Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200828
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
328 AM CDT Sun May 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 327 AM CDT Sun May 20 2018

Convective trends continue to be the primary challenge for the next
24 hours.  A 500 mb low is meandering over the central Plains this
morning.  The low will slowly weaken as it drifts northeast today.
The surface reflection of the upper low is very difficult to discern
due to the influences of three waves of convection stretching from
southern Oklahoma to central Missouri.  However, there is a definite
surface boundary with cooler and drier air to the north across much
of Iowa, central and western Kansas and Nebraska, and warmer and
more humid air over Missouri and Illinois.  Short range guidance is
in pretty good agreement with the mass fields...moving the surface
front slowly east/southeast over the next 24 hours.  The front moves
so slowly, and the synoptic flow is so weak on either side of the
front, that I do not think it will be the primary focus for
convection.  Think mesoscale boundaries from ongoing convection will
be the driver as well as weak isentropic lift over the region will
be the drivers that produce storms.  Should be plenty of instability
with 3000+ J/Kg of MUCAPE and 2000+ MLCAPE this afternoon across
parts of central and eastern Missouri as well as southern Illinois.
There will probably be a diurnal max of activity during the
afternoon and early evening, but can`t rule out widely scattered
thunderstorms overnight tonight as well.  Temperatures will be very
dependent on precipitation trends...but MOS guidance mid 70s to low
80s looks reasonable.

Carney

.LONG TERM...  (Monday through Saturday)
Issued at 327 AM CDT Sun May 20 2018

Chances for showers and thunderstorms will shift into southeast
Missouri and southern Illinois as the NAM/GFS are in good agreement
that the cold front will move slowly southeast across the CWA on
Monday and Monday night.  The front will move back north into the
region on Tuesday and Wednesday, but the global models are showing
upper level ridging moving across Missouri and Illinois which will
keep rain chances low.  Chances for showers and thunderstorms will
increase by late in the week when the global models are showing a
trough moving across the Midwest along a stalled front.

Temperatures will continue be above normal as 850mb temperatures
remain in the middle to upper teens and ECMWF/GFS MOS and GEFS
guidance is 5-10F above normal.

Britt

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1038 PM CDT Sat May 19 2018

Scattered convection should shift east of the taf sites by 06Z
Sunday. A second round of convection may move into COU and the St
Louis metro during the morning hours and into the afternoon ahead
of a surface low and trailing cold front. Conditions should be
VFR outside of the thunderstorms. Surface winds will be somewhat
chaotic due to numerous thunderstorm outflow boundaries across the
area, but generally southeasterly late tonight then shifting to a
westerly direction Sunday morning.

SPECIFICS FOR KSTL: Scattered convection should shift east of STL
by 06Z Sunday. A second round of convection may move into STL
during the late morning hours and into the afternoon ahead of a
surface low and trailing cold front. Conditions should be VFR
outside of the thunderstorms. Surface winds will be somewhat
chaotic due to numerous thunderstorm outflow boundaries across the
area, but generally southeasterly late tonight then shifting to a
westerly direction Sunday morning.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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