Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 291153

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 AM CDT THU SEP 29 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 323 AM CDT Thu Sep 29 2016

Main concern today will be clouds and a chance of showers.  Most of
the area remains clear and any scattered showers are currently
northeast of the area over eastern Illinois.  Do expect cloud that
are rotating around an upper low that is currently over Indiana to
spread westward today.  RAP is showing low level moisture moving
westward today and there will be embedded vort centers moving on the
west side of the upper low.  This will be conducive to both low
clouds overspreading the area and scattered showers developing over
south central Illinois.  The chance of showers will continue into
tonight as a little more pronounced shortwave rounds the upper low
and moves across the region.

Went a bit closer to the warmer NAM MOS temperatures, particularly
over the west have of the CWA where I expect a bit more sun into the
afternoon before it turns mostly cloudy.  Temperatures tonight will
be close to the agreeable MOS guidance which are supported by the
SREF mean surface temperatures.


.LONG TERM...  (Friday through Wednesday)
Issued at 323 AM CDT Thu Sep 29 2016

The upper level low is still expected to wobble over the Ohio Valley
on Friday and Saturday before beginning to lift out Saturday night
and Sunday.  GFS and NAM are continuing to show a well developed
moist conveyor belt on the northern side of the system.  The
moisture feed remains pretty constant through Saturday with a tongue
of 8+ g/kg spec humidity traveling on the 295K isentropic surface
westward from the Atlantic across the Great Lakes and then wrapping
around the low into the Mid Mississippi Valley.  Models continue to
spit QPF under this moisture feed...with highest amounts mainly east
of the Mississippi River.  Based on QPF, it looks like the highest
PoPs should be Friday afternoon, and this makes good synoptic sense
as the low is forecast to meander back from the eastern
Kentucky/Indiana border to the central Illinois/Indiana border. This
will increase broad scale lift over our area and diurnal heating
should do the rest.  Indeed, MOS PoPs are now up in likely and
categorical range along and east of the Mississippi for Friday.  The
low drifts east on Saturday which will likely decrease the areal
coverage of precip...but with plenty of moisture still available and
probably a bit more diurnal heating would expect at least isolated
to widely scattered showers to develop along and east of the

Low should be ejecting northeast on Sunday and even though the GFS
and ECMWF are kicking out an isolated hundredth or two of precip, I
think the day will be dry since the synoptic forcing will be rapidly
moving out of the area.  Medium range for Monday through Wednesday
looks mostly dry under the influence of a strong ridge. Temperatures
should warm up to near seasonal normals...perhaps a bit above normal
Monday and Tuesday as southerly flow increases ahead of the next
trof. Timing on when that trof will move out of the Great Plains
into the Mississippi Valley is currently low confidence with the
ECMWF bringing it in Wednesday but the slower GFS holding back until
later next week.  Think the compromise forecast with PoPs increasing
to slight chance/low chance Wednesday and temperatures warming well
above normal into the upper 70s to around 80 looks good for now.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 652 AM CDT Thu Sep 29 2016

MVFR ceilings are expected to move into St. Louis terminals and
KUIN from the east between 13-15Z. Ceilings are expected to
increase to VFR during the afternoon before decreasing back to
MVFR and IFR late tonight and early Friday morning. There is a
slight chance of showers at the St. Louis area terminals during
the period, however confidence is too low at this time to include
them. Dry and VFR conditions are expected at KCOU.

SPECIFICS FOR KSTL: Ceilings around 2000ft AGL will move into the
terminal by 15Z. This ceiling is expected to raise back up to VFR
by midday before MVFR or IFR ceilings move into the terminal late
tonight and early Friday morning. There is a slight chance of
showers at the site during the period, how confidence is too low
at this time to include them.





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