Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232142
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
342 PM CST Tue Dec 23 2014

.SHORT TERM: (Tonight through Wednesday Night)
Issued at 251 PM CST Tue Dec 23 2014

The main forecast issue remains the impending storm system and
potential for accumulating snowfall. The models have come into
much better agreement with mass fields than they exhibited 24
hours ago. Yet, there are still some large issues with thermal
fields, qpf, and speed of the system that are resulting in lower
than normal confidence and higher than normal uncertainty.

The overall scenario is unchanged. The upper low is in the
development process within the southern portion of the deep
positively tilted trof across OK. The guidance is fairly well
clustered lifting the upper low northeast into southwest MO by
daybreak Wednesday to near St. Louis at 18z and into the lower
Great Lakes region by 00z. In response the surface low currently
located along the front in LA will deepen and move through the TN
and OH Vallies over the next 24 hours. Initially the warm conveyor
belt and mid level frontogenetical forcing will be the key players,
with rain presently across AR increasing in coverage and spreading
northeast into southeast Missouri this evening and northeastward
into far east central MO and southwest IL overnight. Model consensus
indicates the thermal profiles should begin cooling sufficiently
on the western fringe of the rain shield after 09z for the rain to
mix and begin changing to snow. After 12z Wednesday the large
scale forcing/ascent really ramps up in association with the
ejecting mid/upper low and this should result in an upswing in
coverage and intensity of precipitation from east central MO into
south central and central IL, and accelerated changeover to snow.
All of the CWA should be entirely snow by 18z, as the deformation
zone precipitation begins shifting away from the CWA. Snow amounts
are still a tough call and there is lots of room for error. The
mean track of the H85 low would place the most-favored axis of
highest precipitation from around Belleville to Decatur, but
during some of the time the rain will be changing to snow. Also
surface temperatures will be in the 32-35F range and SLRs are
expected to be lower than normal with a wet snow expected. The new
forecast has the highest amounts along the aforementioned corridor
with a general 2-3 inches. I wouldn`t be suprised to see locally higher
totals, especially as you head northeastward into central IL.
Given potential the impacts of slushy/slippery roads and reduced
visibility on holiday travel, we have gone ahead an issued a winter
weather advisory even though these amounts are generally below
those we typically consider for an advisory.

Some residual non-accumulating light snow/flurries may linger from
east central MO into IL during the first part of the evening,
however this will be a short window. The system continues its
quick departure with ridging aloft already by 12z Thursday and low
level warm advection underway after 06z.

Glass

.LONG TERM: (Thursday through Next Tuesday)
Issued at 251 PM CST Tue Dec 23 2014

Pronounced warm advection and backing flow aloft in response to
new upstream amplification will dominate Christmas day. The snow
cover will likely be short-lived as temperatures moderate above
normal Thursday into Friday.

A deep and slow progressive long wave trof will then dominate the
upper air pattern into the weekend featuring seperate northern and
southern stream components. Present indications are we should see
a cold front passage on Friday night with an attendant threat of
precipitation. An overunning pattern could then set-up Saturday
into Sunday with snow possible in the deeper cold air, however the
models are quite variable in their depictions of the important features.

Glass

&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1201 PM CST Tue Dec 23 2014

Mid and high clouds over forecast area through early this evening,
then cigs begin to lower as next system approaches region. Will go
down to MVFR by 03z Wednesday for STL metro area while KCOU/KUIN
will not lower til around 09z Wednesday. KUIN and KCOU will be on
the northern fringes of this system with precipitation mainly in
the form of light snow by 14z Wednesday. As for STL metro area,
precipitation to begin as rain around 06z Wednesday, changing to a
mix of rain and snow by 12z Wednesday, then all snow by 15z
Wednesday. Once snow kicks in cigs to lower to IFR all locations
and persist through rest of forecast period. As for winds, south
winds to veer to the west then northwest by this evening and
pickup a bit by Wednesday morning.

Specifics for KSTL:
Mid and high clouds over forecast area through early this evening,
then cigs begin to lower as next system approaches region. Will go
down to MVFR by 03z Wednesday for STL metro area. Precipitation to
begin as rain around 06z Wednesday, changing to a mix of rain and
snow by 12z Wednesday, then all snow by 15z Wednesday. Once snow
kicks in cigs to lower to IFR and persist through rest of forecast
period. As for winds, south winds to veer to the west then
northwest by 03z Wednesday and pickup a bit by Wednesday morning.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR
     Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis MO.

IL...WINTER WEATHER ADVISORY from 6 AM to 6 PM CST Wednesday FOR Bond
     IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-St. Clair IL-
     Washington IL.

&&

$$

WFO LSX






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