Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181914
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
214 PM CDT Tue Apr 18 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 211 PM CDT Tue Apr 18 2017

Showers left over from a dying MCS over IA has dissipated before
making it into northeast MO.  Diurnal cumulus clouds have developed
across parts of the forecast area this afternoon.  Could not rule
out isolated showers across parts of southeast MO and southwest IL
late this afternoon, although it appears that any significant
diurnal convection will remain southeast of the forecast area.  A
cold front extending from a surface low over central Minnesota
southwest through southeastern Nebraska will drop southeastward to
near the northern MO border by 12Z Wednesday.  Most of the
convection associated with this front should remain north of our
forecast area, but there may be widely scattered showers/storms
across parts of northeast MO and west central IL late tonight and
Wednesday morning. This front will lift back northward on Wednesday
as an upper level trough moves eastward into the northern Plains and
another surface wave develops along the front ahead of it. Warmer
temperatures can be expected tonight and Wednesday with southerly
surface winds in the warm sector. Temperatures will be about 15
degrees above normal.

GKS

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 211 PM CDT Tue Apr 18 2017

GFS/NAM are consistent with moving surface low from western Iowa
into eastern Michigan on Wednesday night and Thursday. The surface
front will move southeast across Missouri and Illinois during this
same time period.  Any showers and thunderstorms that do develop
during the day and evening over the central Plains and the mid
Missouri Valley will likely become more scattered as they move into
the CWA during the late evening and overnight hours.  There will
continue to be a chance of showers and thunderstorms on Thursday
morning, but chances will increase along and south of I-70 in the
afternoon hours.  This when the GFS/NAM are showing increased low
level moisture convergence and instability.  There continues to be
the possibility for a few strong or severe thunderstorms on
Wednesday night or Thursday because of the marginally strong
instability and shear parameters.  The front will then move well
south of the area ending the shower and thunderstorm threat.

Global models are still on track to move the upper low out of the
southwest CONUS and across Missouri and Illinois over the
weekend. The Canadian has now fallen in line with the GFS and
ECMWF in keeping the track of the surface low along the MO/AR
border into the OH River Valley. Consequently, the warm sector
will also stay south of the area this weekend which will have
affect the temperatures and the coverage of thunderstorms that
will be over the area.

Then an upper ridge will begin to build into the area right behind
the trough by Sunday as the surface ridge moves through on Monday.
The models are showing another northern stream shortwave trough and
attendant cold front moving into Missouri and Illinois by next
Tuesday.

Temperatures still look like they will warm above normal ahead of
Thursday`s cold front, but then fall below normal over weekend as
850mb temperatures fall into the 0-5C range.  By Monday and Tuesday,
temperatures will climb back above normal as 850mb temperatures
climb back into the 5-10C range.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1133 AM CDT Tue Apr 18 2017

It appears the MVFR cigs will remain south and west of the taf
sites this afternoon, but there will be VFR, low-mid level clouds.
Any diurnal, air mass convection should remain southeast of the
taf sites. A cold front will drop southward into southern IA and
northern IL tonight, but it appears that the convection associated
with this front will remain north of the taf sites. Mainly just
mid-high level cloudiness tonight and Wednesday morning. South-
southeasterly surface winds will veer around to a south-southwest
direction Wednesday morning.

SPECIFICS FOR KSTL: It appears the MVFR cigs will remain south
and west of the STL area this afternoon, but there will be VFR,
low-mid level clouds. Any diurnal, air mass convection should
remain southeast of the STL area. A cold front will drop
southward into southern IA and northern IL tonight, but any
convection associated with this front will remain well north of
the STL area. Mainly just mid-high level cloudiness tonight and
Wednesday. South-southeasterly surface winds will veer around to
a south-southwest direction Wednesday morning.

GKS

&&

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

$$

WFO LSX



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