Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231809

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
109 PM CDT Tue May 23 2017

Issued at 1134 AM CDT Tue May 23 2017

Cold front extends from west central Illinois into south central
Missouri late this morning. Winds will shift out of the northwest
behind the front this afternoon. Will keep showers and scattered
thunderstorm going along the front as it move across the Missouri
and Illinois. CAMS are showing thunderstorms becoming more
numerous along the front into the afternoon hours. There will also
be an increase in showers over south central Illinois during the
mid-late evening hours as the RAP shows low level moisture ascent
increasing beneath mid level ascent ahead of a shortwave trough.
Otherwise temperatures still look on target for today.



.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 345 PM CDT Mon May 22 2017

Southwest-northeast oriented area of showers and a few thunderstorms
will continue to shift mainly northeastward over the next several
hours. This area of precipitation looks like it is mainly forced by
strong upper-level jet dynamics in conjunction with weak low-level
moisture convergence. These forcing mechanisms begin to shift slowly
southeastward this evening into the overnight hours so have PoPs
also raising across portions of central Missouri into west-central
Illinois during these hours. Very limited instability as of 1900 UTC
so thinking this will be mainly showers, especially after mid
evening when the limited instability we currently have is forecast
to wane nocturnally. Lows tonight look to be near normal for most
locations in the 50s to near 60 degrees.

Cold front will very slowly progress southeastward through the
day on Tuesday with a continued threat for precipitation. Chances
of thunder will increase by midday as midlevel low and its
associated cold pocket aloft move toward the area helping to
significantly increase midlevel lapse rates. Highest chance of
showers and storms because of the timing of the cold front will be
across central portions of the forecast area. Highs on Tuesday
will be dependent on frontal timing as well as
prevalence/thickness of cloud cover and precipitation chances.
Coolest locations are expected to be across northwestern sections
of the forecast area with highs in the low 60s and warmest areas
including portions of southwest Illinois and southeast Missouri
topping out in the low 70s.


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 345 PM CDT Mon May 22 2017

Track of the upper low is a little further west today, with it
moving down along the Mississippi River on Wednesday.  Still looks
like Tuesday night into Wednesday night will have a chance of
showers under the upper low before drier weather is expected as
shortwave ridge moves into the area from the west.  Then it looks
like it may be active Friday into Sunday as a frontal boundary will
become quasi-stationary over the area and several shortwave troughs
move across Missouri and Illinois in west southwesterly flow aloft.
The GFS is showing the atmosphere becoming quite unstable with quite
a bit of deep layer shear early this weekend, so will have to watch
out for a few strong to severe thunderstorms.  ECMWF and GFS begin
to diverge in solutions by Sunday into Monday which brings
uncertainty in the forecast by early next week.

Still have temperatures below normal under the upper low Wednesday
and Thursday, but they will climb back to near normal late week into
the holiday weekend.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1254 PM CDT Tue May 23 2017

Showers and scattered thunderstorms will affect KUIN and the St.
Louis area terminals through this afternoon. MVFR ceilings can be
expected with these showers. VFR ceilings will move into the area
after 00Z before additional MVFR ceilings move back into area
after 12Z underneath and upper level storm system.

SPECIFICS FOR KSTL: Scattered showers and thunderstorms will move
across the area this afternoon. MVFR ceilings can be expected
with these showers. VFR ceilings will move into the area this
evening before additional MVFR ceilings move back into area after
12Z underneath and upper level storm system.





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