Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201743
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1243 PM CDT Sat Sep 20 2014

.UPDATE:
Issued at 1233 PM CDT Sat Sep 20 2014

I`ve added some low pops to portions of southeast Missouri and far
southwest Illinois for this afternoon. It appears a gravity wave
has interacted with a more robust cu field along and south of a
line from St. Clair to Belleville where SBCAPE is from 1500-2000
j/kg and little CIN. This interaction has produced the development
of some spotty showers within the last 30 minutes.

Glass

&&

.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.

2%

.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.

2%

&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1233 PM CDT Sat Sep 20 2014

The main forecast issue is the threat of showers and thunderstorms
this afternoon and evening. A cold front will be pushing into
northern MO later this afternoon and moving southward across the
area tonight. Scattered showers and thunderstorms should develop
across northern MO into central IL this afternoon in a zone
along/ahead of the front. Coverage should be greatest from the
late afternoon into the evening and diminish overnight. Given the
low coverage of storms I have limited confidence in any direct
terminal impacts and thus have kept with VCTS in the TAFS. Gusty
southwest winds will prevail this afternoon in advance of the
front, with gusty north northwest winds on Sunday.

Specifics for KSTL:

The main forecast issue is the threat of showers and thunderstorms
this evening. A cold front will be pushing into northern MO later
this afternoon and moving southward across the area tonight.
Scattered showers and thunderstorms should develop across northern
MO into central IL this afternoon in a zone along/ahead of the
front and move southeast. Coverage should be greatest from the
late afternoon into mid evening and diminish overnight. Given the
low coverage of storms I have limited confidence in any direct
terminal impacts at KSTL and thus have kept with VCTS in the TAFS.
Gusty southwest winds will prevail this afternoon in advance of
the front, with gusty north northwest winds on Sunday.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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