Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232155

355 PM CST Sun Nov 23 2014

.SHORT TERM: (Through Monday)
Issued at 346 PM CST Sun Nov 23 2014

Main concern for tonight will be will there be any snow accumulation
over northwestern parts of the area as rain changes to snow behind a
sharp cold front. Most of the CWA has seen rain today and the
current band which is being forced by strong moisture convergence
and strong ascent ahead of ejecting shortwave trough will move
northeast of the CWA this evening. This will cause a lull in the
precipitation this evening before a second area currently over the
Plains moves into the area later this evening and overnight. This
area is being frontogenetically forced ahead of a second trough
currently over the northern Plains. Attendant cold front that
currently extends across central Iowa into far northwest Missouri
into eastern Kansas will push eastward this evening, reaching St.
Louis by midnight and east of the CWA by 3 am. Forecast soundings
suggest that the rain will mix with and then changeover to snow
after midnight before the frontogenetical band lifts out between
midnight and 6 am. Current indications are that snowfall amounts
will be less than 1 inch along and north of a Shelbina to Quincy
line given the relatively warm soil temperatures (Novelty Missouri
Mesonet 2 inch soil temperature is currently 50 degrees) and the
relative short amount of time that the accumulating snow will
fall. It will be windy late tonight into tomorrow as decent
gradient winds set up behind the cold front. However it does not
appear that we will hit advisory criteria at this time.

Do not expect much clearing tomorrow as forecast soundings show some
minimal CAPE in the lowest levels tomorrow afternoon suggesting some
isolated snow or rain showers will develop in the low level cyclonic
flow. Highs tomorrow are only a few degrees warmer than lows tonight.


.LONG TERM:  (Monday Night through Next Sunday)
Issued at 346 PM CST Sun Nov 23 2014

(Monday night through Wednesday)

Next notable trough that will move through the area will be on
Wednesday which the ECMWF and GFS both show light QPF across the
CWA. Have increased PoPs a bit across the area, with the highest
amounts over the northern CWA where current track of the surface
low will be. Temperatures will stay below normal during the period.

(Thursday through next Sunday)

Overall confidence in forecast during this period is not high as the
both operational GFS and ECMWF and their ensembles have had continuity
issues the past few days with the timing and strength of the troughs
that will move across the area later this week. Latest runs are showing
quasi-zonal flow during the period with shortwave troughs moving quickly
across the area. While the GFS is developing some QPF over the region
on Saturday, this is different than yesterday`s run, so will discount
for now. Will go dry during this period given the inherent difficulties
in timing precipitation in zonal flow.



.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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