Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 122133

Area Forecast Discussion
National Weather Service Saint Louis MO
333 PM CST Fri Jan 12 2018

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 333 PM CST Fri Jan 12 2018

A few sites have reported occasional flurries this afternoon beneath
a persistent stratocumulus deck. The back edge of this deck was
located slightly west of the Mississippi River and clouds extended
eastward through Illinois. Scattered flurries may continue beneath
this cloud deck through the late afternoon but should diminish with
time as the deck simultaneously erodes and shifts eastward.

The western and northwestern edges of the aforementioned cloud deck
were becoming noticeably more cumuliform on satellite imagery and
should begin to dissipate late this afternoon. However, the presence
of this cloud deck for most of the daylight hours means that today`s
high temperatures did not rise as high as previously expected,
especially across the eastern half of the CWA. This means that there
will be a cold start to the nocturnal cooling period, and overnight
temperatures are likely to be colder than previously anticipated.
Overnight lows will probably be at least similar to and more likely
colder than last night. The going temperature forecast may be a
couple of degrees too warm.

The tight pressure gradient across the region will begin to relax
overnight, leading to decreasing wind speeds and gusts late tonight.
That said, given the cold air temperatures, even modest wind speeds
will still contribute to wind chill values of 0 to -10 degrees
during the overnight hours. If the wind speeds are stronger than
expected or if the air temperatures are substantially colder than
forecast, then a Wind Chill Advisory might be needed tonight for
parts of northeast MO and/or west central IL.

A northerly component to the wind direction will likely persist
through tomorrow due to a 1040+ hPa surface high pressure center
building into the upper Midwest. To some extent, highs on Saturday
will depend on how cold it gets tonight and on what the wind speeds
are tomorrow. Conditional climatology for January suggests that a
diurnal rise of around 10-15 degrees would be reasonable for a mix
of light to moderate northerly to northwesterly winds under clear
skies. The going forecast for Saturday may still be a few degrees
too warm.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 333 PM CST Fri Jan 12 2018

(Saturday Night - Sunday)

A weak sheared upper level disturbance/clipper will pass thru on
Saturday night but should result in no sensible wx due to lift being
either too weak or not over any one area for too long.  Models
continue to indicate a stronger clipper on track for daytime Sunday.
 The track of this has shifted more to the west during the past 24
hours to where the accumulating snow potential barely edges our
central MO counties now.  The system itself is still over the
northeastern Pacific and will not reach land until the 12z raobs on
Saturday so this may still shift.  Stay tuned.  For now, though,
potential snowfall from this clipper still looks up to 2" in its
narrow core track (to our west), but less than an inch for our
central MO counties.

Otherwise, the latest intrusion of a cold Canadian high pressure
system will be overhead on Saturday night, resulting in min temps
easily in the single digits and possibly dipping below zero in a few
spots.  Timing will be tricky due to clouds rolling in late and a
southerly flow developing.  On Sunday, look for a warmer day, but
not too much, with plenty of clouds ahead of the next clipper system
rolling in.  Max temps only in the 20s are expected.

(Sunday Night - Wednesday)

Models remain in good agreement on development of a deep upper TROF
swinging thru early next week, but are not quite as confident on
upper LO development, however.  The bottom line effects for our
region remain the same:  another shot of bitter cold air for this
timeframe thanks to a 1050-55mb high pressure system dropping down
that will rival the cold air we had about a week ago, enough of a
combo of cold and wind to where there will at least be a period of
dangerously low wind chills (15 below or colder), with this to be
preceded immediately by what should be a broader swath of snow that
will move thru due to the low level front and approach of the deep
upper TROF.

Most of the forecast area should receive at least some snow
accumulation from the Sunday night-Monday snow event with potential
up to 2" possible.  Tuesday could still see some snow showers but
poor agreement on handling for this has kept it out of the forecast
for now.

Wind chill headlines continue to look like a likely proposition for
Monday night and Tuesday morning for much of the forecast area (a
bigger threat area than 24hrs ago) and may need to be revisited
again for some sections of northern MO and central IL for Tuesday
night and Wednesday morning.

(Thursday - Next Friday)

With the upper TROF exiting and SW flow at the surface for a couple
days, temps should make a decent recovery back to average by the end
of the work week.  Huge differences on how to handle upper level
systems during this time but lower than normal probs for something
directly affecting our region so kept a dry forecast for now.



.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1127 AM CST Fri Jan 12 2018

The back edge of an MVFR cloud deck will gradually shift eastward
during the first 0-6 hours of the valid TAF period, leading to an
improvement to VFR conditions at KSTL/KSUS/KCPS and KUIN by 00z.
Winds will gradually decrease through the period as the pressure
gradient relaxes.





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