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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
551 am CST Fri Dec 19 2014

Short term: (through late this afternoon)
issued at 300 am CST Fri Dec 19 2014

A weak ridge of high pressure which extends from the Great Lakes
region south through the mid-Mississippi Valley is in control of our
weather today. Soundings are showing plenty of low level moisture
trapped under an inversion at around 850mb. The vertical structure
of the atmosphere is much like what we had last weekend with the
moisture stratified in the low levels, so I don't expect much
sunshine for the majority of the area...with the possible exception
of south central and extreme southwest Illinois. Some drier air is
trying to drift in on weak easterly low level flow. Vertical cross
sections show intermittent weak upward motion through the day,
primarily over central Missouri, and short term guidance spits out a
hundredth of quantitative precipitation forecast here and there. Additionally, the hrrr shows some
weak simulated radar echos over parts of central and southeast
Missouri today, so have added in a little patchy drizzle to the


Long term: (tonight through thursday)
issued at 300 am CST Fri Dec 19 2014

(tonight - saturday)

The brunt of a storm system is expected to pass relatively unnoticed
to our south this evening, and if it weren't for some interaction
with a more northerly component, would probably result in no
precipitation anywhere, but such as it is, some very light snow
looks to be possible early this evening for an area primarily south
of stl Metro. This arrangement should be short-lived, with much of
the mid and upper level moisture quickly being shunted off to the
east, leaving behind extensive low cloud cover. At this point,
there is good model consensus on a weak warm air advection, convergence regime
setting up at the low levels that noses into central MO by midnight
tonight and then snaking its way northward towards the uin area by
Saturday morning. Have added in patchy drizzle as a result. For
now, expecting influence of the surface ridge to trump this for stl
Metro and areas S and east. Temps will probably not go very far
tonight with clouds hanging tough, although where the surface ridge
is dominant, should see a steady but slow drop...but for areas to
the north and west of stl Metro where a light S flow will persist, near
steady temps are expected. Speaking of temps, especially to the north
and west of stl Metro where drizzle is expected to develop, readings
will be within a degree or two of the freezing mark and so this will
probably manifest more as patchy fzdz for areas in northeast MO and
west-central Illinois. Prefer the lower MOS temps for Saturday maxes
given cloud cover expected, but they should still rebound into the
upper 30s and low 40s.

(Sunday - wednesday)

A rather deep upper level trough is expected to develop later on
Sunday over the central Continental U.S. And then take a good part of the week
before finally shifting east. Maintained the highest pops for
Monday thru Tuesday, where there is good model consensus on when the
trough will be close enough, yet its axis still just upstream...and
then reduce to slight chances by Wednesday when the trough axis by
this time will have shifted to the east.

While much of the precipitation will be just rain, especially on the
front-end, enough cold air starts working in on Tuesday, especially
aloft, that will allow the atmosphere to sustain a changeover to
snow. The only limiting factor at this point will be surface and/or
boundary layer temps that will ultimately determine the rain vs.
Snow question. GFS and ec MOS surface temps suggest this will not
happen for most locales until late in the day on Tuesday with snow
then expected as the dominant p-type from that point on. This will
also be when the upper trough axis will be crossing thru, and so what
is left will probably not be much, but perhaps light snow accums
will be possible Tuesday night and early Wednesday. This will be
our only realistic chance at a white Christmas, and something to
watch, as a deeper system that digs a bit more south would up the


Models have been advertising for several days now a transitional
period this day which should give US a net result of quiet wx with
near seasonal temps. Nothing has changed with the current model
runs so will let it stand.



Aviation: (for the 12z tafs through 12z Saturday morning)
issued at 538 am CST Fri Dec 19 2014

Expect persistent ceilings across the area today decreasing from
VFR over Illinois to MVFR in eastern Missouri and down to IFR in
central and parts of southeast Missouri. Patchy drizzle is also
possible across the region today, particularly in central
Missouri. Ceilings will likely drop slowly overnight tonight, with
more patchy drizzle over parts of the area.

Specifics for kstl:

Ceilings will likely stay in MVFR range today. Short-range
guidance is indicating that there will be a period of ceilings
below 2000ft for this morning, and this seems reasonable given
weak low level flow from the northeast and a few obs with ceilings
around 1000ft upstream. Could see a little drizzle in the area
today, but greater chances for drizzle should be over central
Missouri. Expect that MVFR ceilings will continue through tonight.



Preliminary point temperatures/pops:
Saint Louis 35 31 41 34 / 10 10 10 5
Quincy 32 29 38 31 / 10 10 10 10
Columbia 35 33 41 34 / 10 10 10 5
Jefferson City 37 34 42 34 / 10 10 10 5
Salem 35 29 41 31 / 10 10 5 0
Farmington 36 30 43 31 / 10 10 10 0


Lsx watches/warnings/advisories:


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