Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201557

1057 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 18z TAFs through 18z Thursday Afternoon)
Issued at 1037 AM CDT Wed May 20 2015

Low pressure and associated rain will move east across the area
today. Rain will end gradually west to east throught the
afternoon. As the upper trof begins digging a bit to the
southeast upper flow will become northwest overight. Main question
is what will the clouds do overnight into Thursday. NAM guidance,
as usual, hits this the hardest with lots of IFR. With some
cyclonic flow aloft and light northerly winds am inclined to keep
at least MVFR ceilings overnight, although IFR is possible at

Specifics for KSTL: Main question will be whether to go IFR
overnight. Rain will continue this afternoon, with the heavier
stuff ending early this afternoon and light, intermittent rain
continuing. IFR clouds have formed behind the rain back to Kansas
City and it looks that will also develop here. Right now am
leaning toward the more pessismistic NAM guidance which keeps IFR
ceilings in until about 14-15z. VFR may not return until 18z or a
little later Thursday.



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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