Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142101

Area Forecast Discussion
National Weather Service Saint Louis MO
301 PM CST Sun Jan 14 2018

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 252 PM CST Sun Jan 14 2018

This afternoon, an upper longwave TROF existed over eastern North
America with a upper longwave RIDGE in the west, yielding NW flow
aloft for our region. Two clipper systems of interest embedded in
this NW flow. The main core of the first one has already pushed
into the deep South, around the Arklatex region, but its northern
lobe has--as advertised--spawned a more northerly and lighter
region of snow showers that has pushed thru our forecast area
today with areas of a dusting (< 1") snow accums, with most a
quarter inch or less. Under mostly cloudy skies (there was a
little sun in a few spots), temps have risen into the mid-upper
20s thanks in part to southerly winds.

The northern lobe will exit our area to the east early this
evening but will lose some punch a bit earlier as the sun sets
shortly, and will yield nothing more than scattered snow showers
with isolated accums of a dusting (< 1/2").

The main attention then turns to our northwest beginning late
this evening, where another clipper system will track thru, along
the leading edge of a polar vortex that will be dropping south
towards our region. This band of snow has already taken shape
across the upper Midwest/Mid-MO river valley, and should
strengthen some tonight as additional upper level support works
in. We are expecting a start time of late this evening for
northeast-central MO and west-central IL, early overnight (1-3am)
for STL metro and late tonight (3-6am) for southern IL and
southeast MO. Once the snow moves in, it should last for about 4-6
hours, with probably the middle 3 hour core or so where
accumulating snow may occur. This will especially intersect the
early part of the morning rush for cities near the I-70 corridor.
Snow totals should be a bit higher north of I-70 where the best
upper support will be, where 1-2" is expected, with 1" or less
south of I-70. Given the expected and potential impacts due to
timing of this event, will hoist a Winter Wx Advisory.

Otherwise, temps tonight will mainly hold near steady in the mid
to upper 20s thanks to clouds and southerly winds, but then crash
into the teens as the cold front passes thru late tonight and
Monday morning. Temps will probably then stabilize for midday and
early afternoon before dropping again heading into Monday night.
Wind chills will drop below zero behind the front but not low
enough for need of an Advisory (-15) until after nightfall Monday.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 252 PM CST Sun Jan 14 2018

A bitterly cold Arctic air mass will spread across MO/IL on
Monday night in the wake of the aforementioned strong cold front.
The combination of very cold air temperatures and moderate
northwest winds will produce wind chill values of -10 to -20
across the area from early Monday evening through late Tuesday
morning. A Wind Chill Advisory has been issued where dangerously
cold wind chill values of near -15 or colder are expected. If the
air temperatures are a few degrees colder than currently forecast
or if the pressure gradient is a bit tighter (leading to higher
wind speeds), then a Wind Chill Warning may be needed across parts
of northeast MO and west central IL for wind chill values of -25
or colder.

The cold air will remain in place on Tue/Tue night as a 1040-1050
hPa surface high slides into the central CONUS. The current
forecast lows on Tuesday may be too warm depending on the location
of the surface high pressure center and the amount of cloud cover.

Winds will begin turning southwesterly late in the day on Wed
once the surface high shifts southward, but the first appreciable
warmup will likely not occur until after the surface high has
settled farther to the south and winds have been persistently
southwesterly for several hours. The first noticeably warmer day
will probably be on Thursday. The warming trend will likely
continue for Fri and Sat, but there is some question about how
warm it will be.

There appears to be little opportunity for widespread precipitation
until at least the end of the week. Models continue to show poor
continuity with and significant disagreement on the upper air
pattern after 00z Thu, therefore there is low confidence in the
temperature and precipitation forecast for the end of the week and
early next weekend.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1148 AM CST Sun Jan 14 2018

Area of lower VSBYs, occasionally into IFR range, along with MVFR
CIGs will affect the TAF sites for much of this afternoon before
this exits to the east or northeast around sundown. A brief break
in the pcpn then expected this evening, with CIGs rising some
before a more steady period of snow expected overnight and parts
of Monday morning ahead of a cold front. VSBYs should settle into
IFR category for much of the time with low-end MVFR CIGs, probably
for much of a 6-hour stretch once it starts. The cold front
passage will be followed by a shift from the NW and gusty winds
and will continue the rest of the day with CIGs probably hanging
in MVFR category, possibly improving to low-end VFR late.



MO...Winter Weather Advisory from 9 PM this evening to noon CST
     Monday for Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford
     MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Knox MO-
     Lewis MO-Lincoln MO-Madison MO-Marion MO-Moniteau MO-Monroe
     MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds MO-
     Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Shelby MO-Warren MO-
     Washington MO.

     Wind Chill Advisory from 6 PM Monday to noon CST Tuesday for
     Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis
     MO-Lincoln MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint
     Charles MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Shelby MO-Warren MO-Washington

IL...Winter Weather Advisory from 9 PM this evening to noon CST
     Monday for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Marion IL-Monroe IL-Montgomery IL-Pike IL-Randolph IL-Saint
     Clair IL-Washington IL.

     Wind Chill Advisory from 6 PM Monday to noon CST Tuesday for
     Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe
     IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-
     Washington IL.



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