Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181634
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1034 AM CST Wed Jan 18 2017

.UPDATE...
Issued at 1033 AM CST Wed Jan 18 2017

Complicated clearing trends this morning. Skies are clearing
quickly over the central part of CWA including the St. Louis metro
area underneath a low level inversion as moisture depth is
relatively shallow. Farther to the north and west moisture depth
is deeper delaying the clearing. Do still expect mid-high clouds
to stream back into the central and southern part of the area this
afternoon ahead of the upper low over the Central Plains. These
cloud trends will cause a disparity in temperatures today where
more sun means warmer temperatures and more clouds mean cooler
temps.

Britt

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 345 AM CST Wed Jan 18 2017

The synoptic pattern early this morning features shortwave
ridging stretching along the MS River, in between a
compact/potent trough across the Ohio Valley and another longwave
trough/cutoff low over the Desert Southwest. The upper-level low
over the Southwest will eject out into the Central Plains by
Thursday morning, bringing the next chance of precipitation to
our region.

Widespread cloud cover continues to plague the region this
morning, with little eastward push of the clearing line noted in
the past hour or so. This is likely due to the weak
lower-tropospheric wind fields given the ridging both at the
surface and aloft. Taking these trends into account, think fog
will be hard to come by this morning, thus have removed mention
from most of the area this morning. Additionally, have kept cloud
cover in through more of the day today, also lowering highs a
couple degrees given the expected lack of solar insolation. The
surface ridge should push east by this afternoon, allowing
southerly flow to commence. This may allow for just enough
mechanical mixing to break up some of the low cloud cover across
portions of central/northeast Missouri late this afternoon,
allowing for a late surge of temperatures into the lower 50s.

For tonight, the aforementioned upper-level low will slide into
the Central Plains which will allow for deep southwesterly flow
to set up over the region. Moisture transport within this regime
will commence late tonight into early Thursday morning, bringing
showers northeast into the region. Very weak elevated instability
is also noted across far southeast MO, thus have included a
slight chance of a thunderstorm in this region after 09Z
Thursday.

KD

.LONG TERM...  (Thursday through Tuesday)
Issued at 345 AM CST Wed Jan 18 2017

Overall, the extended fcst features an active upper air pattn
along with unseasonably warm sfc temperatures (10-20 degrees
above average). The GFS and ECMWF are still in disturbingly good
agreement through 198 hours, but with just enough inter-model
disagreement to avoid a plunge into the atmospheric science
version of the uncanny valley.

The aforementioned upper low will become negatively tilted as it
lifts through the region on Thu/Thu night, bringing showers and
isolated thunderstorms to the area. PW values of 0.8-0.9" are at
+2 SD for January and similar to PW values noted last weekend.
There will probably be some moderate downpours similar to what
was observed at times during last weekend`s freezing rain event,
but this time without the freezing. Temperatures will be well
into the 40s and 50s.

A smaller disturbance intensifies across the south-central plains
on Fri and then lifts northeastward on Fri night and Sat. Moisture
appears to be limited with this system, and the best lift will be
north of the area.

Another upper disturbance will move over the southwestern CONUS
this weekend and induce sfc cyclogenesis over TX/OK. The low
pressure system intensifies and moves eastward/northeastward into
the southeastern CONUS and Ohio Valley late this weekend and early
next week, bringing a chance of rain to mostly the southern half
of the CWA.

Yet another upper disturbance induces yet another sfc reflection
over the central plains early next week, and the resulting low
pressure system then moves northeastward towards the Great Lakes.
This system may bring a chance of rain to parts of the area on
Tue/Tue night.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 513 AM CST Wed Jan 18 2017

Low ceiling trends will be the main focus of the TAF period.
Fuel-alternate MVFR cigs currently plague all sites this morning.
COU may dip to IFR for a period this morning, but expect all other
sites to remain MVFR through much of the day. Could be some
improvement to VFR from west to east this afternoon into this
evening, before additional low clouds move in early Thursday
morning ahead of a system ejecting out into the Plains. This
system will bring showers and possibly even a rumble of thunder to
sites Thursday morning into early Thursday afternoon (into the STL
planning period).

SPECIFICS FOR KSTL:
Fuel-alternate MVFR cigs to persist into this afternoon. Some
uncertainty as to how long they will remain in place, but do have
an improvement to a brief period of VFR conditions this evening.
Additional low clouds (some IFR) will move in Thursday morning
accompanied by showers and perhaps even a rumble of thunder after
12Z.

KD

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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