Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200817

317 AM CDT Sat Sep 20 2014

.SHORT TERM: (Through Tonight)
Issued at 214 AM CDT Sat Sep 20 2014

This prd will focus on approaching cold front that currently
stretches from cntrl MN SW across far NW IA...NE and eventually NE
CO. This feature is being driven by a significant short wave
dropping thru the Grt Lks. This bndry will slowly work south thru
the day. Guidance has slowed this feature over the past 24 hrs. It
is not expected to reach our NE MO counties until closer to
midnight and not clear SE MO until early Sunday mrng. There is some
potential of convection breaking out ahead of the front this aftn
as a vort max and weakness in the sfc pressure field/wind shift may
be enough to initiate at least some isld convection. Even if
SHRAs/TSTMs are able to dvlp this aftn...the better precip
coverage will be this evng thru the overnight hrs. SPC has areas
north of I70 under a slight risk of SVR wx. This looks reasonable
based on temps in the mid/upper 80s with Dps in the upper 60s to
near 70 producing SB CAPE values in excess of 3000 J/kg and MU
CAPE in excess of 2000 J/kg and 0-6km shear values of 30-45kts.
There is also a significant mid and upper level jet max nosing
into IA and nthrn IL from the Dakotas. So, any storms that can
get going have the potential to become svr, with large hail and
damaging winds being the primary threats. I think that there is a
relatively small window for svr convection from late aftn thru mid
evng before the bndry layer cools enough to limit buoyancy. The
NAM indicates significant levels of CAPE, around 1000 J/kg, to exist
thru the night. This solution is an outlier with the remainder of
the guidance indicating CAPE to decrease significantly after 6Z.
So, expect convection to continue south thru the night weakening
in the process.


.LONG TERM: (Sunday through Friday)
Issued at 214 AM CDT Sat Sep 20 2014

The cold front should clear the sthrn CWA around 12Z with any
lingering precip mvng out shortly thereafter. A secondary short
wave will pass just to the NE of the area Sunday. Due to the first
impulse clearing out the moisture, this second shortwave is not
expected to have much impact on the sensible wx other than maybe
some increased cloudiness across our IL counties. Beyond this
feature, high pressure will be in control for the remainder of the
week as an upper level ridge builds across the Plains providing
mostly clear skies and dry conditions. Temps will start the first
half of the week in the low/mid 70s and should moderate to near 80
by Friday. One caveat to this scenario is a closed low off the CA
coast that is fcst to come onshore this wknd and tries to undercut
the ridge but gets absorbed into the mean flow as the week
progresses. I don`t expect this feature to have a significant
effect on the sensible wx attm...but that could change as the week
moves forward.



.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1150 PM CDT Fri Sep 19 2014

Still expect fog to develop within the next few hours at KSUS and
KCPS. Visibilities may drop as low as IFR at times. Otherwise
still expect scattered showers and thunderstorms to develop along
a cold front that will drop southward across the area tomorrow
afternoon and evening. Have introduced VCTS at the terminals to
indicate the best timing for thunderstorms near the airports.
Winds will veer from southwest to northwest behind the front.

Specifics for KSTL: Expect mainly VFR conditions until thunderstorms
move into the area along a cold front after 00Z. If thunderstorms
do move into the terminal, they may have MVFR conditions with
them. Winds will turn out of the southwest by 15Z, and then turn
out of the northwest behind the front on Saturday night.





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