Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181758

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 AM CST Wed Jan 18 2017

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



.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


.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.



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1138 AM CST Wed Jan 18 2017

Large area of clearing has developed around the St. Louis area
terminals with VFR conditions now expected through afternoon hours
into the evening. The clearing line is now close to KCOU, and VFR
conditions will likely develop there early in the afternoon. KUIN
will likely take longer to go VFR as the clearing line is slightly
farther away. MVFR ceilings that are currently over southern
Missouri will likely move back into the area this evening as
moisture streams back into the area ahead of the storm system.
Showers will develop over the area late tonight and tomorrow as
this system moves through Missouri and Illinois.

SPECIFICS FOR KSTL: VFR conditions are expected through early
evening before MVFR ceilings redevelop. Moisture will stream back
into the area causing MVFR ceilings and visibilities to develop
tonight as well as showers as a storm system approaches the area.
Showers and IFR conditions are expected on Thursday as this system
moves across the area.





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