Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230008

708 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/thunderstorms 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 00z TAFs through 00z Wednesday Evening)
Issued at 647 PM CDT Tue Jul 22 2014

Focus remains on thunder potential this evening into the overnight
and possible MVFR CIGs Wednesday morning. Storms are expected to
form along a cold front that currently stretches from ern IA into
NW MO then into NE KS. TSTMs are fcst to dvlp later this evng and
push slowly SE. Think storms will eventually dvlp...but not
confident on coverage or any particular cell impacting one of the
terminals. The front is relatively weak and moving it
will take most of the night for it to clear the terminals. This
means that storms may take longer to clear a terminal than is
currently indicated. TSTMs are expected to dvlp along the front
again tomorrow aftn/evng...but should be well south of the
terminals by then. There is the potential for some post frontal
high end MVFR stratus/stratocu Wednesday morning. If it
dvlps...should not last long...burning off by mid/late mrng. If
any terminal gets rain this evng or overnight...there is the
potential for fog by morning as the bndry lyr remains quite moist
behind the front...esp if the expected low clouds do not form.
Beyond tomorrow morning...diurnal cu and a N/NNE breeze as high
pressure builds into the region.

Specifics for KSTL:

Possible for TSTMs overnight...but not sure of the coverage. There
is the potential for storms to remain near the terminal later than
indicated based on the slow mvmnt of the front. Models indicate
the potential for high end MVFR clouds towards 12Z...but again not
confident on this scenario. If the terminal did get rain...then
fog might be possible given the moist bndry layer behind the
front...esp if the lower clouds do not form.





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