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fxus63 klsx 211210 

Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
610 am 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 850 mb low level jet will begin
developing nearly overhead. Widespread showers and a few
thunderstorms are expected to initially develop and spread across
MO/Illinois tonight.

Based on the orientation of shear vectors compared to the 850 mb
boundary, the primary storm Mode should be linear by the time the
stronger convection reaches the lsx County Warning Area. Although daytime cloud
cover will have limited surface-based destabilization within the
warm sector, most models still depict 700 mb-500 mb 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 Illinois.


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 Iowa into
northwest Illinois during the day, with the attendant cold front sweeping
east across the region and into the western Ohio/Tennessee 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 County Warning Area into the western Ohio Valley. Its faster
solution has the cold front clearing the County Warning Area 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 Illinois. It then has
the cold front stretching through far eastern MO at midday, clearing
the County Warning Area between 21-22z. Other guidance including the European model (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 Illinois 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
Illinois 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 cold air advection 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 Illinois. 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 cold air advection. 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 European model (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 warm air advection 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 12z tafs through 12z Monday morning)
issued at 525 am CST sun Jan 21 2018

Specifics for kcou, kuin: an area of MVFR stratus stretched from
central MO through west central Illinois at taf issuance. Fog was
developing within some of the cloud breaks. Conditions will be
predominantly MVFR during the taf period although some improvement
is expected at kcou between 21/20z - 22/03z. A period of
rain showers/thunderstorms and rain is expected after 03z when a strong low pressure system
approaches the area.

Specifics for kstl, ksus, kcps: overall, there should be an
increase in cloud cover today ahead of an approaching low pressure
system followed by a period of rain showers/thunderstorms and rain after 03z. That said,
ceilings will be highly variable during the first 0-6 hours of the
taf period. The St. Louis Metro area was located between two
separate stratus decks at taf issuance, both of which were making
progress towards the terminals. One stratus deck with MVFR cigs
stretched from central MO through west central Illinois and was nudging
southward towards the terminals. A separate area of IFR cigs
stretched from the Ozarks towards southwestern IL, and some IFR
stratus fragments on the northern edge of the deck were noted on
satellite moving northeastward towards the terminals. On top of
these decks, a separate VFR cloud layer was oriented perpendicular
to the other two and was located over the St. Louis Metro area at
taf issuance, but satellite imagery shows that this area of
clouds is moving steadily eastward. Taken together, the
complicated motions of the three cloud decks suggests that cigs
may vary between IFR, MVFR, and VFR (including brief periods of
skc) during the first 0-6 hours. Fog was also developing within
cloud breaks, and this may affect any of the St. Louis terminals
through 14z.



Lsx watches/warnings/advisories...

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