Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 140548

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1148 PM CST Sat Jan 13 2018

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 226 PM CST Sat Jan 13 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. There were several pieces of energy, or
disturbances, of interest in this fast flow all of which are
currently in western or northern Canada. These will all help to
shape our weather over the next 24 to 36 hours and beyond. At the
surface, the latest bitter cold air intrusion is represented by a
large area of high pressure, centered in Iowa, with northerly
winds for our region. A stubborn area of clouds continues to
persist for much of Missouri, with mostly clear skies in Illinois.
Temperatures across our region ranged from the upper teens to the
lower 20s.

The large field of cloudiness over Missouri has developed holes
from time to time but they fill in with time, and with the
diminishing power of the sun currently, it is anticipated that
this cloud field will mostly survive and maintain a mostly cloudy
to cloudy sky for much of our region tonight. This will, in turn,
limit min temps for most areas, especially in Missouri, where a
forecast above the warmest MOS was preferred. Conversely, a
snowpack field still exists per satellite imagery edging portions
of eastern Marion County Illinois as well as a small portion of
Fayette County and these areas should have a longer period of time
of mostly clear skies and light winds from the presence of the
surface RIDGE, where temps near the lower end of MOS are
preferred, or even lower in the small area where the snowpack
intersects with our CWA. Temps should then become near steady or
slowly rise from west to east with time tonight as the clouds
advect eastward and winds become southerly.

The main energy with the clipper system on early Sunday continues
to track well west of our region to where it will have no impact.
Unfortunately, it is never that simple, with additional weaker
pieces of energy expected to follow behind late Sunday morning and
afternoon along a more easterly track, that should result in
isolated to scattered snow showers during this time. Accumulations
from these will be spotty, but a dusting of a half inch or less
looks reasonable. Max temps in the 20s are expected, leaning
towards the colder guidance with thick clouds expected.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 226 PM CST Sat Jan 13 2018

A disturbance which was located over northwestern Canada this
afternoon will slide southeastward on Sunday, eventually inducing
surface cyclogenesis near the MB/ON border. The entire system will
continue sliding southeastward into the Great Lakes on Sunday
night and begin to occlude on Monday night. This feature will
bring light snow to the LSX CWA from late Sunday night through
Monday morning. Around an inch of snow accumulations is expected
areawide, although some areas could see locally higher amounts
closer to 2 inches. In addition to a period of light snow, a
strong cold front associated with this low pressure system will
bring yet another bitterly cold air mass into MO/IL. Lows on
Monday night will likely fall into the single digits above and
below zero. Wind chill values are expected to reach -10 to -20
across the area, and a Wind Chill Advisory will eventually be
needed for Monday night into Tuesday morning if these conditions
still appear likely over the next few forecast cycles. The
tightening pressure gradient invof the front will also produce
gusty northwest winds, especially on Monday afternoon and

A 1040-1050 hPa surface high quickly builds into the central CONUS
in the wake of Monday`s low pressure system, keeping the cold air
mass in place for Tuesday and Tuesday night. The current forecast
lows on Tuesday may be too warm depending on the location of the
surface high pressure center.

Winds will become southwesterly after the surface high settles
south of the area. This will contribute to warming temperatures
during the middle and end of next week.

There appears to be little opportunity for widespread
precipitation until at least the end of next week when models
depict one or more weak PV anomalies moving across the central
CONUS. However, there is low confidence regarding precipitation
chances at this time due to poor model agreement regarding the
location, strength, and existence of these features.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1133 PM CST Sat Jan 13 2018

VFR flight conditions are expected overnight as waves of clouds
from 5000-8000 ft spread northwest to southeast across the area.
An upper level disturbance is still on track to traverse the
region on Sunday. The latest guidance suggests the threat of snow
may be a bit higher than previously expected and have added light
snow and MVFR flight conditions to all of the terminals, starting
first at KCOU around mid-morning and in the afternoon elsewhere.
I don`t think it will be snowing continuously as there will likely
be scattered areas of light snow impacting the terminals. The
visibility could certainly be lower at times. Another round of
more widespread light snow will impact the region late Sunday
night into Monday morning, after the valid time of most of the
TAFS. Winds will be light overnight, veering and becoming
southerly on Sunday.


VFR flight conditions expected tonight into Sunday morning with
waves of clouds from 5000-6000 feet. Scattered areas of light
snow could impact the terminal late on Sunday afternoon accompanied
by MVFR flight conditions. I don`t think it will be snowing
continuously due to the scattered nature, however the visibility
could certainly be lower at times. Another round of more widespread
light snow will impact the terminal late Sunday night into Monday
morning, near the end of the valid time for KSTL. Winds will be
light overnight, veering and becoming southerly on Sunday.





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