

There are still some uncertainties with respect to the details on
Friday, but the growing consensus among the deterministic and
ensemble guidance is that we should see a potent cold front
gradually advance through the CWA on Friday afternoon and night.
Ahead of the front it looks to be another very warm day with highs
well into the 80s and pushing 90 in spots. The air mass ahead of the
front will be very unstable, but it also looks like there may be a
substantial cap as well. Since large scale support to reduce the cap
lags the front, the coverage of storms that can initiate along the
front is in question. The degree of instability and deep layer shear
of 30-40 kts would support a severe threat with any storms that do

From Friday night into Saturday it appears the precipitation
distribution should become anafrontal with the precipitation in
the post-frontal air mass. This is supported by increasing large
scale lift associated with the advancing upper trof as well as
increasing frontogenesis. The NBM exceedance guidance along with
numerous members of the GEFS and EPS show post-frontal precipitation
across a good portion of the CWA much of Saturday, especially the
southeast half of the CWA. Much cooler air featuring below normal
temps is expected on Saturday in the wake of the front and with
the clouds and precipitation. The below normal temperatures will
continue into Sunday as the upper trof traverses the mid-upper MS
Valley, and surface high pressure builds across the region.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 628 AM CDT Tue May 17 2022

VFR flight conditions are expected today and into this evening
with increasing clouds through the period. While there could be a
spot shower or storm late this afternoon and this evening across
parts of central MO, the main threat of showers and storms will
hold off until overnight and into Wednesday morning. At that time
a complex of showers and storms originating across the Plains will
progress through western MO and into the area. I have added
thunderstorms into the TAFS with this package. Flight conditions
could certainly be lower at times once the storms arrive and they
could also have stronger gusty winds.





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