Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191145
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
545 AM CST Fri Jan 19 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 334 AM CST Fri Jan 19 2018

Weak upper ridging combined with persistent S/SW winds around the
periphery of a surface high pressure center over the southeastern
CONUS will help boost high temperatures into the mid 40s to low
50s today. Low dew points will also contribute to efficient
warming of the air mass. Cloud cover will increase late tonight
ahead of a developing warm front associated with an approaching
low pressure system. Patchy fog is possible late tonight given
recent snow melt, but wind and T/Td profiles in BUFKIT suggest
that stratus is the most likely outcome.

Kanofsky

.LONG TERM...  (Saturday through Thursday)
Issued at 334 AM CST Fri Jan 19 2018

Saturday-Saturday Night

A deep progressive upper trof will advance through the southwest
U.S. and into the southern/central Rockies Saturday into Saturday
Night. The mid Mississippi Valley Region during this time will be
dominated by gradually backing west to west-southwest flow aloft,
while the lower troposphere will feature a prominent low level WAA
regime demarcated by southerly surface flow veering to southwest
from around 2-5 kft. This pattern will support above normal temps,
however stratus will likely hold the intensity of temps in check.
Present indications are that shallow low level moisture return
(largely at or below 2500 ft AGL) will commence on Saturday with the
accompanying stratus overspreading the CWA. The pattern will pretty
much remain status quo Saturday Night with continued increase of
shallow low level moisture, supporting extensive stratus and likely
fog and some drizzle as well.

Sunday-Monday

The upper trof will continue to strengthen with a closed low
developing and moving into the southern and central Plains on
Sunday, moving into the MS Valley on Monday. Overall the models are
not too far off from one another, sans the ECWMF is a bit slower and
has the upper low lagging to the southwest of the GFS by Monday
evening. The expectation is stratus and fog/drizzle will persist
into Sunday morning, however by Sunday afternoon any precipitation
will become more showery as low level flow backs resulting in
deepening low level moisture. Above normal temperatures will
continue on Sunday with the persistent low level WAA regime in
place. Certainly the most active weather should be centered on
Sunday night and into Monday. The surface low will deepen moving
from eastern KS on Sunday evening into southwest IA on Monday
morning with the attendant cold front sweeping into eastern MO.
There should be a large upswing in precipitation/coverage within the
warm sector ahead of the advancing cold front on Sunday Night in
response to increasing large scale ascent associated with the upper
system and the organizing warm conveyor belt. A narrow axis of
weak and predominately elevated instability is also present during
the evening which could bring a few thunderstorms to portions of
central and eastern MO. If the slightly faster GFS/NAM are indeed
more correct, the main band of pre-frontal precipitation will be
located well to the east of the CWA on Monday morning and the
majority of the CWA will be in the dry slot. The surface low will
then move northeast and cold front will sweep through the
remainder of the CWA with gusty and veering west-northwest surface
winds and CAA in the wake of the cold front/lifting surface low.
Any threat of additional precipitation, which could be rain
changing to snow, will be associated with wrap around precipitation
impacting northeast MO and west central IL from late morning into
the afternoon and early evening.

Tuesday-Thursday

Heights aloft will be on the rise as the upper trof departs to the
east-northeast with flow aloft from the west or west-northwest and a
few weak migratory impulses traversing the area. There doesn`t
appear to be any sensible weather with these impulses and the period
looks to be largely dry. Temperatures will be moderating from near
normal to above normal as the week progresses.

Glass

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 525 AM CST Fri Jan 19 2018

SPECIFICS FOR KCOU, KUIN: VFR conditions are expected for the
first 0-12 hours. Winds will remain southerly to southwesterly
with gusts up to 20 kts. Stratus is expected to spread northward
after 20/06z, but there is some question about whether it will
reach as far north as KUIN.

SPECIFICS FOR KSTL, KSUS, KCPS: VFR conditions are expected for
the first 0-12 hours. Winds will remain southerly to southwesterly
with gusts up to 20 kts. Stratus is expected to spread northward
and reach the St. Louis area terminals after 20/09z, although
there is some question about whether ceiling heights will be VFR
or MVFR.

Kanofsky

&&

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

$$

WFO LSX



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