Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 130024
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
624 PM CST Fri Jan 12 2018

.UPDATE...
Issued at 620 PM CST Fri Jan 12 2018

Made a quick update for this evening for clouds and flurries. Flow
in the cloud-bearing layer across far eastern MO into IL has
become more northerly and this has halted any eastward clearing of
the stratus. Present indications are the western edge of the stratus
will continue to skirt far eastern MO while blanketing southwest
IL. Clearing is more likely from north to south, with the back
edge in west central IL at this time. On and off flurries will
persist until the stratus departs.

Glass

&&

.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.

Kanofsky

.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.

TES

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 534 PM CST Fri Jan 12 2018

MVFR stratus blankets much of western IL and the western edge is
near KUIN then runs south to between KSUS and KSTL. This makes
for a problematic cloud forecast and I have increased clouds into
mid evening or so, however they may persist a bit longer at KSTL
and KCPS and the overcast cigs may come back into KSUS. Occasional
flurries will also accompany the stratus. Once the stratus clears
out later this evening, I think VFR flight conditions will prevail
with varying amounts of mid-high clouds moving into the region
from the Plains. Gusty northerly winds will continue into the
first part of the evening, gradually diminishing late evening and
into overnight.

SPECIFICS FOR KSTL:

MVFR stratus blankets much of western IL and the western edge is
near KUIN then runs south to between KSUS and KSTL. This makes
for a problematic cloud forecast and I have increased clouds into
mid evening or so, however they may persist a bit longer at KSTL.
Occasional flurries will also accompany the stratus. Once the
stratus clears out later this evening, I think VFR flight conditions
will prevail with varying amounts of mid-high clouds moving into
the region from the Plains. Gusty northerly winds will continue
into the evening, gradually diminishing late evening and into
overnight.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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