Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220950
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
350 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 QPF 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 TROF 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 NW that quickly dropped into the bottom of the TROF
and used it to close off a new LOW near the AR/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 NW 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 IL and
persisting until evening.  The western edge looks to be over Metro
STL East.  The elongated upper TROF 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 IL 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 TROF becomes re-established over the central CONUS 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 06z TAFs through 06z Monday Night)
Issued at 1103 PM CST Sun Dec 21 2014

Focus continues to system to impact the region on Mon. Latest
guidance, while not having a great handle on current conditions,
continues to suggest prev TAFs are on track. Believe conditions
will deteriorate shortly after sunrise Mon. Cigs shud lower to low
end MVFR as well as visbys reduced into MVFR range due to either
RA, BR or both. Latest indications suggest RA may have a more SHRA
component, but during dry periods, while brief, visbys shud still
be reduced due to BR. Have low confidence on visbys on Mon and may
drop to IFR at times. Confidence in IFR at SUS/CPS remains low and
will keep trends going for now. Winds will veer to eventually swly
Mon evening with visbys and cigs improving with this wind shift.

Specifics for KSTL: VFR and dry expected thru the overnight hours
with conditions deteriorating shortly after sunrise. Confidence in
IFR conditions is still too low and will therefore keep sct cloud
deck mentioned for now. Conditions will improve Mon late
afternoon or evening as winds become swly.

Tilly

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX



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