Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200944

444 AM CDT Wed May 20 2015

.SHORT TERM:  (Through Tonight)
Issued at 316 AM CDT Wed May 20 2015

A broad area of rain progresses across the forecast area today in
association with a wave of low pressure riding along a
quasistationary frontal boundary. The precipitation is forced and
maintained by a strong southerly H85 LLJ interacting with a
quasistationary boundary. Isolated embedded thunderstorms are
possible. The upper vort center behind the surface wave moves into
weakly confluent flow and merges with a separate vort center which
originates over Manitoba. The resulting trough forms over Iowa
tonight and moves to the east or slightly south of east. Surface
high pressure will have already spread into the forecast area by
this time and no additional precipitation is expected with the
trough overnight, although the slight increase in lift could
prolong any ongoing showers across the eastern forecast area
between 00-06z.

The forecast area remains on the cool side of a quasistationary
boundary today and extensive clouds/precip will limit insolation.
These unusually cool conditions might set or tie some climate
records. Please refer to the climate section below.


.LONG TERM:  (Thursday through Tuesday)
Issued at 316 AM CDT Wed May 20 2015

High pressure remains in place on Thursday and Friday. An upper
ridge also builds over the central US on Friday then amplifies on
Saturday ahead of a deepening trough over the western half of the
US. Several disturbances lift through the trough between Saturday
and Monday. These disturbances should bring a period of unsettled
weather to the forecast area, especially between Sunday and
Tuesday. Similar to the last two weekends, day to day
precipitation chances will depend both on synoptic scale
shortwaves and on any MCVs which form upstream. Refinements are
likely. Temperatures begin to warm up again on Saturday when winds
become southerly around the retreating surface high. A more
noticeable jump in warmth is expected on Sunday after a warm
frontal passage.



.AVIATION:  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1155 PM CDT Tue May 19 2015

Deteriorating conditions are expected beginning late tonight and
continuing into Wednesday morning from west to east across our
region as first rain, then lower CIGs move in. At some point, we
should see IFR conditions at COU, with STL metro continuing to be
on the threshold, but have expanded IFR mention into SUS and STL
with this issuance. Otherwise, low-end MVFR will be the rule for
much of Wednesday. Prefer the faster timing presented with the
18z/00z GFS model run, which brings rain in fast and is consistent
by and large with the lifting mechanisms present in these systems.
We should still see a bit of drizzle at the tail-end of the main
area of rain and this will move in about the same time as the
lower CIGs. Otherwise, NE surface winds will prevail for much of
this period.

Specifics for KSTL: Faster rain timing still on track and have
now added IFR into the mix by afternoon. Unknown how fast low
clouds will move out, though, with several solutions possible.
Have continued to go with a slightly optimistic approach for
improvement in the evening but kept the non-VFR conditions all
night. Should still see some drizzle to accompany the IFR CIGs
during the afternoon.



Issued at 316 AM CDT Wed May 20 2015

Record cool high temperatures for May 20th are as follows.

St. Louis, MO....55/1924 (KSTL)
Columbia, MO.....57/1924 (KCOU)
Quincy, IL.......57/1967 (KUIN)
Farmington, MO...66/1954 (co-op station)




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