Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202116

316 PM CST Sat Dec 20 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 302 PM CST Sat Dec 20 2014

Clouds will persist early tonight and perhaps start to decrease
from south to north by morning. Overnight lows should be similar
to last night in most locations.


.LONG TERM:  (Sunday through Next Saturday)
Issued at 302 PM CST Sat Dec 20 2014

The primary forecast issues revolve around a large low pressure
system and a deep trough which are expected to affect the area
between Mon and Wed.

The ongoing gradual warming trend (highs today were generally about
5 degrees warmer than yesterday, which in turn was a few degrees
warmer than the previous day) will persist on Sunday and Monday.
Monday`s highs in the mid-40s to lower 50s will be around 10
degrees above average for this time of year.

The aforementioned low pressure system develops over the northern
plains on Sunday and Sunday night, eventually closing off on Mon or
Mon night within a deep trough over the central CONUS. This system
will dominate our weather through Wed, and widespread rain is likely
as it moves into the region on Mon and Mon night. Forecast PW values
of between 0.6" and 0.8" by 00z Tue are above the 75th percentile
and approaching +2 SD for December. A cold front is expected to move
through parts of MO/IL on late Mon night or Tue morning, which is
slower timing than the models were predicting ydy. By Tue
afternoon or evening, some of the rain will probably switch over
to light snow within the colder post-frontal air mass.
Temperatures return to near or below average behind the front.

Meanwhile, a second vort max is forecast to drop into the central
CONUS trough and induce a new surface wave along the cold front
somewhere in TX/AR/LA (a third, smaller vort max interacts with
the northern upper low center during this time). The resulting
southern system then lifts northeastward on Tue and Tue night, but
there are sufficient differences between the GFS and ECMWF to
preclude getting more specific than SChc/Chc PoPs with a generic
rain/snow mix.

After the trough rotates away on Wed night or Thu, a brief period
of upper ridging is anticipated ahead of the next disturbance. The
12z runs of the ECMWF and GFS are in better agreement on the large
scale pattern compared to 24hrs ago, but the forecast with this
system will still depend on what will have happened with the
previous system, and confidence remains low.



.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1145 AM CST Sat Dec 20 2014

Seeing some holes in the clouds today, some making their way
towards STL metro area. But for the most part continue to see MVFR
cigs most locations, though did add tempo mention of IFR cigs at
KUIN for a few hours this afternoon. Winds to persist from the
south through the forecast period.

Specifics for KSTL:
Seeing some holes in the clouds today, some making their way
towards STL metro area. But kept MVFR cigs for now. Winds to
persist from the south through the forecast period.





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