Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 222046

346 PM CDT Tue Jul 22 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 330 PM CDT Tue Jul 22 2014

There is a considerable amount of uncertainty in the convective
evolution and coverage tonight. Dew points in the 70s, strong
heating and steep mid level lapse rates are currently contributing
to SBCAPE of 3500-6000 j/kg across the northwest 2/3rds of the CWA
with the highest values across central and north central MO. There
also is a significant cap in place due to warm mid level temperatures.
The big question is when and how much of the cap will be reduced
as the front moves southward out of Iowa and southeast Nebraska
into northern MO this evening. Model solutions including the
convection allowing models are quite varied on QPF and implied
coverage. Recent runs of the HRRR are quite stingy on coverage
with isolated storms at best. I don`t have a great feel for what
is going to occur, but the deterministic models suggest that height
falls/cooling aloft should be sufficient to weaken the cap and
allow for scattered development along front, mainly late this
evening and especially overnight. I have attempted to follow this
trend with the greatest coverage along/east of the MS river. The
severe threat is conditional on development early enough this
evening to realize the big CAPE, and the main threat would appear
to be damaging winds given the warm air aloft.


.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 330 PM CDT Tue Jul 22 2014

The cold front should lie roughly from south central IL to near
St. Louis into southwest MO at 12z and exit the CWA by early
afternoon. There will continue to be a threat of scattered showers
and storms ahead of the advancing front. Temperatures should be
cooler tomorrow with the main thrust of cooling and drying in the
afternoon. Unseasonably deep upper troffing in the eastern U.S.
will keep a large surface high dominating the area Wednesday night
into Friday, with most of this period dominated by lower humidity
and below average temperatures.

Beginning late Thursday night and into Friday the threat of precipitation
will return in association with elevated showers/thundestorms due to
strong warm advection/moisture transport via a southwesterly LLJ ahead
of the slowly retreating warm front and in association with impulses
aloft dropping southeastward along the eastern periphery of the
upper high. A cold front will then drop through the area Saturday
afternoon-Sunday as a upper trof digs southeastward from the upper
MS valley and into the Great Lakes bringing another threat of
showers and thunderstorms. This upper wave will be part of large
scale amplification of the upper air pattern across NOAM and will
feature another deep trof over the eastern U.S. through the first
part of next week and send out July with another period of below
average temperatures.



.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 131 PM CDT Tue Jul 22 2014

Upper level high retreating some to the west will allow fairly
strong short wave to drop into into the eastern US bringing
northwest flow back to the region. Associated surface cold front
will move through late tonight and Wednesday morning. Forecast
issue is when, where and if thunderstorms develop. Cooling aloft
has to occur before the instability disappears. No model is
convincing enough toexclusively follow. Have blended and followed
previous forecast putting storm at UIN about 00z and gradually
moving south, into STL at 06z. Models and soundings suggest some
mvfr cielings wednesday am.

Specifics for KSTL: Nothing out there has convinced me to change the
previous forecast. CWSU concurs to keep the prevailing
thunderstorms from 06 - 08z...with a couple of hours of mvfr
ceilings Wednesday am.





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