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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
1152 PM CDT Wed Sep 17 2014

issued at 659 PM CDT Wed Sep 17 2014

Noticed some light returns on radar which looked a lot like
drizzle. Sure enough, light drizzle is now occurring here in Weldon
Spring. Looking at NAM x-sections and rap soundings, seeing a
fairly deep layer of low level moisture between the surface and
about 800mb coincident with weak lift centered around 850mb.
This lift continues though at least 03z, ending between 04z-06z.
Have therefore gone ahead and added patchy drizzle to the forecast
for this evening for most areas east of Columbia and south of
Bowling Green MO/Pittsfield Illinois.



Short term: (through late tonight)
issued at 311 PM CDT Wed Sep 17 2014

Old peripheral upper level energy from the mesoscale convective system that is now pushing
into northern Arkansas 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 Kansas and extreme
southwest MO, which has been nearly stationary due to the mesoscale convective system rain,
will try to move northeast later tonight and should generate a new
round of rain showers/thunderstorms and rain 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 Illinois 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 Illinois 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 06z tafs through 06z Thursday night)
issued at 1143 PM CDT Wed Sep 17 2014

MVFR flight conditions remain widespread from southwest Illinois across
eastern MO where they deteriorate to IFR across central and
western MO. Further north where skies cleared earlier, additional
fog and stratus is quickly developing. Conditions should deteriorate
overnight at KUIN and kcou with dense fog/LIFR flight conditions
expected. For the St. Louis Metro terminals I am expecting it to
remain status quo given the current T/TD spreads and thus MVFR
flight conditions. All terminals should see improvement beginning
around 15z and then continued rising of ceiling heights and cloud
cover from late morning into the afternoon.

Specifics for kstl:

Little change expected overnight with MVFR flight conditions
persisting. Anticipating improvement in cig heights beginning
around 15z, and then continued rising of ceiling heights and
reduction in cloud cover from late morning into the afternoon.



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