Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 141020

Area Forecast Discussion
National Weather Service Saint Louis MO
420 AM CST Sun Jan 14 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 420 AM CST Sun Jan 14 2018

Light snow was spreading southeastward through western MO early this
morning.  This activity was associated with weak low-mid level warm
air advection ahead of an approaching shortwave over southeast
Nebraska.  Much of this precipitation should weaken or dissipate as
it shifts eastward into our forecast area with drier low level
conditions on the western periphery of the surface ridge moving east
of the region.  It still appears that there may be up to one half
inch of accumulation in parts of central MO this morning with lesser
amounts further east this afternoon of a dusting or less across
northeast MO into west central IL and possibly as far east as the
western portion of the St Louis metro area.  The light accumulation
should be north and west of STL today, with only a few flurries
elsewhere. Despite the cloud cover limiting solar insolation today,
low level warm air advection and southerly surface winds will lead to
slightly warmer high temperatures this afternoon, albeit still below
normal for mid January.  A deepening upper level trough over the
northern Plains will send a cold front southeastward into our
forecast area late tonight.  A band of snow is expected along and
just behind this front which will impact northeast MO and west
central IL this evening.  This band of snow will shift southward
into central MO late tonight, including the St Louis metro area.
Accumulations tonight along and north of I-70 should be around one
half inch. Precipitation type should be all snow, but there is still
some uncertainty as to the timing of the band of snow and the exact
amounts, but it should be a relatively quick shot of mainly light
snow.  Colder air will advect into northeast MO late tonight behind
the strong cold front with strong and gusty northwesterly winds.


.LONG TERM...  (Monday through Saturday)
Issued at 420 AM CST Sun Jan 14 2018

Models continue to be in good agreement overall thru Wed. Period
begins with NW flow aloft as an upper low develops over the Great
Lakes region. Mdls still struggle with the upper low impacting the
region on Thurs into Fri. Fri night and beyond, mdls place the
region generally under a SW flow with a strong warm-up ahead of the
next approaching system. Timing differences with the approaching
trof ejecting into the Plains on Sat exist, tho the ECMWF is a slow
outlier attm.

Focus remains on the snow potential Mon, turning the dangerously
cold temps and wind chill values thru Tues night. Mdls have sped up
timing of the front Mon morning. There are some differences among
guidance, but it is possible that the accumulating snow may come to
an end by 12z Mon north of a KVIH to KSTL to K3LF line. Have kept
higher PoPs further north and west than much of the guidance
currently suggests due to this uncertainty. Mdls have been hinting
at a potential minimum in snow accumulation, approximately thru the
middle of the CWA and perhaps further west. The main upper trof
helps support lift across the ern portions of the CWA. Frontogenesis
shud help support some lift across the wrn half of the CWA, but this
forcing is focused further west and only partially builds into the
area. However, during the afternoon hours, this forcing builds
across srn portions of the CWA, tho there is uncertainty in this as
well. Bottom line still remains the same at this point. Still
expecting widespread accumulations of around one inch with some
areas seeing up to 2 inches possible.

Mdls continue to depict a very strong sfc ridge building into the
region behind the front mentioned above. With fresh snow expected to
be on the ground, have continued trending twd the cooler guidance.
However, questions in cloud cover Mon night leads to uncertainty in
how cold temps may drop. With the dry slot thru the area, the going
forecast may still be too warm. This uncertainty continues into Tues
night as the sfc ridge builds deeper into the area. With stronger
winds behind the front Mon night, still expect wind chill values of -
15 to -25F across most of the area. With lighter winds Tues night,
wind chill values shud be warmer, tho may still hover around -15F
across the nrn third of the CWA.

With winds becoming swly possibly as early as Wed, temps shud begin
to warm and continue to do so thru the end of the period. Have kept
the remainder of the forecast period dry with the uncertainty in the
upper low on Thurs into Fri mentioned above. Some guidance generates
QPF across the area Fri night into Sat ahead of the approaching
system. Believe this is due to the strong moisture advection into
the area, but with the moist ground and warm airmass building into
the region, can not rule out drizzle. Regardless, will keep dry PoPs
for this time for now.



.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.



Saint Louis     29  22  24   2 /  30  60  60   0
Quincy          25  13  16  -3 /  40  60  10   5
Columbia        28  20  20  -1 /  50  60  20   0
Jefferson City  29  22  23   1 /  50  60  30   0
Salem           25  23  28   1 /  20  70  60   5
Farmington      25  22  29   3 /  20  20  60  10




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