Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211831
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1231 PM CST Fri Nov 21 2014

.UPDATE:
Issued at 1230 PM CST Fri Nov 21 2014

Clouds continue to move northeast across the area at midday and
expect that most of the CWA will be cloudy by late afternoon.
Radar is showing that the rain is currently entering far western
Missouri at the moment. Expect rain chances to increase late this
afternoon into early this evening as low level moisture
convergence increases and ascent increase ahead of a shortwave
trough currently moving eastward over the central Plains.

Britt

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 355 AM CST Fri Nov 21 2014

Surface ridge to continue moving off to the east ahead of next
weather system. Winds to become southeasterly by midday as weak warm
front lifts north through area. This will bring in slightly warmer
temps to the region with temps rising above freezing most locations
by 18z. In the meantime, low level moisture will be on the increase,
but will take awhile to moisten the column as there is a lot of dry
air to overcome. Will see showers develop over southwestern and
southern Missouri and spread to the north and east, moving into
central MO and southern portions of forecast area by 17z.
Fortunately temps will be above freezing when the rain does move in
so no mixed precipitation expected. Precipitation will be scattered
in nature. Highs will be in the upper 30s to mid 40s.

Byrd

.LONG TERM:  (Tonight through Thursday)
Issued at 355 AM CST Fri Nov 21 2014

(Tonight through Sunday)

Rain chances should be on the increase as we head into the weekend
as moisture and dynamics slowly intensify over the region.   Having
a hard time pinning down too many specifics in PoP trends for
tonight and Saturday as models indicate weak, broad forcing and lift
along with a gradual but limited increase of moisture, so getting
the impression that a scattered/chance type PoP is the best way to
go for tonight, with model consensus indicating a slightly better
chance of rain over southern sections of the CWA by Saturday.  It`s
not totally out of the question that there could be a threat of some
freezing rain for a very brief period of time early this evening in
our far northern/northeast counties if precip becomes intense
enough to generate some evaporate cooling, but based on
conservative hourly temp trends believe any period of temps aob
32 would be quite brief.  Obviously, we will keep an eye on precip
and temp trends during the afternoon.

Certainly, greatest rain chances will be late Saturday night and
especially on Sunday, when main upper level system and associated
dynamics swing into the area from the southern Plains.  While
moisture increase may be a struggle tonight and into Saturday, by
early Sunday ams should be quite moist, with forecast soundings
indicating lower troposphere nearly saturated aob 500mb.  Because of
this moisture and strong lift have gone with categorical PoPs over
the entire area for Sunday.   Have also maintained mention of
thunder similar to what`s in going forecast, and although models are
suggesting some CAPE on Saturday and Saturday night in the elevated
warm advection regime, believe primary thunder threat will be
associated with the main system on Sunday.

Based on aforementioned lift and moisture this system should be a
decent rain producer across the region; widespread rains of 1 to 1.5
inches can be expected in the late Saturday night-Sunday time frame,
with isolated higher amounts certainly possible.

Another round of precip, albeit lighter, is expected on Sunday
night. Although main shortwave is forecast to push east of the
Mississippi River by early Sunday evening, there is good agreement
in the 00z models that strong shortwave digging into the central
CONUS in the wake of the first system will generate a band of
mid-level frontogenetical forcing that will produce a secondary area
of precip from sw MO into s IL.  Although this secondary band of
precip will be falling into a colder ams, at this time it appears
that temps will remain warm enough that it will remain liquid.
However, if precip where to become intense enough a drop in the
freezing level could make ptypes a bit more problematic in some
areas.

(Monday-Thursday)

Secondary area of precip should wind down from west to east on
Monday, but 00z medium range models are in fairly good agreement
that another fast moving shortwave will drop into the deepening long
wave trof over the central CONUS by Wednesday, resulting in a slight
chance of light snow over northern sections of the CWA.

Solutions diverge considerably by Thanksgiving, with ECMWF
indicating a deeper longwave trof over the Ohio Valley and a deeper
intrusion of cold air into our area, while GFS hints at a less
amplified upper air pattern which keeps the baroclinicity...and
potential overrunning precip threat...over our area.  For now have
maintained extended initialization guidance, which indicates some
low PoPs during this time.

Truett

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1230 PM CST Fri Nov 21 2014

Low based VFR or high MVFR will continue to move northeast across
the area this afternoon. Rain will begin to move into the area
later this afternoon into the evening hours, though the coverage
is not expected to great enough to warrant more than VCSH at this
point. Later this evening and overnight, a low level jet is
expected to develop over the region which will bring LLWS
conditions at all of the airports. These conditions will last
until around 14Z Saturday morning. Then low based VFR or high MVFR
are expected during Saturday with a low chance of rain.

Specifics for KSTL: Expect mainly low VFR conditions through the
day today until this evening when MVFR ceilings move into the area
with a chance of rain. Expect LLWS conditions overnight with
persistent MVFR ceilings before ceilings rise to VFR during the
day with a continued low chance of rain.

Britt

&&

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

$$

WFO LSX





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