Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230607
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1207 AM CST Sun Nov 23 2014

.UPDATE:
Issued at 639 PM CST Sat Nov 22 2014

Have made some refinements to the POPS tonight into early Sunday
morning. The main axis of rain/showers this evening will reside
across southeast MO across southwest and south central IL within
the warm moist conveyor belt and ahead of a subtle impulse in the
southwest flow aloft. Further north and northwest across northeast
and central MO the chance of precipitation appears rather low
given the lack of any forcing, and if any occurs it will likely be
isolated or in the form of sprinkles or drizzle. The precipitation
threat will ramp up overnight however in central and northeast MO,
especially after 08-09z with large scale ascent associated with a
shortwave currently over TX moves northeastward into the area and
low level flow backs refocusing the lift westward. Lift and
showers/rain associated with this wave will overspread the entire
area early-mid Sunday morning.

Glass

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 347 PM CST Sat Nov 22 2014

Scattered showers are already moving across the southern half the
CWA this afternoon in an area of modest 925-850mb moisture
convergence ahead of the trough currently over the southern High
Plains.  Expect rain to become more widespread late this afternoon
into the early evening hours over the southern two thirds of the CWA
as the showers currently over southwest Missouri continue to move
northeast into the area.  This is being forced by an area of
stronger low level moisture convergence that the HRRR shows moving
across the area between 22-03Z.   While there will still continue to
be scattered showers elsewhere behind this area through late
evening, it still appears that better chances for more widespread
rain will move in from the west after midnight.  By that time, large
scale ascent and moisture convergence will be increasing ahead of the
aforementioned southern Plains trough that will be ejecting
northeastward ahead of longwave trough.  With clouds and warm air
advection tonight, stayed with lows that were close to going
temperatures and MOS guidance.

Britt

.LONG TERM:  (Sunday through Next Saturday)
Issued at 347 PM CST Sat Nov 22 2014

Rain and a few thunderstorms are expected on Sunday as a southern
stream, negatively tilted shortwave moves from eastern TX
northeastward into the TN and OH Valley regions, while a northern
stream upper level trough/low moves slowly east southeastward
through the northern Plains.  The models were depicting upper level
divergence over our area between a strong upper level jet streak
across the southern Plains and a weaker upper level jet streak over
the Great Lakes region.  The models have some discrepencies as to
the exact track of the deepening surface low, but at this time it
appears that it will move northeastward through or just north of the
STL area around 00z Monday.  There will be ample instability for at
least isolated thunderstorms on Sunday into Sunday evening,
especially across southeast MO and southwest IL.  Strong low level
cold air advection can be expected Sunday night after the passage of
the low and trailing cold front.  The threat for precipitation will
continue Sunday night as shortwaves rotate through our area south of
the main upper level low/trough over the northern Plains.  As the
colder air filters southeastward into the region the rain may change
over to light snow before ending late Sunday night/early Monday
morning across portions of northeast and central MO as well as west
central IL.  Most of the precipitation will shift east of our
forecast area by Monday, although there may be some lingering light
snow or flurries mainly across west central IL.  Much colder
temperatures can be expected for Monday, with nearly steady
temperatures, mainly in the middle 30s to lower 40s as the NAM and
ECMWF models drop the -8 degree C 850 mb isotherm southward to STL.
The GFS model is even colder and quicker bringing the colder air
southeastward into our forecast area.  An unseasonably cold period
is expected through Tuesday night as a large amplitude positively
tilted upper level trough moves eastward through the central US.
The GFS and ECMWF models are still hinting at the possibility of a
northwest flow shortwave and associated weak surface low to bring a
quick shot of rain/snow to our area late Tuesday night and
Wednesday.  The models, particularly the GFS are not as cold for
Thanksgiving Day with the coldest air remaining just north of our
forecast area, although a strong surface ridge is still forecast to
build southeastward into the region from the northern Plains on
Thanksgiving.

GKS

&&

.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1136 PM CST Sat Nov 22 2014

Specifics for KCOU, KUIN: An approaching low pressure system will
bring widespread rainfall and IFR conditions to the area on
Sunday. Winds will become northwesterly behind a cold front
towards the end of the 24-hr TAF period at KUIN. Just beyond the
end of the TAF period, a brief period of light snow is possible
while precipitation is ending.

Specifics for KSTL, KSUS, KCPS: Despite several hours of
persistent light rainfall at night along with steadily increasing
surface dew points, ceilings have remained predominantly VFR at
the St. Louis metro area TAF sites over the last 6 hours. Most
perplexing. In any event, confidence in cig/vis trends is lower
during the first 0-6 hours of the TAF period compared to later,
when a strong low pressure system will bring widespread rainfall
and IFR conditions to the area. Conditions will deteriorate with
time, especially after 10-12z. Winds will become westerly to
northwesterly and increase behind a cold front towards the end of
the 30-hr TAF period.

Kanofsky

&&

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

$$

WFO LSX





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