Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231755

1155 AM CST Sun Nov 23 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 331 AM CST Sun Nov 23 2014

Surface low to begin lifting northeastward towards forecast area
today. Will see showers on the increase this morning and spread
north across the region. Some instability today so there is a chance
of thunderstorms mainly along and south of I-70.  As for
temperatures, will depend on cloud cover and rainfall. But feel that
temps will rise into the mid 50s to low 60s by this afternoon.


.LONG TERM:  (Tonight through Saturday)
Issued at 331 AM CST Sun Nov 23 2014


While specific precip trends are certainly tricky tonight and into
Monday, latest synoptic and hi-res guidance suggests the general
scenario will be that this afternoon`s band of rain and embedded
thunderstorms will rotate east out of the area this evening, as
additional precip begins to form to our west as secondary shortwave
drops south in the wake of today`s system.  This secondary area of
precip should then wind down to spotty areas of light rain/light
snow during the day on Monday.

Yesterday`s guidance as well as 00z model output is now suggesting
more precip lingering in the cold air late tonight and early Monday
than was indicated by earlier runs.  I have not made any major
changes to the timing of the changeover from rain to snow over the
northwest half of the CWA in the 06-12z time frame; this may be a
bit too fast, but didn`t want to discount the intensity of the
CAA. In the far NW counties, it appears that this strong CAA will
cause freezing levels to drop enough to support the rain changing
over to all snow for a few hours late tonight and into early
Monday morning, and based on this have added some minor accumulations
of under an inch in our extreme northwest counties.

Min/max temps for tonight/Monday will be based on non-diurnal, 3
hourly temp trends during this time frame.

No major changes in going forecast trends heading into midweek, with
dry and chlly weather on Tuesday followed by a brief warmup and low
PoPs for the fast-moving, moisture-starved system on Wednesday.


Medium range guidance has come into better agreement with a less
amplified solution for Thanksgiving Day, suggesting a ribbon of
light precip over northern sections of the CWA as shortwave
interacts with baroclinicity that is draped across the region.
Pattern suggests that there should be a strong temperature
gradient from north to south across the region, with highs in the
30s over the cold air in the north while approaching 50 in the
warmer air over southern sections of the CWA.

This agreement jumps the tracks heading into the latter half of the
week, with GEM and ECMWF forecasting broad trofing over the central
CONUS while the GFS maintains a more quasi-zonal look.  Of course,
given the intensity of the baroclinic zone this somewhat subtle
difference will make a huge impact on sensible weather trends, and
especially on temperatures.  This discrepency can be seen in the
surface prog forecasts for Friday morning; ECMWF and GEM have broad
and cold Canadian high over the plains and into the upper
Mississippi Valley with north/northeast winds over our FA, while GFS
as surface low over the northern plains and southerly winds over our
area.  MOS guidance based on these solutions offer a more
quanitative and stark measure of this discrepency...a whopping
20-25 degree difference on Friday`s highs between the colder
ECMWF and warmer GFS. For now have maintained earlier (colder)
trends, based on consensus of GEM and ECMEF.



.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1133 AM CST Sun Nov 23 2014

Widespread high-end IFR ceilings will be the rule at all TAF sites
through the day and into tonight. Visibilities may also dip into
high-end IFR range at times with heavier rain, particularly before
00Z. Surface low pressure will track just northwest of region
tonight, with winds becoming west to southwest. Arctic high
pressure then rapidly build in behind main cold front from 06Z to
12Z Monday. Remaining precipitation should mix with then change to snow
before ending at KUIN and KCOU TAF sites Monday morning, however
drier air arrives to the south over metro St. Louis area just as
temperatures get cold enough to support snow at the surface. A few
flakes may mix in there as well, but it should not be significant
so have not included in this set of TAFS. West winds will become
strong behind arctic front Monday morning and will likely gust
into the 25-30 knot range. Ceilings should improve into MVFR range
behind the front as well. As temperatures throughout the column
continue to cool during the day, some brief snowshowers are
possible Monday afternoon.

Specifics for KSTL: IFR ceilings will likely last though the
night, with periods of IFR visibilities this afternoon in pockets
of heavier rain. Main cold front moves in around 12Z Monday, but
atmosphere dries out by then in critical cloud layer cold enough
to form snow, so the precipitation should shut off by then. Post
frontal winds will be a factor, and will gust in the 25 to 30 knot
range after 12Z Monday. MVFR ceilings behind cold front will
likely last through this TAF period.





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