Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 061150
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
550 AM CST Fri Dec 6 2019

.SHORT TERM... (Through Late Sunday Afternoon) Issued at 348 AM
CST Fri Dec 6 2019

An upper level shortwave is currently passing through the region,
driving a surface trough and cold front through the mid-Mississippi
Valley. Weak WAA ahead of the front has resulted in light rain
across much of southern MO and IL. Behind the front, high pressure
is building into the Northern Plains, with a thin layer of low
stratus across much of the north central CONUS.

Northwest upper level flow will take hold across the central CONUS
today and remain in place before turning zonal on Saturday night.
The NW flow will help drive the cold front through the region
today, with high pressure moving in behind the front. The upstream
post-frontal low stratus will also build into the mid-Mississippi
Valley this morning and remain in place into at least the early
afternoon before mixing and advecting out of the region. The
combination of clouds and northerly winds will drop today`s highs
about 15 degrees compared to what we saw yesterday.

Skies will remain mostly clear tonight, which coupled with calm
winds beneath the surface high, should allow tonight`s lows to drop
into the 20s across much of the region. As low level high pressure
slides east of the area on Saturday, winds will swing around to the
south. This will draw warmer air back into the region, pushing high
temps back into the 40s and 50s on Saturday, and perhaps the 60s on
Sunday in central and southern MO.

BSH

.LONG TERM... (Sunday Night through Thursday) Issued at 348 AM
CST Fri Dec 6 2019

By Sunday evening, guidance is in consensus showing a broad upper
level trough across the western CONUS. A shortwave embedded in the
flow ahead of the trough will gradually drive cyclogenesis across
the Central Plains on Sunday night. The shortwave, cyclone, and
associated cold front will push through the region on Monday, before
the parent upper trough drives a secondary shot of low-level cold
air into the region on Tuesday.

This pattern will result in a handful of impacts for the mid-
Mississippi Valley. First up, several GEFS, SREF, and ECMWF ensemble
members are showing precip developing in the warm, moist flow
ahead of the surface low early on Monday morning. At least a
chance of precip will then persist until the front clears the area
on Monday night, and temps may even be cool enough to generate
some light snow before the precip ends. Secondly, a tight
pressure gradient on the back side of the low will result in stiff
northerly winds on Monday night and Tuesday morning. Finally, the
most noticeable impact will be the much colder cold temps behind
the front. Blended guidance is showing high temps on Tuesday near
or just above freezing. GEFS guidance is even a tick colder, with
the mean remaining below 30 through the day on Tuesday. Lows on
Wednesday morning will be the coldest of the week, dropping into
the teens across the region. High pressure will gradually slide to
the east of the region on Wednesday, keeping things quiet and
cool through midweek.

BSH

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 441 AM CST Fri Dec 6 2019

A cold front was just southeast of STL early this morning, and
will continue moving southeastward today. Post frontal MVFR
cloudiness around 1500 feet in height has advected into UIN early
this morning and should spread into COU and into the St Louis
metro area later this morning. This shallow and thin low level
cloudiness will gradually rise in height and scatter out late this
morning and early this afternoon. The surface wind will become
north- northwesterly as well as strengthen and become gusty in the
St Louis metro area later this morning as it already has in UIN
and COU. The surface wind will weaken by late afternoon as a
surface ridge drops southeastward into MO from the northern
Plains.

SPECIFICS FOR KSTL: A cold front was just southeast of STL early
this morning, and will continue moving southeastward today. Post
frontal MVFR cloudiness around 1500 feet in height has advected
into UIN early this morning and should spread into STL later this
morning. This shallow and thin low level cloudiness will
gradually rise in height and scatter out early this afternoon.
The surface wind will become north-northwesterly as well as
strengthen and become gusty later this morning as it already has
in UIN and COU. The surface wind will weaken by late afternoon as
a surface ridge drops southeastward into MO from the northern
Plains.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     46  29  49  38 /  10   0   0   0
Quincy          40  24  46  36 /   0   0   0   0
Columbia        43  26  49  38 /   5   0   0   0
Jefferson City  44  26  51  38 /   5   0   0   0
Salem           48  28  47  36 /  30   0   0   0
Farmington      49  27  50  35 /  30   0   0   0

&&

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

$$

WFO LSX


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