Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 101137

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
637 AM CDT Thu Aug 10 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 322 AM CDT Thu Aug 10 2017

Isolated showers were developing across north central Missouri early
this morning.  They were being generated ahead of a shortwave trough
over the Upper Midwest.  Isolated showers and thunderstorms will be
possible across central and northeast Missouri to west central
Illinois through this morning as this shortwave moves through.  A
better chance of showers and thunderstorms will occur this afternoon
as both the GFS/NAM are showing enough low level moisture
convergence developing from southeast Missouri into southwest
Illinois.  The HRRR seems to confirm this as it is depicting
scattered thunderstorms developing in this same area between 19-24Z.
Scattered thunderstorms will also be possible overnight along a cold
front that will move southeast through the area.

Forecast soundings today show mixing once again up to the 850-800mb
layer.  MOS guidance looks good in most areas both today and tonight.


.LONG TERM...  (Friday through Wednesday)
Issued at 322 AM CDT Thu Aug 10 2017

Latest guidance for Friday appears to be in good synoptic agreement
with the front moving from near the Quincy/Hannibal area in the
morning through most if not all of the forecast area by Friday
evening.  While the mass fields are in good overall agreement, there
is quite a discrepancy in QPF for Friday.  Most of the models print
out precip along and south of I-70 with the NAM being the most
widespread and producing the heaviest amounts of QPF.  Think this is
primarily due to grid-scale convective feedback upstream earlier in
the run which spins up a much stronger surface-850mb low along the
front which moves across southern portions of the area Friday.  Will
therefore concentrate on the GFS/ECMWF/GEM solution.  Given the
modest instability and relatively weak forcing, think slight chance
to chance PoPs are still reasonable.

Model differences continue for Saturday through next
Wednesday...both run-to-run and between models.  The GFS and GEM
push the front well south of the area over northern Arkansas and far
southwest Missouri by early Saturday morning. While the EC is
similar, it allows the front to drift north Saturday evening as low
pressure develops over the central Plains. The low moves along the
front as it drifts northeast Saturday night and Sunday producing a
20-30kt low level jet. Resulting moisture convergence produces
widespread precipitation across the area from late Saturday night
through Sunday night.  The GFS has a stronger low level high
pressure system over the Midwest north of the front which holds the
front and resulting precip over southwest Missouri and northern
Arkansas.  Given uncertainties am sticking close to ensemble
guidance for PoPs Saturday night through Sunday.

The EC is dry for Monday through Wednesday as high pressure moves
across the Midwest into the Ohio Valley.  The GFS has a similar
synoptic pattern, although the high is further north-northeast and
the front is correspondingly closer to our forecast area.  GFS
therefore has a higher chance for rain over our southern zones for
Monday night through Tuesday night.  Again, no real reason to prefer
one model over the other.  The synoptic pattern with the large high
to our north-northeast favors a dry forecast, so will keep the tail
end of the long-range dry for now for all but a few counties over
the eastern Ozarks.

The persistent long wave trof over eastern North America will
continue to favor near or slightly below normal temperatures at
least through Tuesday. Models begin to break down the upper level
pattern toward the middle of next week allowing a ridge to move over
the Mississippi Valley. This will likely lead to warming
temperatures for Wednesday.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 622 AM CDT Thu Aug 10 2017

Scattered showers and isolated thunderstorms over western and
central Missouri will move east this morning and affect KCOU.
Additional showers and thunderstorms are possible late this
afternoon and this evening over eastern Missouri and southwest
Illinois that may affect the St. Louis area terminals. A cold
front will move southeast across the area late this afternoon and
tonight which will bring a chance of showers and thunderstorms to
most of the terminals. Any storms will have the potential to
produce MVFR or possible IFR conditions as well gusty winds.


There is a chance of showers and thunderstorms this afternoon and
again tonight. Any storms will have the potential to produce MVFR
or possible IFR conditions as well gusty winds.





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