Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220443

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 PM CDT Sun May 21 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 240 PM CDT Sun May 21 2017


Cooler and drier air continues to filter into the bi-state region
late this afternoon. Stratocumulus deck should continue to advect
mainly eastward, with its edges dissipating this evening with loss
of daytime heating. This should leave the CWFA clear to mostly clear
overnight tonight with high pressure across southern Missouri in
control. Should be quite a cool night with favorable conditions for
raditional cooling in close proximity to sfc anticyclone. Expect
lows in the 40s for most areas, with the exception of portions of
southwest Illinois as well as the urban heat island of St. Louis in
the low 50s. Favored terrain may even bottom out near the 40 degree


Surface high will slide up the Ohio River Valley fairly quickly on
Monday with return flow around the western periphery of the high. An
area of showers with a few thunderstorms should move northeastward
out of Kansas, with the center of the precipitation axis heading
toward portions of northeastern Missouri and west-central Illinois
by Monday afternoon. This area of showers/storms will be aided by
weak moisture convergence at low levels, as well as strong jet
dynamics associated with the right-entrance region of a 100+kt 300-
hPa jet streak transversing the mid-Mississippi Valley.

Temperatures on Monday should be quite a bit warmer than that of
today for most of the area with south/southwest winds expected at
the surface along with 850-hPa temperatures rising to around +10C.
Highs are expected to be in the 70s or near to slightly below normal
for the date. Coolest conditions are forecast to be across portions
of northeastern Missouri where the earlier onset of cloudy
conditions, and eventually the likelihood of precipitation, should
help keep highs from only topping out around 70 degrees.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 240 PM CDT Sun May 21 2017

Global models are still in good agreement that a large upper low
will drop down through the upper Midwest early this week into
Illinois by late Wednesday.  It still looks like a shortwave trough
and an attendant cold front will swing around the south side of the
the upper low on Tuesday bringing a chance of showers and
thunderstorms.  While there may be a brief respite in the rain over
parts of the area on Tuesday night, showers will pick back up on
Wednesday as the upper low moves over the area.  Then a shortwave
ridge will move across the area on Thursday night providing dry time
late Wednesday night into Thursday night before a pair of shortwave
troughs bring chances of showers and thunderstorms on Friday and
over the weekend.

Still looks relatively cool for mid-late May with highs only in the
60s to around 70 Tuesday through Thursday under the upper low. Highs
will get back closer to normal by the end of the week as 850mb
temperatures climb back to around 15C with southwesterly low level



.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1142 PM CDT Sun May 21 2017

VFR conditions are expected through the period. After a mostly
clear night with just a few passing high clouds, VFR cigs will be
on the increase through the day on Monday as a shortwave
approaches the region. This system will bring showers and a few
thunderstorms to UIN and perhaps COU Monday afternoon, but the
precipitation should weaken by the time it approaches the St.
Louis metro sites. After fairly light SSW winds tonight, they will
increase with gusts of 15-20 knots out of the SSW on Monday.





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