Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170432
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1132 PM CDT Sat Sep 16 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 320 PM CDT Sat Sep 16 2017

The approaching cold front has slowed its progression into the
forecast area allowing plenty of sun paired with a southerly surface
flow to get temps into the upper 80s and low 90s. Temps tonight will
be  warmer than last night by a few degrees due to the incresing
cloud cover overnight and the south wind. Precip will begin from the
NW and progress to the SE. Models are having some difficulty in the
timing of the precip but based on the movement of the front precip
should begin in the early morning for parts north and west and into
the metro area by late morning. Temps for Sunday look to reach the
mid 80s in the metro and SE and the upper 70s to the NW due to the
increased cloud cover.

Walsh

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 320 PM CDT Sat Sep 16 2017

The period from Sunday night through Tuesday morning appears to be
the stretch with the greatest probability of precipitation in the
longer range period that extends into next weekend. During this time
frame a series of weak, low-amplitude migratory disturbances will
traverse the region within the west-southwest flow aloft. The
intensity and timing of these features and overall ramfications on
the forecast is difficult to gage owing to convective enhancement in
the models. That said, there prognosis along with a persistent
southwesterly LLJ of varying magnitudes, and the presence of a plume
of steep mid level lapse rates contributing to a warm moist unstable
air mass will provide a favorable environment for deep convection. A
cold frontal boundary which is forecast to move into the region on
Sunday will sag south/southeast with time. The NAM continues to
appear overly agressive with its southward push of this front
given the absence of a prominent surface high to its north or
northwest flow aloft. Thus a solution closer to the GFS and ECMWF
which only bring the front to near I-70 on Sunday night appear
more logical. Overall the forcing features are weak during this
time frame and it won`t precipitate continuously, but there appear
to be several more favored time frames. The first would be late
Sunday night into Monday morning associated with one of the
migratory disturbances and the front/residual boundaries. The
second would be Monday night associate with another migratory
disturbance and the retreating front. Late Tuesday into early
Wednesday looks like a min in precipitation chances. In general
the temps should be above average, but this could be modulated by
clouds and ongoing precipitation.

The global models have come into better agreement with the large
scale pattern from mid-late week. A broad upper low and trof centered
through western NOAM will result in a largely zonal flow component
into the MS valley region which will help push an initial cold front
into the western CWA late Wednesday. However shortly thereafter
the large scale flow undergoes amplification with a deepening of
the western trof and ridge aloft building from the lower MS
Valley thru the Great Lakes. This will essentially result in a
road block and slow down any progession, and the front is forecast
to die/frontolysize over the area on Thursday. From Thursday into
Saturday the sensible weather looks predominately dry, maybe a
spot storm, and even warmer with above average temperatures.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1123 PM CDT Sat Sep 16 2017

Cold front extending from northwestern WI southwest through the
OK Panhandle will move southeastward through the taf sites late
tonight and Sunday. Scattered showers and thunderstorms across
northwest MO should weaken some as they move towards UIN and COU.
For now will just include VCSH in COU and UIN tafs late tonight.
Mainly just mid-high level cloudiness outside of the showers
late tonight. A weak south-southeast surface wind can be expected
late tonight. MVFR post frontal cloud ceilings are expected in
UIN Sunday morning and possibly also at COU as well. Scattered
showers and thunderstorms may impact the St Louis metro area
Sunday afternoon. The surface wind will veer around to a northwest
direction Sunday after fropa.

SPECIFICS FOR KSTL: Cold front extending from northwestern WI
southwest through the OK Panhandle will move southeastward
through the STL area late Sunday morning and afternoon. Scattered
showers and thunderstorms across northwest MO should weaken and
likely dissipate as they move towards the STL area. Mainly just
mid-high level cloudiness late tonight. A weak south-southeast
surface wind can be expected late tonight. Scattered showers and
thunderstorms may impact the St Louis metro area Sunday afternoon.
The surface wind will veer around to a northwest direction Sunday
afternoon after fropa.

GKS

&&

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

$$

WFO LSX


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