Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172012

312 PM CDT Wed Sep 17 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 311 PM CDT Wed Sep 17 2014

Old peripheral upper level energy from the MCS that is now pushing
into northern AR will continue to generate isolated-scattered
showers into the early evening before it either weakens too much to
generate anything or it exits our area to the east.

Otherwise, a warm frontal boundary over southeast KS and extreme
southwest MO, which has been nearly stationary due to the MCS rain,
will try to move northeast later tonight and should generate a new
round of SHRA/TSRA over areas mainly to the south and west of STL
metro.  Thunder chances up our way look meager enough to leave out
for now.

Clouds are starting to break in southwest IL now and this trend
should continue and even quicken as the upper level energy
generating the showers there comes to an end or exits.  This will
leave the leftover low cloud mainly to the west of the MS river and
the light easterly flow will begin to erode the eastern edge of
this, albeit slowly, this evening.  By late tonight, though, a
favorable fog/stratus setup along the periphery of the surviving low
cloud in central-southeast MO may actually allow the leftovers to
grow a bit back to the east again.

Went with a more aggressive cooling on the IL side of the river
tonight due to expected clearing, while preferred the warmer MOS in
central MO.  In-between, tended to favor the cooler MOS guidance as


.LONG TERM:  (Thursday through Next Wednesday)
Issued at 311 PM CDT Wed Sep 17 2014

Surface ridge to continue building in on Thursday. Could see some
lingering light showers over far southern portions of forecast area
in the morning. Otherwise, clouds to clear out from northeast to
southwest with highs in the low 70s. Lows Thursday night will be in
the 50s.

By Friday, surface ridge moves off to the east allowing warmer
southerly flow to return to the region. However, next weather system
to move in on Saturday with showers and thunderstorms possible and
persist through Saturday night before tapering off on Sunday. Highs
on Friday will be in the mid to upper 70s, in the low to mid 80s on
Saturday, then begin to cool back down on Sunday as next ridge of
high pressure builds in.

Cooler weather to persist through the middle of next week with highs
in the 70s and lows in the 40s and 50s.


.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1230 PM CDT Wed Sep 17 2014

Region of sub-VFR ceilings, whose eastern edge is near the MS
river, will begin to erode this afternoon and shrink back to the
west as easterly flow combined with dissipating SHRA kick in more.
This will result in some intermittent MVFR ceilings across STL
metro sites with much more persistent sub-VFR conditions at COU,
unfortunately. May eventually see a break in COU very late this
afternoon and into parts of this evening, but looking like IFR
conditions are likely once again later tonight there. Depending on
how well we clear out further east into STL metro and UIN, may see
some fog development in an otherwise favorable setup with light
east winds and damp ground. Threw out initial numbers on VSBYs
with this being understood conditionally--if clearing is more
substantial in those areas, will need to go more widespread IFR
VSBYs. A return to VFR is anticipated across all sites by midday
Thursday, except perhaps COU.

Specifics for KSTL: Intermittent MVFR ceilings likely for the next
2-3 hours before easterly flow and time of day should push the
eastern edge backward. Period of VFR then expected for late this
afternoon and early this evening before some fog is expected to
develop. How well we clear out will determine how bad the fog gets
as this is an otherwise good setup for it given light easterly
flow, lack of previous day mixing, and recent rainfall. A return
to VFR then expected late Thursday morning.



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