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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
603 am CST Mon Dec 22 2014

Short term: (through late this afternoon)
issued at 350 am CST Mon Dec 22 2014

Full latitude trof over the Great Plains will continue to deepen
today. All guidance is pointing toward widespread light rain as the
low level jet ramps up ahead of the trof. Models show moderate
moisture convergence at 850mb and widespread quantitative precipitation forecast across the area.
Highest rainfall amounts will likely be along and north of the I-70
corridor with amounts up to 1/2 inch. With a warmer start to the
day and south-southeast flow, should see temperatures a little
warmer than yesterday, tho widespread rain will keep highs in the
mid 40s to around 50.

Carney

Long term: (tonight through sunday)
issued at 350 am CST Mon Dec 22 2014

(tonight - Tuesday evening)

An upper level trough will already be established over the plains by
this time with a strong impulse rotating around its base that should
move thru a good chunk of the forecast area during the evening
hours. This should initially result in widespread rain for most
areas, especially areas immediately downstream from this impulse,
such as areas mainly to the north of I-70. MOS guidance low-balling
this too much and went well above MOS for pops, with most areas
forecast with likely to categorical pops.

In the wake of the impulse this evening, a dry slot is then expected
to take hold and persist from late tonight thru Tuesday evening.
Preferred small diurnal ranges with this setup, which favors the
warmer MOS temps for tonight and cooler MOS temps for Tuesday.

Pcpn-types thru this period are expected to be rain.

(Late Tuesday night - Christmas eve)

For the first time in several days, a good model consensus has
developed on the final phase for this system. Previously the GFS
was the outlier, being the sole model that took a strong disturbance
from the Pacific northwest that quickly dropped into the bottom of the trough
and used it to close off a new low near the Arkansas/OK border late
Tuesday night and then track it northeastward into Indiana Wednesday
evening. Well now the ec and Gem have come onboard with this
solution, giving greater credence to this--abandoning their previous
advocation of an open wave.

The key disturbance from the Pacific northwest was coming onshore when the
00z models were initialized and so a convergence of solutions was
anticipated.

Even if this has a stronger possibility of happening, though, there
are still issues. But at least it looks to solve the pop problem--
with what should be a deformation pcpn axis of developing by
daybreak Wednesday primarily over southern and eastern Illinois and
persisting until evening. The western edge looks to be over Metro
stl east. The elongated upper trough axis that does result, however,
will probably still spread light pcpn over areas further to the west
though, with the enhanced amounts in the deformation zone in the
east.

The issues still remaining are a pcpn-type one, with what still
looks to be sufficient cold air aloft to then defer all pcpn-type
issues to the lowest 2kft, especially how cold surface temps manage
to get. The problem is that there is very little cold air to tap
other than what gets manufactured by the system itself. MOS will
typically handle temps with these systems not very well, and so
undercut values by at least a category, as we should rise very
little during the day, especially under the anticipated deformation
zone. This should give US surface temps in the mid 30s mostly, with
some upper 30s. The atmospheric setup, despite temps above
freezing, should support mostly snow. But warm ground and surfaces
with these above freezing temps should severely limit any snow
accums, if any occur at all.

So...it looks like a dusting of snow on grassy areas is what is
expected to occur for most locations, with the best potential for
grassy area accums being under the def zone in parts of
southern-eastern Illinois where up to an inch is possible. As it stands
now, this should not affect travel much.

If a colder solution than this presents itself, snow accums will
have to be substantially increased.

(Christmas)

Models continue to advertise a transitional pattern this day with
quiet wx and temps returning to above normal levels.

(Friday - sunday)

The upper trough becomes re-established over the central Continental U.S. With
weak disturbances and a cold front moving thru early in the period.
Have small pops to cover for now, with a return then to below normal
temps for next weekend in the wake of the front.

Tes

&&

Aviation: (for the 12z tafs through 12z Tuesday morning)
issued at 538 am CST Mon Dec 22 2014

Deepening trof of low pressure over the Great Plains will bring
rain and lowering ceilings and visibilities to the area. A wide
area of rain stretching from central through eastern Missouri into
southwest Illinois has developed, and MVFR flight conditions are
already being reported across most of eastern Missouri. Expect
ceilings to drop below 2000ft by mid morning and into IFR across
most if not all of the area by late morning or early afternoon.
IFR flight conditions with intermittent light rain will continue
through the day and into early evening. Expect rain to taper off
from south to north and ceilings should improve marginally as the
rain ends. Some guidance brings flight conditions up to VFR,
however I feel this is unlikely at this time given the time of
year and amount of low level moisture which will remain in place.

Specifics for kstl:

Expect lowering ceilings and visibilities this morning as rain
affects the terminal. Feel pretty confident that the ceiling will
drop below 1000ft this morning, but am not as confident in the 16z
timing in the current taf. This is based on a consensus of
guidance products, and the time could easily slide one way or
another. Once ceilings drop to IFR, expect the low ceilings to
prevail until evening and then improve marginally when the rain
ends. Some guidance brings flight conditions up to VFR overnight, however
I feel this is unlikely at this time given the time of year and
amount of low level moisture which will remain in place.

Carney

&&

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

$$

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