Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 281723

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1223 PM CDT WED SEP 28 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 311 AM CDT Wed Sep 28 2016

Cold front is getting ready to enter the northern CWA, and will push
rapidly across the CWA by mid morning per the RAP.  Regional radars
are showing that the scattered showers are currently staying close
to an upper low over northern Illinois.  May still be a slight
chance of showers early across the Illinois counties as the cold
front moves through the area as the RAP is showing some weak mid
level ascent and low level convergence along the front.  HRRR
reflectivity is showing some weak returns over south central
Illinois along the front. Highs today will be much cooler with cold
air advection behind the front. Went with a compromise of MOS highs
which is supported by the SREF ensemble temperatures.

Expect mainly dry weather tonight with some clouds lingering over
Illinois in close proximity to the upper low which will be moving
south across Indiana.  Expect lows tonight to fall to close to the
agreeable MOS lows.


.LONG TERM...  (Thursday through Tuesday)
Issued at 311 AM CDT Wed Sep 28 2016

Guidance has been consistently moving the upper low a bit
further west for the past several runs. It should be near the
Indiana/Kentucky border Thursday morning and it will spin over the
Ohio Valley at least through Saturday. The low starts lifting out on
Saturday night according to medium range guidance and should be
swallowed up into the longwave pattern over New England and
southeast Canada by Monday evening. For the Mid-Mississippi Valley
this means north-northwest flow through the depth of the troposphere
through Saturday.  850mb temps hover in the 7-10C range through
Sunday which mixes down to upper 60s to mid 70s each day.

Guidance is spitting out some light QPF primarily during the
afternoon and evening for Thursday through Saturday mostly along and
east of the Mississippi River.  This looks reasonable given the
strength of the low and the multiple lobes of vorticity which rotate
around it through the weekend.  Additionally, isentropic fields show
a nice feed of moisture on the cool conveyor belt off the Atlantic
with c-defs less than 10mb on the 295K and 300K isentropic surfaces
feeding back into the Mississippi Valley. Since models are in good
agreement now with this QPF and the aforementioned moisture feed,
will go ahead and bump up PoPs across parts of eastern Missouri and
Illinois for Thursday through Saturday.  Will keep Sunday dry for
now with the low moving off and removing the moisture feed from the
east.  Am continuing to lean heavily on cool MOS numbers and 850mb
mix down temperatures for highs through Saturday rather than
standard blended guidance due to more extensive cloud cover and
probable showery precip.  Should see fewer clouds Sunday with an
upper ridge building overhead...and then return flow Monday and
Tuesday with temperatures rising back above normal.



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1219 PM CDT Wed Sep 28 2016

Strong nly winds will continue thru the afternoon, diminishing
this evening with sunset. Field of MVFR cigs across nrn IL and IA
shud continue to break up and lift as it attempts to move south
this afternoon. Nly winds will pick up again Thurs morning with an
area of MVFR cigs impacting STL/CPS during the late morning hours.
Have kept out of SUS for now as it looks questionable as to if
these cigs will reach STL/CPS. Have kept in the TAF for now as
confidence is not high enuf to remove.

With a clear sky tonight further west, namely COU area, is some
potential for fog development. However, with mixing anticipated
thru the night, fog threat shud be further west.





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