Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181743

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 AM CST Mon Dec 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 311 AM CST Mon Dec 18 2017

Temperatures were well above freezing early this morning with light
drizzle reported at KSTL.  KLSX was also operating in VCP-31 which
is picking up on the drizzle this morning.  RAP soundings are
showing deep moisture up to 800mb through 12Z before it moves out
from west to east through late morning.  Have gone with patchy
drizzle and low clouds over the entire area through early morning,
with the drizzle ending from west to east through mid morning, and
the low clouds clearing out this afternoon.  Then skies will remain
partly cloudy tonight with some clouds moving back in over southeast
Missouri and southwest Illinois overnight associated with a southern
stream storm system.

Highs today will be held in check initially by the low clouds, so
only expect about a 10 degree diurnal rise today.  Lows tonight will
still be well above mid December normals and will be close to MOS


.LONG TERM...  (Tuesday through Sunday)
Issued at 311 AM CST Mon Dec 18 2017

Key Points for Tuesday - Christmas Eve:

1) Above average temperatures Tuesday-Thursday, and several days
will be well above normal

2) Light rain possible across the far southern CWA Tuesday Night-
Wednesday morning

3) Colder air arrives late Thursday Night into Friday, but large
model divergence Thursday Night-Christmas Eve leads to limited
confidence in details as well as confidence in the chance of
wintry precipitation/snow

The latest model runs are still showing the upper air pattern over
NOAM will undergo considerable evolution over the next 7 days.
With each new model cycle come changes and variations from the
previous cycle covering the longer range portion of the forecast
from late week into Christmas Day. Overall the pattern will
transition from a defined split flow regime with a fast broadly
cyclonic flow across the northern tier and progressive southern
stream closed low/trof in the southwestern U.S. early Tuesday, to
a highly amplified pattern by next weekend.

Tuesday appears to be one of those days with well above normal
temps. A cold front will stretch across southern IA at daybreak and
will move south-southeast during the day. Present indications are
the front will be north of I-70 at midday, and just clearing the CWA
by early evening. West-southwest lower tropospheric flow and good
low level WAA will prevail ahead of the front and this should
translate to unseasonably mild temps, with highs around 15-20
degrees above average.

In the wake of the front on Tuesday night, cooler air and the
associated surface high pressure system will continue to settle
south-southeastward into the Mid MS Valley and western OH Valley.
Meanwhile the southern stream upper low/trof located in the
southwest U.S. at the start of this period will progress across the
southern Plains and into the lower MS Valley Tuesday night/early
Wednesday morning, exiting into the TN Valley by midday Wed. The
models have come into better agreement with this system and the
NAM and GFS have trended a bit north compared to solutions from
the model runs 24h ago. They are now in agreement with the ECMWF
that the northern periphery of the deformation zone precipitation
shield will impact portions of far southern MO and southern IL
with light rain, including the extreme southern counties of the
LSX CWA Tuesday night into Wednesday morning. The slowly
retreating surface high will exert a strong influence and result
in cooler temperatures on Wednesday, but still above normal.

Low level WAA will begin to get underway on Wednesday night in
association with continued retreat of the surface high pressure
system and falling surface pressure in the central Plains, the later
of which is in response to large scale height falls attendant with
the digging/deepening upper low/trof in the Great Basin. The upper
low will bottom out by late Thursday near the Four corners region,
and it is after this time period where we begin to see increasing
spread and divergence between the GFS and ECWMF. Thursday itself
should be another of those unseasonably mild days with well above
average temperatures. Heading into Thursday night and Friday, a cold
front will track across the region. However differences in
speed/strength of the ejecting and weakening 4 corners trof result
in both speed/timing differences with the cold front as well as
its orientation. Both models have the front into eastern MO at
daybreak on Friday with a threat of prefrontal warm sector showers
starting with the advancing front on Thursday night. The GFS has
a stronger and more progressive trof and thus blasts the more N/S
cold front through the CWA by early Friday afternoon. Alternatively,
the ECWMF is weaker and slower with the ejecting trof which leads
to a more NE-SW orientation, slower passage, and a threat of
post-frontal precipitation Friday into Friday night. Some of this
precipitation could be in the form of snow or a wintry mix across
parts of central and northeast MO with this solution. The CMC GEM
also adds credence to the ECWMF solution, however at this time
range and given the run-to-run variability our confidence is low
in any specific solution.

A big surface high and increasingly deep and cold air will then
settle south-southeast through the region thru the weekend. The
magnitude of the cold air will depend on the character and
orientation of a deep upper trof that will dominate into the final
week of December. A highly amplified upper air pattern will exist
across NOAM with a ridge in the eastern Pacific into eastern AK,
and a pronounced vortex near Hudson Bay and southward extending
large deep upper trof. There are big differences in the location
of the upper trof axis and whether the CWA is near the base of the
trof like the GFS, or the trof axis is to the west and we are
within w-sw flow aloft like the ECWMF and CMC GEM. These later 2
models suggest that weak disturbances within the flow aloft,
combined with ULJ dynamics, and strong low-mid level frontogenetic
forcing could produce a broad NE-SW oriented band of snow
impacting portions of central and northeast MO and west central IL
in the Saturday night into Christmas Eve time frame. Given the
time range we are talking about and model/cycle variability,
uncertainty is high and predictability low at this time.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1128 AM CST Mon Dec 18 2017

Area of high-end MVFR cigs will continue to move ewd and
dissipate, becoming VFR by mid afternoon. Concern turns to area of
IFR stratus and fog across OK south into TX. Guidance differs
regarding if this stratus will advect newd and if it does, how
far. IF the stratus holds together and develops back nwd, the most
likely terminal impacted is KCOU with KUIN possible twd sunrise
Tues. At this time, these clouds shud remain north and west of

SPECIFICS FOR KSTL: Cloud bases are expected to lift and break up
thru the afternoon becoming VFR mid afternoon. Winds will pick up
somewhat thru this afternoon before diminishing again this evening
remaining generally sely.





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