Area Forecast Discussion
Issued by NWS St. Louis, MO

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981
FXUS63 KLSX 222111
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
311 PM CST Thu Feb 22 2018

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 309 PM CST Thu Feb 22 2018

Still have persistent fog across northeast MO and west central IL
this afternoon with a low level stratus cloud deck across the entire
region.  Another batch of showers, now moving northeastward into
southwestern MO ahead of a southwest flow shortwave, will spread
into much of the forecast area this evening.  The highest QPF should
occur across southeast MO into southwest IL tonight in a region of
strong low level temperature advection and moisture convergence on
the nose of a strong southwesterly low level jet.  There will likely
be embedded, elevated convection/storms across this area.  Will
begin a Flood Watch across parts of southeast MO into southwest IL
tonight with one half inch to one inch possible and more rounds of
convection expected into the weekend.  This area has already had
some streams and creeks into flood due to recent rainfall.
Temperatures will be steady or rise a little this evening into the
overnight hours as the surface winds veer around to a southeast
direction and surface dew points slowly rise. Temperatures will be
warm enough that the precipitation type will be all liquid rain,
even across northeast MO. By 12Z Friday most of the precipitation
should shift southeast of our forecast area.  Will have a break in
the rain for much of Friday with the next batch of showers moving
into parts of central and southeast MO already by late Friday
afternoon. Rising cloud heights Friday afternoon, although not
clearing should lead to warmer temperatures with highs about 5 to 10
degrees above normal for late February.

GKS

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 309 PM CST Thu Feb 22 2018

Forecast concerns Friday night into the weekend include the
potential for freezing rain, locally heavy rainfall, and possible
severe thunderstorms.

Rain will begin to move back northward on Friday evening as both the
GFS/NAM show upper trough entering the high Plains.  There will be a
pronounced increase in low level moisture convergence over the
region on Friday night into Saturday morning that will act to
increase coverage of the showers across the area from south to north
during the night.  There will be the chance for some freezing rain
over parts of northeast Missouri and west central Illinois late
Friday night and early Saturday morning as temperatures fall to
around freezing which could cause a light glazing of ice.

Showers and the threat for thunderstorms will increase on Saturday
as the upper trough moves into the Plains causing ascent to increase
across the area.  GFS/NAM/ECMWF all show the surface low moving
across northern Missouri into northern Illinois with some
uncertainty with just how far north the surface warm sector will
move on Saturday and Saturday evening.  This will have implications
on how far north thunderstorms, especially strong to severe
thunderstorms could develop.  The latest SPC SREF and CIPS analogs
support the SPC SWO Day 3 which depicts the risk for a few strong to
severe thunderstorms across southeast Missouri into south central
Illinois. In addition to the severe risk, will continue the flood
watch through Saturday evening.  PWATs will continue to be
abnormally high Friday night into Saturday evening, and there will
be potential for training over saturated soils which will could
cause flooding.

After Saturday night, dry weather is expected Sunday into Tuesday as
subsidence builds in the wake of the exiting trough and a large
surface high moves across the area.  The global models are showing
the next chance of rain the middle of next week as another trough
enters the central CONUS causing additional moisture to be drawn
northward from the Gulf.  Temperatures during the period will be
above normal.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1150 AM CST Thu Feb 22 2018

Should see some improvement in the visibilities now that the
drizzle has ended, although there was still fog at the taf sites
except in COU. May see a little improvement in the cloud heights
this afternoon, but will still likely remain below 1000 feet.
Showers will spread eastward into the taf sites this evening with
a few embedded, elevated storms possible in the St Louis metro
area. This precipitation will shift east-southeast of the taf
sites by 12Z Friday. Should see a gradual improvement in the cloud
heights on Friday. East-northeast surface wind will veer around
to a southerly direction late tonight, and to a westerly
direction Friday morning after passage of a weak cold
front/surface trough.

SPECIFICS FOR KSTL: Should see some improvement in the visibilities
now that the drizzle has ended, although there was still fog. May
see a little improvement in the cloud heights this afternoon, but
will still likely remain below 1000 feet. Showers will spread
eastward into the STL area this evening with a few embedded,
elevated storms possible. This precipitation will shift east-
southeast of STL by 12Z Friday. Should see a gradual improvement
in the cloud heights on Friday, possibly into the VFR catagory
late Friday afternoon. East-northeast surface wind will veer
around to a southerly direction late tonight, and to a westerly
direction Friday morning after passage of a weak cold
front/surface trough.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through Saturday evening for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

IL...Flood Watch through Saturday evening for Randolph IL-Washington
     IL.

&&

$$

WFO LSX



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