Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 282203

403 PM CST Wed Jan 28 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 402 PM CST Wed Jan 28 2015

Shortwave over the Nebraska panhandle and eastern CO and associated
surface low ahead of it over eastern Nebraska will move eastward
through northern MO and southern IA tonight.  The surface low will
drag a cold front southeastward through our forecast area late
tonight and Thursday morning.  Most of the models have the
precipitation north of the surface low track, although the ECMWF
model does generate light qpf across IL already this evening in the
warm air advection regime ahead of the shortwave.  The models were
also depicting upper level divergence centered over northeast MO and
west central IL, mainly late tonight in the left exit region of an
upper level jet streak dropping into southwest MO.  The quality and
depth of moisture is quite limited with this system so will only
include slight chance pops across west centrl and southwest IL,
mainly late tonight into Thursday morning.   A tight surface
pressure gradient along with a strong southwesterly low level jet
will lead to relatively strong surface winds for the nightime
hours.  Low temperatures will be above normal tonight as the strong
cold air advection behind the front will not impact most of the area
until Thursday morning.


.LONG TERM:  (Thursday through Next Wednesday)
Issued at 402 PM CST Wed Jan 28 2015

Colder conditions are expected on Thursday due to low level cold air
advection with strong and gusty northwest winds behind the cold
front, along with post frontal low level cloudiness to limit solar
insolation.  Little diurnal temperature range is expected with some
areas likely having nearly steady or slightly falling temperatures
during the late morning and afternoon hours.  These clouds should
clear out Thursday night as subsidence increases over our area as a
strong surface ridge over the northern Plains builds southeastward
into the region.  Cold conditions will continue Thursday night and
Friday with near normal temperatures for late January.
Precipitation is expected to spread into our area by Saturday
afternoon and evening as moisture spreads into our region ahead of
the southwestern US upper level low, and as a strong northern stream
shortwave approaches from the northern Plains.  Initially it appears
that surface temperatures will be warm enough Saturday afternoon
into the evening that any precipiation at this time should fall as
liquid rain.  As a cold front sags southward through our forecast
area Saturday night into Sunday as the upper level trough gradually
deepens over the northern Plains and Great Lakes region the
precipitation should change over to snow.  The ECMWF model has most
of its QPF along and south of I-70 Saturday night and Sunday.  The
GFS is similar although it does have a little more qpf across
northern MO and west central IL compared to the ECMWF.  There has
been plenty of model differences along with run to run
inconsistencies with this weekend storm system and hence it has been
difficult to determine precipitation amounts and types across our
area.  For now it does appear that at least a portion of our
forecast area will see accumulating snow Saturday night and Sunday,
but the snowfall amounts and where the maximum axis of snow will
occur is still in question.  Much colder, below normal temperatures
can be expected for Sunday night through Monday night with the
models dropping the -12 degree C 850 mb isotherm southward into our
forecast area Sunday afternoon.  Monday and Tuesday will likely be
dry as snow associated with northwest flow shortwaves should be
north of our area, and any precipitation associated with the
southwest US upper level low finally moving eastward through the
southern Plains and Gulf Coast region will be well south of our area.



.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1137 AM CST Wed Jan 28 2015

VFR flight conditions are expected through this evening with winds
being the main issue. A tight pressure gradient ahead of an
approaching surface low and cold front will result in gusty
southeast winds this afternoon, which should diminish around or
just after sunset. Decoupling of the lower atmosphere after sunset
and the development of a stout south-southwesterly LLJ will
result in LLWS conditions for much of the evening and into the
overnight hours until the cold fropa. The cold front will move
through the TAF sites from northwest to southeast in the 08-12z
time frame accompanied by gusty northwest winds and MVFR flight

Specifics for KSTL:

VFR flight conditions will persist this afternoon and evening with
gusty southeast winds into the early evening. The surface wind
will diminish during the evening and this will result in the onset
of LLWS as a stout south-southwest LLJ evolves. A strong cold
front will then move through the terminal in the 11-12z time frame
accompanied by gusty northwest winds and MVFR flight conditions.





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