FXUS63 KLSX 212046

Area Forecast Discussion
National Weather Service Saint Louis MO
346 PM CDT Mon Aug 21 2017

.SHORT TERM... (Through Late Tuesday Afternoon)
Issued at 344 PM CDT Mon Aug 21 2017

Scattered afternoon thunderstorms are mainly being driven by
afternoon heat and humidity as well as a pair of outflow boundaries
left over from this morning's storms. Temperatures have rebounded
back into the upper 80s and low 90s in the wake of the eclipse which
has invigorated convection. Had a few reports of wind gusts to 40-
50 mph, frequent lightning, and brief heavy downpours as the cores of
the storms fall out, but deep layer effective shear is only 25-30kts
at this time, so think widespread severe weather is unlikely even
with the fairly high MLCAPE values of 2500 J/Kg indicated on the SPC
mesoanalysis at this time. Should see these afternoon storms
diminish this evening as we lose daytime heating.

Attention turns to the cold front over the Northern Plains tonight.
This front will push southeast into northern Missouri and west
central Illinois between 08-11Z. Showers and thunderstorms look
likely along and ahead of the front after midnight tonight. Guidance
seems to be in good agreement that a line of storms will move into
the area ahead of the front after 06Z and move southeast to near the
I-70 corridor by between 09-12Z. The line slows as elevated
instability wanes toward morning...with scattered thunderstorms
likely continuing across the southern 1/2 to 2/3 of the forecast
area on Tuesday before the front finally pushes all the way through
the area on Tuesday night.


.LONG TERM... (Tuesday Night through Next Monday)
Issued at 344 PM CDT Mon Aug 21 2017

Should have lingering showers and storms southeast of the STL area
Tuesday evening until the cold front shifts southeast of our
forecast area and a surface ridge builds southeastward into the
region from the northern Plains. Cooler and less humid air will
filter into the area Tuesday night leading to lows about 5 to 10
degrees below normal. This will begin a period of relatively cool
and dry weather from Wednesday into the weekend as surface ridging
remains locked in place from the Great Lakes region southwest into
eastern MO with any convection likely remaining west and north of
our forecast area. Although weak northwest flow shortwaves will
move through the region it appears that the low levels will be too
dry to generate any precipitation. The ECMWF model does develop an
upper level trough over the northern Plains on Sunday, but it looks
like any precipitation associated with this feature will stay north
of our forecast area. Highs will typically be around 80 degrees
with lows in the upper 50s to around 60 degrees during this period.



.AVIATION... (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1242 PM CDT Mon Aug 21 2017

Primary concern is thunderstorms through the period. Current
thinking is that scattered storms will continue to develop north
of the I-70 corridor this afternoon. Some isolated storms are also
possible across parts of southeast Missouri over the eastern
Ozarks. Afternoon storms should diminish this evening. Attention
turns to a cold front tonight. The thunderstorms will develop
along the front and drop southeast across the forecast area after
05-06Z. MVFR flight conditions and possibly IFR conditions in
heavier storms will be possible until late Tuesday morning.


Isolated storms are possible this afternoon as an outflow boundary
moves through the terminal. Any storms that form will be capable
of brief MVFR/IFR conditions in heavy rain. Afternoon storms
should dissipate during after sunset. A cold front will move into
Missouri tonight with thunderstorms along and ahead of it. Current
thinking is that the best chance for thunderstorms will be between
10Z and 12Z. MVFR and possibly IFR conditions will be possible
with the strongest storms. Should see flight conditions improve in
the late morning.





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