Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160436

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1036 PM CST Wed Nov 15 2017

Issued at 906 PM CST Wed Nov 15 2017

Cold front has moved into far southern Illinois and southeast
Missouri with a secondary front over northern Missouri and
central Illinois. Latest observations and GOES-16 imagery are
showing fog over southern Illinois and southeast Missouri. Do not
expect this fog to spread northward as drier air continues to
advect into the area. Winds will turn light late tonight into
early Thursday as surface ridge currently over the Plains moves
into the area. Going lows still look on track for tonight.



.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 315 PM CST Wed Nov 15 2017

The surface cold front stretching from near Chicago to around Fort
Smith Arkansas at 200 PM will continue moving southeast this
afternoon and should be out of our forecast area between 300 PM and
400 PM.  The southern edge of a strong high pressure system which is
centered over Canada will spill into the Missouri and Mississippi
Valleys tonight and Thursday.  The pattern aloft is not especially
amplified, and is actually dominated by a shortwave ridge which is
currently moving off the eastern Rockies.  This ridge will move east
tonight and Thursday...keeping the coldest air bottled up in Canada.
However, the southern end of this airmass is still pretty cool,
and we should experience Thursday morning lows similar to those
that occurred this morning across parts of eastern North and South
Dakota. Temperatures should bottom out in the mid 20s to mid 30s,
which is well below normal. The ridge aloft builds overhead on
Thursday and wind at the surface swings around to the east-
southeast. With the ridge overhead, warm advection aloft and
veering surface winds, we should see a decent warmup into the mid
40s to mid 50s.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 315 PM CST Wed Nov 15 2017

The synoptic pattern at the start of the long term period will
feature two troughs of note, once exiting the eastern seaboard and
another approaching the northwestern CONUS. This regime will place
the Midwest within ridging aloft between the two systems, a pattern
which will turn more northwesterly aloft through the period.

The main focus of the period will be on a strong storm system slated
to move through the region Friday into Saturday. A broad warm
advection regime and perhaps a weak embedded shortwave trough will
aid in lift and associated shower development Thursday night into
Friday (Canadian/Euro more bullish with coverage than the GFS).
There may be a brief decline in shower coverage Friday afternoon,
but a strong PV anomaly will approach the area by Friday night. This
approaching system will help to deepen a surface low over the
Central Plains and consequently sharpen a cold front over portions
of MO and KS.  Showers and even a few thunderstorms will break out
initially north of this boundary Friday evening into the overnight
hours, with showers and a few thunderstorms increasing along/ahead
of the boundary early Saturday morning as it begins to surge
southeast. Despite the impressive kinematics associated with this
system, very minimal amounts of surface-based instability should
help to limit the severe weather threat. That being said, it will be
gusty ahead of and behind this system due to the tightening pressure
gradient. Wind gusts could exceed 30-35 mph at times, especially
Saturday morning into early Saturday afternoon.

Precipitation will have moved southeast of the region by Saturday
afternoon (ECMWF still remains about 6 hours slower than the GFS
solution). After highs in the upper 50s/low 60s on Friday, they will
be substantially cooler Saturday and especially into Sunday (in the
40s) as surface ridging moves into the region.

Confidence begins to decrease by the beginning and middle of next
week as solutions diverge. The ECMWF brings another PV anomaly
southeast into the southern CONUS, cutting it off from the main
westerlies by the middle of next week which would keep a fairly mild
and quiet pattern in place for the Midwest. The GFS solution shows
the same PV anomaly but is much more progressive with it,
allowing it to cut out an upper-level trough over much of the
CONUS which would bring cool and fairly active conditions to the
Midwest. Given the vast uncertainty, have stuck with a general
model blend for now until trends become more clear.



.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1034 PM CST Wed Nov 15 2017

Dry and VFR conditions are expected through the period. Winds will
turn light and variable as high pressure moves through the area.


Dry and VFR conditions are expected through the period. Winds are
expected to turn light and variable after 12Z.





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