Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 281018

418 AM CST Fri Nov 28 2014

.SHORT TERM:  (Today through Saturday)
Issued at 332 AM CST Fri Nov 28 2014

Cold high pressure over the Central U.S. will drift east today and
return flow will bring warmer air to the Mid Mississippi Valley.
Expect temperatures to rise to near seasonal normals in the upper
40s to low 50s.  With southerly flow we should see increasing low
level moisture.  The last several model runs have been pretty
consistent in bringing high RH into the area at or below 925mb.  Now
the SREF is in on it showing high probabilities of low ceilings
across the area for much of the day Saturday.  Have therefore
increased cloud cover and lowered temperatures slightly for
Saturday.  Models continue to spit out light QPF in this strong
moisture return pattern; have kept forecast dry though since this is
a known issue with the models in these situations.  BUFKIT soundings
indicate weak subsidence in the moist layer, so I feel the chance
for precip is very low.


.LONG TERM:  (Tonight through Thursday)
Issued at 417 AM CST Fri Nov 28 2014

The shortwave currently over British Columbia will deepen and slide
into the northern Plains Saturday night.  This will allow a very
cold but shallow airmass to dip into the Upper Midwest.   Guidance
has been pretty consistent with the speed of the resulting cold
front, which should stay north of our area Saturday night.  Should
continue to be cloudy/mostly cloudy with a decent southwest wind
overnight.  This would tend to keep temperatures up overnight so
tried to stick closer to warmer guidance.

Cold front will dip across the area on Sunday driven by a
1035-1040mb Canadian high.  Typically, the models are too slow with
the infiltration of these kinds of very cold airmasses.  To try to
compensate for this, have leaned heavily on 2m temperatures from the
NAM which seem to be about the coldest and most aggressive with the
cold advection.  With that in mind, front should be through Quincy
by 18Z, and through the STL Metro before 00Z.  While temps ahead of
the front should remain mild, but behind the front temperatures
should drop rapidly into the 30s and 40s.  Meanwhile, expecting
frontogenesis and low level moisture convergence to produce light
rain along and south of the I-70 corridor.  BUFKIT soundings from
all guidance members are pretty consistent in showing this cold
airmass won`t be deep enough to produce snow, but rather freezing
rain initially late Sunday night, then possibly mixed with sleet on
Monday before ending.  Not confident yet in a mix, so have let
surface temperatures determine precipitation type in the forecast
and just kept a "rain changing to freezing rain" forecast for this
forecast cycle.  Patchy light freezing rain south of I-44 should end
Monday by late afternoon or early evening.

Medium range guidance seems to be in better agreement for Tuesday
through Thursday this morning.  Unfortunately they`re in better
agreement that the flow aloft will be quasi-zonal, which is not good
for accuracy in the medium range.  The general pattern looks like we
should see slowly moderating temperatures after the big chill on
Monday and Monday night as the strong high pressure system moves off
to our east-northeast and the low level flow again turns to the
south.  The ECMWF still brings a second cold front through the area
on Wednesday which would keep us cooler than the GFS, but since the
two models agree to disagree on this point, I stuck close to
ensemble MOS guidance temperatures through the period.



.AVIATION:  (For the 06z TAFs through 06z Friday Night)
Issued at 1145 PM CST Thu Nov 27 2014

VFR conditions and dry weather will continue thru the valid TAF
period. A ridge of high pressure has now moved to our east and has
initiated a light easterly flow. This will then strengthen and
veer south after daybreak on Friday...and then slacken off a bit
while remaining from the south Friday night. LLWS conditions
should be briefly met over in COU and UIN early Friday morning as
a 40-45kt low level jet moves overhead and then mix out by late
morning as gusts. May see LLWS conditions again later Friday night
for all TAF sites but will deal with that on later issuances.

Specifics for KSTL: VFR and dry weather thru much of the period.
VFR CIGs will exit late tonight as winds veer from the south and
become gusty after daybreak on Friday. A moderate south wind will
continue into Friday night. LLWS conditions of a marginal nature
could threaten Friday night and will merit some watching for
future issuances. Along with the low level jet developing for
Friday night, model data continues to strongly suggest non-VFR
stratus developing late in the night. Have introduced CIGs at the
1500ft level for now, with data supportive to a degree of IFR



Saint Louis     49  40  58  53 /   0   0   5   5
Quincy          45  37  52  47 /   0   0   5   5
Columbia        53  42  61  51 /   0   0   5   5
Jefferson City  56  43  62  52 /   0   0   5   5
Salem           44  37  55  54 /   0   0   5  10
Farmington      48  40  57  54 /   0   0   5  10




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