Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 310941
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
341 AM CST Sat Jan 31 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 327 AM CST Sat Jan 31 2015

Mid and high clouds to remain over region ahead of main weather
system. In the meantime, low level moisture and warm air advection
are on the increase on nose of low level jet. So rain will continue
to develop and spread eastward through much of our forecast area
today, especially north and west of St. Louis metro area. It will
mainly be in the form of rain but could see a brief period of snow
mixed in across central and northeast Missouri this morning. As for
high temperatures, they will be in the upper 30s to mid 40s.

Byrd

.LONG TERM:  (Tonight through Friday)
Issued at 327 AM CST Sat Jan 31 2015

(Tonight-Monday)

Obivously, main concern is ptypes and potential snowfall amounts
with storm system pushing into the region.  00z runs have continued
warming trend of low level temp profile in advance of the storm, and
given that this has been the trend for 4 straight model cycles
forecast has been adjusted "warmer" as well.  This adjustment will
manifest itself in a slower transition to all snow, and reduced
snowfall amounts over the central third of the CWA...along the I-70
corridor.

Certainly, best area for accumulations in our CWA will be our far
northern counties, and the key to the forecast is how soon the
precip transitions from rain to snow.  NAM forecast sounding for UIN
indicates some low level cooling that would cause the switch to
occur as early as 03z, while GFS is warmer/slower with cooling.
It`s also interesting, and perhaps a bit worrisome from a heavy
snowfall viewpoint, that both the NAM and GFS are nearly isothermal
at 0C at 12z up to 900mb, and a slight wiggle either warmer or
colder will have a dramatic impact on when the changeover occurs and
ultimately how much snowfall.

Based on this uncertainty have tried to be a bit conservative with
snowfall forecasts across the north, but working the numbers in
various ways still yields snowfall totals of around or just above 6
inches along and north of an Edina-Mount Sterling line, with
snowfall totals dropping off quickly as you head south...to no more
than an inch along a Mexico-Vandalia IL line.  Based on these
forecast amounts, will go with a Winter Storm Warning for our far
northern counties for tonight into Sunday evening, with roughly a 2
county "buffer" of a winter weather advisory for lesser snowfalls of
2-4 inches for the same time period.   Along and south of
I-70...snowfall totals should be under an inch.  These snowfall
totals include any snowfall from tonight, through Sunday, and into
early Sunday evening.

This is our best estimate of expected snowfall with this storm, and
its important to point out that all indications are that there will
be an extremely tight snowfall gradient from extreme southeast
Iowa into northern MO and west central IL, with snowfall totals
varying greatly over a very short distance. We`ve attempted to
reflect this gradient in the forecast, but in all likelihood the
distance between several inches of snow and just a dusting will be
much tighter than indicated. Any slight change in the expected
gradient will have a very large impact on snowfall totals at any
given location in the warning and advisory areas.

Winds are forecast to increase fairly dramatically in the wake of
the surface low sunday afternoon and evening over northern parts
of the CWA. Given that much of the snowfall will be quite wet
don`t believe that blowing snow will be much of an issue, but will
be watching out for this threat as some of the accumulating snow
towards the end of the event becomes much drier.

The light snow that lingers into Sunday evening should end no later
than midnight, followed by a cold Canadian high that will drop lows
into the single digits and teens by Monday morning.   00z guidance
supports going forecast trends that temps will remain below normal
on Monday as the high pushes into the mid-Mississippi Valley, and
this will be followed by a fairly strong...but brief...warmup on
Tuesday with highs rebounding into the upper 30s and 40s.

(Wednesday-Friday)

Shortwave rotating through base of broad trof over the CONUS will
usher in another shot of cold air for the last half of the work
week.  By Thursday, highs will only be in the 20s to lower 30s, with
only a modest rebound in temps by Friday. Have continued some
slight chance PoPs with passage of shortwave on Wednesday, with
low PoPs continuing over southern sections of the CWA into
Thursday as GFS suggests this area could be on northern fringes
of system working its way from the southern Plains into the southeast
U.S.

Truett

&&

.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1127 PM CST Fri Jan 30 2015

Specifics for KUIN, KCOU: VFR for at least the first 0-6 hours and
possibly longer. Clouds will lower and thicken ahead of an
approaching low pressure system. Regional radar loops and METARS
show that rain is spreading eastward across the region and should
begin at KCOU during the morning and then at KUIN during the
afternoon hours. KUIN is far enough north that the earlier arrival
of colder air at the terminal would mean a longer duration of
snowfall, and several inches of snow could accumulate at KUIN between
Saturday night and Sunday night. At this time, only minor snowfall
accumulations are expected at KCOU, and the transition from rain
to snow is beyond the end of the valid TAF period.

Specifics for KSTL, KSUS, KCPS: VFR for at least the first 0-6 hours
and probably through the first 12 hours. Clouds will lower and
thicken ahead of an approaching low pressure system. A prolonged
period of rain is expected on Saturday. When colder air arrives,
it might cause the rain to mix with or change to snow very late on
Saturday night into Sunday.

Kanofsky

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 9 PM this evening to 9 PM CST
     Sunday FOR Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

     WINTER STORM WARNING from 9 PM this evening to 9 PM CST Sunday
     FOR Knox MO-Lewis MO.

IL...WINTER WEATHER ADVISORY from 9 PM this evening to 9 PM CST
     Sunday FOR Pike IL.

     WINTER STORM WARNING from 9 PM this evening to 9 PM CST Sunday
     FOR Adams IL-Brown IL.

&&

$$

WFO LSX





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