Area Forecast Discussion
Issued by NWS St. Louis, MO

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758
FXUS63 KLSX 171744
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1144 AM CST Sun Dec 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 330 AM CST Sun Dec 17 2017

Large area of light rain continues to slowly move northeast early
this morning across Missouri.  This area is being forced by low
level moisture convergence underneath mid level ascent ahead of a
upper level low.  This area of rain will continue to move northeast
this morning, but models are showing that the band of rain will lose
its areal coverage as it passes through. This will be because the
upper low is opening up so the mid level ascent will weaken at the
same time the low level jet will also be weakening.  Will only keep
a brief window of likely PoPs moving northeast across the area this
morning with mainly dry conditions this afternoon and this evening
as shortwave ridging moves over the area.  Model guidance shows low
level moisture will stay over the area through tonight, with the
potential for patchy fog and some drizzle overnight.

With clouds and the chance of rain, highs will not be as warm today
as they were yesterday, though they will be above mid December
normals.  Went with highs close to both the MOS and SREF mean temps.
Lows tonight will be closer to normal highs.

Britt

.LONG TERM...  (Monday through Saturday)
Issued at 330 AM CST Sun Dec 17 2017

The models are still showing considerable evolution in the upper air
pattern over NOAM over the next 7+ days, and with the 00Z model
cycle the GFS and ECMWF have come into better agreement in the
overall TRENDS. The upcoming week begins featuring a defined split
flow regime with a fast broadly cyclonic flow across the northern
tier and closed low in the southwestern U.S.. By next weekend there
has been an increasing trend of a highly amplified pattern across
NOAM with a vortex in the vicinity of Hudson Bay and southward
extending broad deep trof through the Nation`s midsection. In
terms of sensible weather this evolution would largely support
above average temperatures for much of the upcoming week, with
below average temperatures closing out next weekend.

As stated above, the week ahead will feature above normal temps
through Thursday. The big potential caveat to the intensity of the
mild temps on Monday will be clouds. Stratus will likely blanket the
area at daybreak, and maybe some patchy fog/drizzle. Southwest-
westerly lower tropospheric flow will prevail and increase some
through the day, resulting in pronounced WAA and also helping with
the clearing process. The GFS suggests west-east clearing in the
afternoon whereas the NAM is a bit slower from late afternoon through
evening. If the faster solution is correct then highs could be a
good 3-5 degrees warmer than currently forecast. Tuesday will feature
a cold front moving across the area. The front will be located in
the vicinity of the IA/MO border at 12Z, moving to near I-70 by
midday, and clearing the CWA by early evening. Good low level WAA
will continue ahead of the front, and at least immediately behind
the front the air is not all that cold. Thus another day of mild
December temps, around 15-20 degrees above average.

Surface high pressure will continue to settle southward into the
area on Tuesday night, then gradually retreat to the east on
Wednesday. This air mass will be cooler than the preceding days, but
still above average. There has been some model disagreement on the
potential for some light rain late Tuesday night into early
Wednesday associated with southern stream upper low/trof moving from
the southern Plains into and through the lower MS Valley. The
NAM/GFS continue to show a weaker system and keep any precipitation
south of the area, more along the track across AR into the TN
Valley. The ECWMF while showing some southward shift, continues to
have a bit stronger solution and generates some light precipitation
across southern MO including the far southern CWA, primarily late
Tuesday night.  Heading into the later part of the week, both the
GFS and ECMWF are now slower with the late week cold fropa associated
with the deepening and eventually progressive positively tilted
western upper trof as it phases with the northern stream. Both
models keep the front west of the CWA through mid evening on
Thursday, and then vary slightly on the orientation and speed of
the front into and through the CWA late Thursday night into Friday
morning. The slower solutions and later arrival of deeper cold
air now suggest that there may be little to no wintry post-frontal
precipitation.

The increasing trend of the ECMWF and GFS towards a highly amplified
pattern over NOAM and dominance of a deep broad upper trof by the
end of the week into early next week suggest: 1) we could be in
store for some very cold air, well below normal and 2) any snow will
be light and associated with disturbances rotating within the
cyclonic upper flow of the trof. This is very far off in the
forecast and it will be something to monitor in the days ahead.

Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1134 AM CST Sun Dec 17 2017

VFR currently over most of the area are expected to fall to MVFR
this afternoon, and then do low MVFR between 1000-1900 feet by
early evening in most locations. Low MVFR will persist into Monday
morning with some patchy drizzle and potentially IFR ceilings,
though most guidance does not produce a lot of IFR. Should see
some improvement in flight conditions late Monday morning into the
early afternoon especially over central and northeast Missouri.

SPECIFICS FOR KSTL:

VFR flight conditions are expected to prevail for the next few
hours at Lambert. The MVFR ceilings to the southwest of the
terminal will eventually move in, but timing is uncertain at the
moment. There will probably be a little light drizzle which may
also affect visibility...but I am less certain of visibility
impacts than the lowering cigs. Once MVFR cigs roll in, MVFR will
prevail through Monday morning. CIGS are expected to drop to
between 1000-1900ft this evening...but again am uncertain on
timing.

Carney

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     48  43  57  43 /  40  10   5   5
Quincy          46  40  52  38 /  50  10   5   0
Columbia        48  41  55  42 /  30  10   5   5
Jefferson City  49  41  56  42 /  20  10   5   5
Salem           46  41  54  42 /  50  10   5   5
Farmington      45  40  56  43 /  30  10   5   5

&&

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

$$

WFO LSX



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