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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
559 am CST sun Dec 28 2014

Short term: (through late this afternoon)
issued at 326 am CST sun Dec 28 2014

A quiet day weather wise with seasonal temps, though could be bit of
a tricky temp fcst with clouds potentially hindering temps across
the sern County Warning Area. High pressure will build into the area today.
Persistent stratus deck has been slow to move east early this mrng.
Current relative humidity progs show the slow east/southeast progression will continue thru
this mrng...potentially not clearing the stl Metro area until closer
to 18z and early/mid aftn for cntrl/SW Illinois. Meanwhile...a thick
mid/high cloud deck continues to stream NE out of the base of the
upper trof across the sthrn plains. These clouds are fcst to affect
locations along and southeast of a cou to uin line this mrng shifting
quickly to the southeast thru the aftn as the upper trof axis weakens as it
passes the MS rvr this evng. Due to the potential for two thick
cloud decks to affect areas S and east of the stl Metro area today in
combination with continued weak cold air advection thru the aftn may mean temps
will not rise much more than a few degrees from mrng lows. Attempted
to account for the impact of the thicker cloud cover but may still
be too warm. Sfc ridge axis approaches by late aftn allowing winds
to become lght/vrb.

2%

Long term: (tonight through saturday)
issued at 326 am CST sun Dec 28 2014

(tonight through wednesday)

00z nwp output continues to support going forecast trends that
depict Arctic air surging across the forecast area in the Monday
night-Wednesday time frame. Temp trends with the arrival of the
Arctic airmass are the primary forecast concern.

Fairly typical late December air mass should mean seasonal late December
temps tonight and into Monday, but the arrival of the cold air will
produce a big drop in temperatures from late Monday night, through
Tuesday, and into Tuesday night. 00z runs are in very good agreement
with earlier solutions in the the timing and intensity of the cold
air, so only very minor adjustments needed to going temp forecasts.

As mentioned yesterday, given the intensity of the cold air
temperatures on Tuesday will likely struggle to warm much from their
early morning lows. For most parts of the forecast area forecast will depict
no more than a 5 degree climb, and it may be that temps will be
nearly steady if clouds are thick enough.

This coldest air of the winter will drop Tuesday night's lows into
the single digits and teens, and limit wednesday's highs to the
20s.

Precipitation during the period looks to be fairly light, and should
primarily be in the form of very light snow or flurries behind the
Arctic front Monday night into Tuesday morning.

(Thursday through saturday)

Medium range solutions are still cutting off upper low over the West
Coast by midweek, and they are coming into better agreement with the
movement of this system has we head into the end of the upcoming
week. By Friday evening, medium range consensus is that upper low
will be over the Texas Panhandle, with the upper low then opening up
and lifting into MO by Saturday morning. Warm advection developing
well in advance of the upper system, and in the wake of the
retreating Arctic airmass, should be enough to bring a precipitation
threat to at least the southern half of the forecast area as early as Thursday
night, with this precipitation threat expanding on Friday and Friday
night as warm air advection intensifies and large scale lift ramps up with the
approach of the upper system.

Overall, the 00z solutions are offering a "warmer" forecast for our
cwa, with the 850mb low forecast to track from Kansas into north Illinois on
Friday night. If this does verify, the threat of heavier snowfall
will be to the northwest of the 850mb low track and generally north of our
cwa, with the bulk of the wintry precipitation in our forecast area tied to the
warm air advection-induced precip into the retreating Arctic air Thursday night and
Friday. I have added a bit more to the wintry mix over southern
areas of the County Warning Area for Thursday night and Friday morning with the
addition of sleet due to the pronounced elevated warm layer over this
part of the forecast area in all medium range solutions. Have maintained a
rain/snow mix elsewhere for now as thermal profiles elsewhere are
less clear-cut.

In spite of the similarity of the 00z solutions, I very much
anticipate additional and different iterations of the evolution of
this system.

Truett

&&

Aviation: (for the 12z tafs through 12z Monday morning)
issued at 521 am CST sun Dec 28 2014

Pesky lingering MVFR cigs continue to impact the terminals early
this mrng. The stratus has been slow to clear overnight and this
trend is expected to continue today. The edge of the lower clouds
is very near kcou and should be clearing around the time the fcst
becomes valid. Lower cigs should clear KUIN in an hour or so but
back edge of the MVFR deck is not visible due to higher clouds
streaming overhead. Backed off the clearing time for the stl Metro
terminals and may still be too optimistic with some guidance
indicating it may be closer 18z before the lower cigs scatter out.
After the MVFR cigs move ESE, there will still be mid/high clouds
overhead. These should quickly exit to the southeast thru the aftn. Sfc
high builds in thru the day and passes overhead tonight allowing
winds to become lght/vrb.

Specifics for kstl:

MVFR cigs will likely hang around most of the mrng but timing is a
wag due to higher cloud cover obscuring the lower deck. Mid/high
clouds should slide southeast drng the aftn. Sfc ridge moves overhead
tonight allowing winds to become lght/vrb and then Erly as the
ridge axis moves east tomorrow.

2%

&&

Lsx watches/warnings/advisories:
MO...none.
Illinois...none.
&&

$$

Weather forecast office lsx

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