Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182313
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
613 PM CDT Fri Oct 18 2019

.SHORT TERM...  (Through Late Saturday Night)
Issued at 259 PM CDT Fri Oct 18 2019

An embedded shortwave on the leading edge of an upper level trough
has begun to build into the High Plains, spurring the development of
a surface low across the eastern Dakotas. The shortwave will
continue to drift eastward overnight before passing over the mid-
Mississippi Valley tomorrow in the late morning or early
afternoon. Around the same time, a weak cold front trailing the
surface low will move into the region, which coupled with the
support of the shortwave, will likely provide enough lift to
generate at least scattered showers as both features move into
central MO. However, the surface front is expected to weaken and
become diffuse at it moves further into our region and moisture
ahead of the front will be limited. Therefore I have continued to
keep the highest shower chances confined to central and NE MO.
This areas will also be noticeably cooler than areas further south
due to mostly cloudy skies for much of the day. Shower chances
will tail off tomorrow afternoon and evening as the front weakens,
but partly to mostly cloudy skies will linger across the area
through the night.

BSH

.LONG TERM...  (Sunday through Next Friday)
Issued at 259 PM CDT Fri Oct 18 2019

A pronounced trough will dig into the northern Plains on Sunday
before deepening into a closed low as it moves into the upper
Mississippi Valley on Monday. Surface low pressure will also
deepen and move across the northern Plains in response to this
trough, establishing a strong pressure gradient across the
Mississippi Valley on Sunday. This will help quickly advect warm
moist air back into the region, pushing Sunday`s highs back to
around 70 and pulling surface dew points to around 60 by Monday
morning. As the upper low becomes stacked atop the surface low and
moves into the upper Mississippi Valley on Monday morning, a
trailing cold front will sweep through the Midwest.

This combination of lift along the front, lift on the periphery of
the upper trough, and enhanced low level moisture, is likely to
generate showers and perhaps storms in the mid-Mississippi
Valley. However, there are still modeled differences in cyclone
track, timing, and intensity, which in turn raises questions about
when exactly those showers and storms will occur. The most likely
scenario is the front passes the region some time on Monday
morning, limiting the instability available to help generate
thunderstorms. However, confidence is high that deep and low layer
shear will be very strong along the front. Should storms develop,
this strong shear may be sufficient to intensify storms to severe
levels, including a threat for tornadoes. All that said, the conditional
nature of the severe threat and the uncertainty described above
will preclude me from mentioning severe chances outside of this
discussion.

Behind Monday`s system, generally northwest upper level flow will
become established across the region for much of next week, with
the possibility of multiple embedded shortwaves passing the
region. However, forecast guidance is in poor consensus regarding
these impulses, and the low levels should be quite dry through at
least mid week. This has resulted in a dry forecast from Monday
night through Thursday, by which time there is some signal for
another deep cyclone to pass through the region. The NW flow in
the wake of Monday`s storm will likely keep temperatures on the
cooler side through much of next week.

BSH

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 602 PM CDT Fri Oct 18 2019

Mainly dry and VFR conditions are expected through mid morning on
Saturday. Then showers will move into central and northeast
Missouri as well as west central Illinois including KCOU and KUIN
during the late morning and afternoon hours. Heavier showers may
reduce visibilities and ceilings to MVFR by tomorrow afternoon.
Winds will gradually veer from southeast to west through the
period.

SPECIFICS FOR KSTL:

Dry and VFR conditions are expected through the period. Winds will
remain out of the southeast through 12Z Saturday morning before
veering to the west by Saturday evening.

Britt

&&

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

$$

WFO LSX


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