Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211816

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1216 PM CST Sun Jan 21 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 407 AM CST Sun Jan 21 2018

An approaching upper disturbance and its induced surface reflection
will affect the region today/tonight. The disturbance was located
over the Four Corners region at 09z per water vapor imagery.
Although some patchy drizzle is possible today, the best chance for
widespread precipitation will occur after 21-00z when the upper PV
anomaly has moved farther eastward. By early this evening, large-
scale ascent will have increased markedly downstream from the upper
trough axis, an approaching surface boundary will provide additional
low-level forcing for ascent, and a strong H85 LLJ will begin
developing nearly overhead. Widespread showers and a few
thunderstorms are expected to initially develop and spread across
MO/IL tonight.

Based on the orientation of shear vectors compared to the H85
boundary, the primary storm mode should be linear by the time the
stronger convection reaches the LSX CWA. Although daytime cloud
cover will have limited surface-based destabilization within the
warm sector, most models still depict H7-H5 lapse rates of at least
6 deg C/km after 00z along with around 50kts of 0-6km shear. These
values are sufficient to support a few strong/severe thunderstorms
even with limited CAPE.

Highs today are expected to be in the upper 50s to low/mid 60s
across the area. Overnight lows will be coolest (40s) in central MO
and warmest (around 50) near the Mississippi River and into
southwestern IL.


.LONG TERM...  (Monday through Saturday)
Issued at 407 AM CST Sun Jan 21 2018

The general scenario on Monday into Monday Night has changed
little except for some timing/speed issues. A deepening surface low
will track northeastward from northwest MO through southeast IA into
northwest IL during the day, with the attendant cold front sweeping
east across the region and into the western OH/TN Valley by evening.
Overall it appears the models have slowed a bit from previous runs
with the eastward progression of the cold front. The GFS is the
fastest solution with the cold front extending across eastern MO at
12Z and the warm conveyor belt precipitation located from the far
eastern part of the CWA into the western OH Valley. Its faster
solution has the cold front clearing the CWA by early afternoon.
Alternatively, the NAM is a good deal slower with the cold front
extending through central MO at 12Z and the warm conveyor belt
precipitation located from extreme eastern MO into IL. It then has
the cold front stretching through far eastern MO at midday, clearing
the CWA between 21-22Z. Other guidance including the ECMWF is a bit
more supportive of a slower solution. Following this scenario there
would be another window of thunderstorm potential across far eastern
MO into western IL with the passage for the actual surface cold
front due to a narrow axis of surface-based instability resulting
from heating within the dry slot along with modest mid level lapse
rates. Wrap around precipitation consisting of rain/showers will
then spread back across central and northeast MO into west central
IL during the afternoon in response to mid level frontogenesis and
large scale ascent associate with east-northeast moving upper low
and southward trailing vort max.  Continued CAA and deepening of the
cold air will occur during the evening with a surface trof/more
prominent northwest wind shift denoting the main thrust of colder
air. This should result in a changeover from rain to snow and
potential for some very light accumulations primarily from central
through northeast MO into west central IL. The tightening surface
pressure gradient will support both increasingly gusty southerly
winds ahead of the front, and strong/gusty west-northwest winds in
the wake of the cold front and lifting surface low.

A seasonably cold day is expected on Tuesday with near normal
temperatures. Deep cyclonic flow will be present through the morning
with slow clearing of clouds, gusty northwest winds, and weak CAA. A
low-amplitude short wave will track across the upper-mid MS Valley
within the west-northwest flow aloft on Tuesday night, however
present indications are that little if any precipitation will
accompany it. Weak surface high pressure will then drift east across
the area on Wednesday with temperatures beginning to moderate. A
better warm-up will proceed during the later part of the week as a
pattern change progresses with ridging aloft on Thursday and the
return of south-southwest low level flow/WAA. A progressive long
wave trof will evolve over the western U.S. heading into Friday
and this will bring the return of southwest flow aloft. Presently
the ECMWF and GFS disagree on timing/location of a lead impulse
tracking into the area within the southwest flow on Friday and any
associated precipitation potential. Moisture return at that point
is weak despite a prominent WAA regime. Current timing in the
models would have a much better chance of rain Friday night into
Saturday as the upper trof moves into the Plains bringing a series
of weak impulse across the area, an advancing cold front, and
improved moisture transport.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1149 AM CST Sun Jan 21 2018

First concern today is MVFR and IFR conditions over parts of
northeast Missouri and south central Illinois. Guidance has not
been handling the stratus and fog very well over the past 24 to 48 have little confidence in it now. Current thinking is
that the VFR conditions across central and eastern Missouri will
continue to spread north-northeast this afternoon. The low clouds
and fog over the eastern Ozarks into south central Illinois will
likely erode and lift...but may not entirely clear out. Once
flight conditions get to VFR, expect VFR to prevail until the
evening. A broken line of thunderstorms is expected to develop by
early to mid evening over southwestern Missouri and move east.
MVFR ceilings and visibilities will likely prevail as this area of
rain moves through tonight...and there may well be pockets of IFR
with the heavier storms. Storms will also be capable of locally
damaging wind gusts to around 50kts. Expect flight conditions to
improve Monday morning after the rain ends.


Primary concern for this afternoon is the possibility that MVFR
ceilings could return earlier than forecast in the TAF. The short
range model guidance has not been handling the ceilings very well
over the past couple of days, and in fact most of the guidance
says that Lambert should still be MVFR. Seeing some redevelopment
of low level clouds right now...but it looks mostly scattered
right now. Current thinking is that VFR conditions will prevail
until sometime this evening. A broken line of thunderstorms is
expected to move into the vicinity of the terminal between 05Z and
07Z tonight, bringing the a return of lower ceilings, moderate and
possibly brief periods of heavy rain, and potentially strong wind
gusts up to 35-45kts. MVFR conditions are expected to prevail
until the rain ends toward sunrise.





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