Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 070940
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
340 AM CST Sat Dec 7 2013
.SHORT TERM: (Through Late This Afternoon)
Issued at 340 AM CST Sat Dec 7 2013
Unseasonably cold weather expected today despite full sunshine as
strong 1043 mb surface ridge centered over South Dakota and ern
Nebraska shifts ewd. The 850 mb temperatures at noon will only be
about -8 to -11 degrees C over our forecast area. Will go below the
MOS temperature guidance across sern MO and swrn IL where there is
significant snow cover. The high temperatures today will be at
least 20 degrees below normal for early December.
.LONG TERM: (Tonight through Friday)
Issued at 340 AM CST Sat Dec 7 2013
A longwave TROF will begin this period situated to our west, but
will gradually propagate east across the CONUS during early-mid
week, with the axis passing thru our region Monday night. By
mid-late next week, a much flatter flow pattern is expected to be in
place. Overall this signals the very cold air will continue thru at
least the middle of next week with some moderation to take place
The highlights will feature a couple of quick-hitting systems. The
first will be later tonight and into early Sunday night but will
track much further to the NW than this past system. It will begin
as a WAA/frontogenetical dominated region of lift pushing up from SW
MO and expanding N-NE towards the I-70 corridor late tonight. The
lift will be sufficiently deep with this first phase, and enough ice
in the cloud, for resultant pcpn to be snow. Despite what should be
a brief first round, could see light accums to a half inch thanks to
high anticipated liquid-to-snow ratios from the very cold
atmosphere. The models all diverge and move out-of-phase the
various lift from this first round by late Sunday morning and
re-focus with the main system that will pass thru further NW. Pcpn
with this will again be all snow for mainly NE MO and W IL and
should continue into the afternoon before pulling away to the N-NE.
Elsewhere for much of Sunday, we are looking at a lot of lo cloud in
place with very little above it and some weak lo level lift, which
unfortunately, still points towards FZDZ potential. Have maintained
FZDZ in the forecast as patchy coverage and with the very cold
surfaces already in place, what FZDZ that does form should have
little trouble forming a light glaze. This should all come to an
end with a cold front sweeping thru later Sunday night and
reinforcing the cold air.
The other item of interest is centered around Monday night.
Broadscale lift is expected to increase with passage of the longwave
TROF axis thru our region, and combined with favorable quadrant from
a 180kt h300 jet streak and intensifying frontogenesis to focus the
lift, EC/GFS/SREF and now 06z NAM are pointing at potential for
narrow band(s) to form across central-southern MO and southern IL.
Something like this is hard for the models to get a good handle on 3
days out and will take a cautious approach and introduce flurries
for now. But something to watch.
.AVIATION: (For the 06z TAFs through 06z Saturday Night)
Issued at 1119 PM CST Fri Dec 6 2013
VFR through at least the first 6-12 hours of the 24-hr TAF period.
Northerly winds will veer easterly during the day as a high
pressure center moves from the northern Plains towards the Great
Lakes. After several hours of mostly clear skies, clouds will
lower and thicken ahead of an approaching disturbance, especially
after 08/00z. Beyond the end of the valid TAF period, MVFR cigs
with light snow and/or freezing drizzle are possible.
Specifics for KSTL: VFR through at least the first 6-12 hours of
the 30-hr TAF period and probably even longer. Northerly winds
will veer easterly during the day as a high pressure center moves
from the northern Plains towards the Great Lakes, and clouds will
lower and thicken over KSTL after 08/00z ahead of an approaching
disturbance. MVFR cigs with light snow and/or freezing drizzle are
possible during the last 3-6 hours of the valid TAF period with
precipitation chances extending beyond 30 hours.