Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201207

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
607 AM CST Sat Jan 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 342 AM CST Sat Jan 20 2018

Gaps in the cirrus showed that the spatial extent of stratus at 08z
across KS/OK/AR/TN/KY closely followed the RAP`s depiction of
condensation pressure deficits at 900 and 925 hPa. This stratus deck
is expected to continue spreading northward over the next several
hours. Weak lift within the saturated cloud layer may produce patchy
drizzle at times, and areas of fog are expected overnight as the
boundary layer continues to moisten.

Persistent S/SW winds around a high pressure center located
over the southeastern CONUS and ahead of a developing surface low
over southeastern CO will maintain the flow of warmer air into the
region today and tonight. Yesterday`s high temperatures reached the
50s areawide with only limited cloud cover noted on satellite
imagery. Similarly warm highs are expected again today even beneath
the stratus, which is consistent with the diurnal rise of ~10-15
degrees suggested by conditional climatology. If the stratus does not
spread as far northward as currently anticipated, then the going
forecast of highs in the low/mid 50s is likely 5-10 degrees too
cool. Tonight`s lows will likely be a few degrees warmer than last
night due to the cloud cover.


.LONG TERM...  (Sunday through Friday)
Issued at 342 AM CST Sat Jan 20 2018

A deep upper trof will move east-northeast from near the 4
corners region through the central/southern Plains Sunday into
Sunday Night, continuing into the MS Valley on Monday, and then
weakening some as it moves into the Great Lakes/OH Valley Monday
Night-early Tuesday morning. In response, a surface low initially
in southeast CO will deepen and move into central KS by early
Sunday evening. The low will then continue deepen moving northeast
into southwest IA by early Monday morning and into the Great
Lakes by Tuesday morning.

Stratus/fog associated with shallow low level moisture will
blanket the region on Sunday morning in advance of the cyclone and
attendant cold front. Weak low level WAA should also result in
some drizzle. By Sunday afternoon the precipitation should become
more showery as low level flow backs resulting in deepening low
level moisture. Despite the extensive cloudiness, WAA should
contribute to above normal temperatures. And if we get breaks in
the cloud cover, then the temps could be even warmer than
currently forecast. It still appears the most active weather
should be centered on Sunday night and into Monday. Showers should
blossom in coverage on Sunday evening within the warm sector
ahead of the advancing cold front in response to increasing large
scale ascent associated with the upper system and the organizing
warm conveyor belt. Advective processes will also contribute to an
axis of weak and predominately elevated instability supporting
the potential for isolated to scattered thunderstorms,
particularlyfor central and eastern MO. The models vary slightly
with the eastward speed of the cold front across the region, and
the model consensus suggests it should stretch across eastern MO
at 12Z Monday. The main band of pre-frontal precipitation will be
located well ahead of the cold front, extending either across the
eastern CWA in IL over possibly even further east from eastern IL
into Indiana while the remainder of the CWA is located within a
dry slot. The cold front will then sweep across eastern MO and
western IL by early-mid afternoon with gusty and veering west-
northwest surface winds and CAA in the wake of the cold front and
lifting surface low. It still appears that light precipitation
will move back into central and northeast MO during the afternoon,
and into west central IL during the evening within the wrap-
around region of the deepening cyclone. This precipitation appears
to be associated with the cyclonic branch of the warm conveyor
belt and frontogenetic forcing. Initially it would be rain but as
the cold air deepens it should change to snow during the later
part of the afternoon and into Monday night as the system
continues to pull northeast.

Rather deep and broad cyclonic flow will be present in the wake of
the departing system Tuesday into Wednesday. Temps on Tuesday should
be pretty close to normal owing to lots of clouds early on and the
initial impacts of the post-frontal cold air mass, with some slight
moderation on Wednesday as surface high pressure dominates. The
remainder of the week however will feature a warming trend as low
level flow becomes southerly/WAA commences and as ridging builds



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 528 AM CST Sat Jan 20 2018

SPECIFICS FOR KUIN: A stratus deck was moving towards KUIN and
should reach the terminal during the first 1-4 hours of the TAF
period. Once the stratus deck arrives, conditions will likely
remain MVFR thereafter before falling to IFR after 21/06z. Patchy
drizzle and fog are expected across the region after 21/03z. Winds
will remain southerly to southwesterly through the TAF period.

SPECIFICS FOR KCOU: A stratus deck was moving towards KCOU and
should reach the terminal during the first 0-3 hours of the TAF
period. Although conditions will likely remain MVFR for the rest
of the TAF period, there may be some improvement to high-end MVFR
after 18z. The KCOU ASOS has not reported a visibility for the
last 4 hours (a ticket is already open). Although the total
observation concept will suffice for monitoring ceiling heights
with the incoming stratus deck, it will be insufficient for
monitoring visibility conditions which are expected to vary
widely across the region after fog and patchy drizzle have
developed later today. For this reason, "AMD NOT SKED" has been
included in the 12z TAF. Winds will remain southerly to
southwesterly through the TAF period.

SPECIFICS FOR KSTL, KSUS, KCPS: A stratus deck was moving eastward
and should reach the St. Louis metro area terminals before 18z.
There may be some improvement to high-end MVFR or perhaps low-end
VFR between 20/21z and 21/00z based on BUFKIT soundings. Patchy
drizzle and fog are expected across the region after 21/03z. Winds
will remain southerly to southwesterly through the TAF period.





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