Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230601

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1201 AM CST Fri Feb 23 2018

Issued at 859 PM CST Thu Feb 22 2018

Scattered showers will continue to move up from southwest
Missouri into the area through Friday morning. Rainfall amounts
will range from around a tenth of an inch along the interstate 70
corridor to up to a quarter of an inch over parts of southeast
Missouri and southwestern Illinois. Light amounts are expected
north of the interstate 70 corridor. Areas of dense fog have
developed over portions of northeast/central Missouri and western
Illinois. A dense fog advisory has been issue through 9am as fog
is expected to persist through Friday morning.



.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.


.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.



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1144 PM CST Thu Feb 22 2018

LIFR ceilings with fog and pockets of IRF ceilings will persist
through Friday morning. Scattered showers will begin to tamper off
from west to east early Friday morning. Ceilings will lift to
MVFR and persist through Friday night. Light winds expected
through Friday night.

SPECIFICS FOR KSTL: IFR ceilings and Fog will lift to LIFR by
Friday morning. Scattered showers over the terminal will tamper
off after 08z. Ceilings are expected to lift to MVFR by Friday
afternoon before falling again to LIFR near the end of the period.



MO...Dense Fog Advisory until 9 AM CST Friday for Knox MO-Lewis MO-
     Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

     Flood Watch through Saturday evening for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST Friday for Adams IL-Brown IL-
     Pike IL.

     Flood Watch through Saturday evening for Randolph IL-Washington



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