Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 222147

347 PM CST Sat Nov 22 2014

.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.


.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



.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1235 PM CST Sat Nov 22 2014

Have added tempo groups for showers later this afternoon and early
this evening at the STL metro TAF sites as an area of showers over
southwest Missouri moves northeast. MVFR ceilings are expected
through 10-15Z when a steadier rain is expected to develop. Once
this steady rain moves into the area, IFR conditions are expected
on Sunday at all TAF sites.

Specifics for KSTL: Expect ceilings between 2000-3000ft through
22Z. Then an area of showers will move into the terminal from the
southwest which may cause ceilings to fall below 2000ft into the
early evening hours. Then expect most of the night to be dry with
just scattered showers in the area before lower ceilings and more
widespread showers move into the area after 09Z, with steady rain
and IFR conditions moving into the area by mid morning Sunday.





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