Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210520
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1120 PM CST Sat Dec 20 2014

.UPDATE:
Issued at 924 PM CST Sat Dec 20 2014

Clouds are lingering over the entire CWA this evening and will
likely do so into the overnight hours as we are experiencing one
of the longest nights of the year. Latest satellite observations
has shown that low clouds are expanding slowly  eastward over
east central/southeast Illinois while there has been very little
movement over southeast Missouri. Latest ACARS sounding from KSTL
shows a decent inversion between 900-850mb with plenty of moisture
below it, and the RAP keeps plenty of low level moisture over the
CWA tonight, so left cloudy skies over the CWA the rest of the
night. Also raised temperatures tonight, particularly in the
southern and eastern counties where clearing was previously
forecast.

Britt

&&

.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.

Kanofsky

.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 yesterday. 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.

Kanofsky

&&

.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1117 PM CST Sat Dec 20 2014

Focus will continue to be cigs thru the period. Guidance continues
to handle the current trends poorly across the region. Have
therefore continued low confidence in cig heights, but believe
overall trends are reflected in TAFs. Believe conditions will
remain largely as-is except UIN becoming MVFR in the next hour or so.
Cigs shud slowly lift Sun around Noon or during the afternoon.

Specifics for KSTL: Expect cigs to remain in higher MVFR range
thru much of the forecast period with sely to ssely winds aob 10
kts. Believe cigs will lift to VFR Sun afternoon, however, exact
timing remains uncertain.


&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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