Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220610

1210 AM CST Sat Nov 22 2014

Issued at 1008 PM CST Fri Nov 21 2014

Weak lift and mid level moisture associated with a low-amplitude shortwave
currently located in western MO combined with lift via a southwesterly
LLJ has generated some showers this evening. These showers were
currenly located to the west of the MS river across southwest and
south central IL, moving to the northeast at 30-35 kts. The showers
should exit the CWA by midnight as the shortwave continues to
migrate eastward in the west-southwest flow aloft, also taking the
mid level moisture with it. In its wake during the overnight
hours, the atmosphere will be bottom heavy with moisture confined
to the low levels. This moisture stratification along with continued
weak ascent via low level WAA would support patchy light rain or
drizzle. Accordingly I have lowered pops a bit and refined the
precipitation mode.



.SHORT TERM:  (Through Late Tonight)
Issued at 349 PM CST Fri Nov 21 2014

Will continue the chance of light rain as low level moisture
convergence and large scale ascent increases ahead of shortwave
trough currently moving towards us across the central Plains.  There
are two small areas of rain that have developed ahead of this over west central Missouri which will move into central
and northeast Missouri later this afternoon and a second over
northern Arkansas which will move into southeast Missouri.  Rain
chances will increase areawide this evening as aforementioned trough
moves across Missouri and Illinois.  Chances will continue through
the night as a 50kt low level jet sets up over the region which will
provide more low level moisture convergence.  At this time it still
appears that precipitation from this event will be mostly liquid as
dewpoints are steadily rising as rain moves into the area.  Both the
HRRR and RUC are showing that the surface wet-bulb temperatures will
be at or above freezing during the onset of precipitation through
early this evening with temperatures continuing to rise through the
late evening hours.


.LONG TERM:  (Saturday through Next Friday)
Issued at 349 PM CST Fri Nov 21 2014

At least scattered light showers can be expected Saturday and
Saturday night due to low-mid level warm air advection and as a
southwesterly low level jet continues to bring low level moisture
into the area.  The best chance of rain along with the highest
rainfall amounts should occur on Sunday as a southern stream
negatively tilted shortwave moves through the southern Plains and
into the TN Valley region, while an upper level low moves
southeastward into the northern Plains.  There will be upper level
divergence over our area between an upper level jet streak across
the southern Plains and another upper level jet streak across the
Great Lakes region.  A surface low will develop and deepen as it
moves northeastward through the St Louis area around 00z Monday.
There should be at least isolated thunderstorms across portion sof
central and southeast MO and southwest IL.  Temperatures will be
much warmer this weekend and above normal for a change.  Much of the
precipitation should shift east of our forecast area Sunday night as
colder and drier air filters southeastward into the region behind
the surface low and trailing cold front.  There may be some
lingering precipitation Sunday night into Monday morning as
shortwaves move through the area south of the upper level low which
will progress slowly eastward into the Great Lakes region.  The rain
may change over to some light snow across northeast MO and west
central IL late Sunday night and Monday as colder air continues to
advect into the region.  Temperatures will return to below normal
values beginning Monday with both the GFS and ECMWF models dropping
the -8 degree C 850 mb isotherm down to STL by 18z Monday.  Below
normal temperatures are expected from Monday through Thanksgiving
with little if any precipitation as a deep upper level trough moves
slowly eastward through the central US.  The ECMWF model does drop a
northwest flow, clipper type shortwave and associated weak surface
low southeastward through our area which could bring the potential
for a brief period of snow on Wednesday, but the GFS is much weaker
with this feature and only has very light QPF.  A strong surface
ridge will build southeastward into MO for Wednesday night and
Thanksgiving Day with unseasonably cold temperatures expected.



.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1134 PM CST Fri Nov 21 2014

MVFR conditions are expected to prevail overnight and for most of
tomorrow. Some improvement is possible at KCOU between 22/21z and
23/06z before conditions deteriorate again. Light drizzle is
possible overnight. Widespread rain is likely just beyond the end
of the 24-hr TAF period.

Specifics for KSTL: Although conditions may occasionally lift to
VFR for brief periods overnight, MVFR cigs are expected to prevail
until mid to late morning. Light drizzle is also possible.
Widespread rain is likely towards the end of the 30-hr TAF period
and beyond.





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