Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 122117

Area Forecast Discussion
National Weather Service Saint Louis MO
317 PM CST Sun Nov 12 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 316 PM CST Sun Nov 12 2017

Visible satellite is showing an extensive area of clouds covering
the area as water vapor imagery is showing a mid level trough moving
across Missouri and Illinois.  Some patchy drizzle is be reported
over the area ahead of the trough.  This trough will move east of
the area this evening, but the clouds will be slower the clear out.
The clouds will clear out of northeast and central Missouri as well
as west central Illinois this evening before the rest of the area
clears out overnight.  The combination of light winds and clear
skies will likely lead to some patchy fog development in these areas
late tonight and early on Monday morning.  The fog will burn off
tomorrow morning. Clouds will increase from the west by afternoon as
another weak shortwave trough approaches the area from the west.
Still expect a mainly dry day though as the atmosphere will be dry
through a deep layer with limited ascent.

Temperatures will be below normal through tomorrow as a relatively
cold airmass still exists over the area.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 316 PM CST Sun Nov 12 2017

Warmer temperatures can be expected Monday night as the surface
ridge shifts east of our area and surface/low level winds become
southerly/southwesterly.  Return moisture will be spreading
northward into the region as well, and this coupled with low-mid
level warm air advection may bring patchy light rain or drizzle to
northeast and central MO and west central IL Monday night and
Tuesday.  The models were consistent that more significant showers
will move through our area late Tuesday night and Wednesday morning
as an upper level trough moves eastward through the northern Plains
and Great Lakes region and sends a cold front southeastward through
our forecast area late Tuesday night and Wednesday. There will be a
break in the precipitation Wednesday night and Thursday morning,
then the front will lift back north-northeastward as a warm front
and bring showers and a few storms to our area mainly Thursday night
and Friday morning. The operational GFS and the ECMWF models
continued to have timing differences with the deepening upper level
trough moving through the northern Plains and Great Lakes region
Friday through Saturday and the associated deepening surface low and
trailing cold front.  The GFS was quicker bringing the cold front
southeastward through our forecast area Friday afternoon and evening
along with convection along it.  The ECMWF model was slower, not
bringing the cold front through the forecast area until late Friday
night and Saturday morning.  For now will lean toward the quicker
GFS, and hence lower the pops late Friday night and go dry on
Saturday. Unseasonably warm conditions can be expected on Friday
after passage of the warm front, but this will be short lived as
colder air filters southeastward into our area Friday night and
Saturday morning after the passage of the cold front.  At this time
it appears that the precipitation will end after frontal passage
Friday night before temperatures get cold enough for any snow.
Colder and drier conditions can be expected next weekend as a
surface ridge moves eastward through the region with northwesterly
upper level flow.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1115 AM CST Sun Nov 12 2017

MVFR conditions will dominate for the TAF period for all terminals
except UIN and COU. UIN will see IFR through Monday morning
before ceilings begin to break. Cloud deck at COU will rise this
afternoon to MVFR but patchy fog is expected overnight and bring
the site down to IFR. Mid level clouds through the overnight for
the metro TAF sites with a broken low deck possible this
afternoon.Cannot rule out patchy fog for all other terminals but
confidence is low at this point. Will follow up next TAF cycle.
Winds will remain under 10kts from the north/northwest for the


MVFR for most of the period. Mid level clouds will remain until
near noon tomorrow when skies begin to clear. There is a broken
low cloud deck in the area but should lift by mid afternoon.
Cannot rule out patchy fog overnight if ceilings break earlier but
confidence is low at this point.





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