Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 241355
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
755 AM CST Wed Dec 24 2014
Issued at 755 AM CST Wed Dec 24 2014
Band of precipitation on track per short term discussion from
earlier this morning. The adjustments to the forecast are with
pcpn-types and snow amounts, favoring more rain and minimal snow
accums if any.
Surface temps are not falling fast enough in the band to reach its
target value of 35F or less and the window of opportunity is
closing fast with band already pulling out in southeast MO and
should roll up thru the rest of southern IL this morning.
Also, enhanced reflectivities of 35 to 40 dbZ seen on radar
returns over downtown STL but this is the melting layer supported
by 12z RAOBs...at 1500-2000ft and will be very stubborn to budge
at this point.
.SHORT TERM: (Through Late This Afternoon)
Issued at 351 AM CST Wed Dec 24 2014
As greatly advertised, a deep upper level TROF does indeed exist
over the central CONUS early this morning. However, there really
is no true closed circulation at this time anywhere. Rather a
sharply defined shear axis is creating a quasi-circulation over
northwest MO. The strong disturbance that was supposed to create a
new southern LOW center by this time is over southern AR.
Satellite has filled in with cloudy skies across the forecast
area, but not too long ago, there existed a region of mostly clear
skies back into northeast and central MO which allowed sfc temps
to cool into the lo-mid 30s. Other areas remained in the upper 30s
and lo 40s with the warmest air over in the Salem IL area. Radar
mosaic shows the system pcpn finally showing its hand, with banded
pcpn and pcpn reaching the ground forming from Farmington, MO to
Metro STL East to just west of Litchfield, IL. Some additional
echo is forming still further to the west over south-central MO in
response to the approaching shear axis but no pcpn being reported
but rather this is mid cloud.
The models still are anticipating the energy and focus on this
complex system to eventually transfer to the south and east, but
the current model consensus does not expect this to occur until
late this morning if not midday. By this point, the pcpn will be
exiting our region and there will be very little impetus to push
the pcpn back westward into the colder air until it is too late.
As a result, we are looking at a more easterly QPF axis and less
of a push to create snow as a pcpn-type.
Have forecasted less pure snow and instead now believe a rain-snow
mix will be the best it can do even with accounting for dynamical
cooling processes. Without a clear closed circulation center
before it is by our area, it is just going to be too tough with
several other factors already fighting it, such as warm ground and
an initially warm boundary layer.
Snow amounts have been scaled back to an inch or less and that
could be overdone...with the western edge cutting thru roughly
from Litchfield, IL to Belleville, IL. Factoring in also the
previous forecast, have decided to continue the Winter Wx Advisory
for a smaller region which includes Metro East and parts of
southwest IL, with the Advisory cancelled for STL City and MO
sections. If snow falls in the western Metro it should not
accumulate on roads with minimal to none on grassy surfaces with
rates not expected to be much.
Add to all of this some scattered -SHSN/-SHRA with region of deep
cyclonic flow passing into our region later this morning and into
the afternoon from western MO, but accums if any will be
Went on the cool side of MOS, but that should still yield maxes in
the upper 30s or so.
.LONG TERM: (Tonight through Tuesday)
Issued at 351 AM CST Wed Dec 24 2014
Any lingering light snow in southwest IL will taper off tonight as
the low pressure system continues moving away from the area.
Sun-starved inhabitants can look forward to a present from the
atmosphere on Thursday, as skies will likely be partly to mostly
sunny for at least part of the day. Daytime high temperatures will
be around 5-10 degrees above average for this time of year. In
addition, there should be a bit of a SW or S breeze due to the
tightening pressure gradient ahead of the next low pressure
system, which is tentatively expected to affect MO/IL between
Friday night and Sunday night.
Ahead of this system, WAA will lead to daytime temperatures of
around 10-15 degrees above average on Friday. The associated cold
front will then move through the region on Friday night and
Saturday. The initially warm temperatures will keep the ptype as
pure rain on Friday night, with snow becoming possible on Saturday
once temperatures have cooled within the column.
A new H5 trough develops over the western CONUS early next week,
but there are differences between the GFS and the ECMWF on how the
vorticity maxima eject out of the trough, leading to different
depictions of the large-scale pattern between Monday and Tuesday
An Arctic intrusion is anticipated during the middle of next week
around the beginning of the new year. Such an intrusion is
consistent with recent teleconnection signals.
.AVIATION: (For the 12z TAFs through 12z Thursday Morning)
Issued at 535 AM CST Wed Dec 24 2014
Area of IFR CIGs is moving thru the forecast area now and should
move out by early afternoon, yielding to MVFR category. The MVFR
CIGs are then expected to persist until late tonight when backing
winds from the SW should push them out of here. The main band of
precipitation`s western edge has revealed itself and it should
primarily affect CPS out of the TAF sites with STL receiving only
sprinkles for a short period this morning. Rain will be the main
pcpn-type here with some snow mixing in possible for CPS. This
main band expected to rapidly pull eastward by 16z. Spotty
sprinkles or flurries are then expected from the west...affecting
UIN and COU for brief periods this morning and early
afternoon...and STL metro sites perhaps later this afternoon. The
very light and spotty nature of this makes it hard to predict and
will, for now, forego mention in TAFs. Otherwise, look for NW
winds, at times gusty this afternoon, to back SW later tonight.
Specifics for KSTL: Area of much lower CIGs is about ready to
pounce on the airport and have started with these lower CIGs right
out of the gate. Pcpn now expected to be mainly east but a few
sprinkles likely for a few hours this morning. The lower CIGs,
expected to be in the low-end MVFR range, are then expected to
persist thru the day and into tonight with clearing anticipated
around 09z as low level winds back from the SW.