Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 050536
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1136 PM CST Wed Dec 4 2013
.SHORT TERM: (Through Tonight)
Issued at 333 PM CST Wed Dec 4 2013
Focus tonight will be dropping temps with fropa as well as some
light dz this evening.
The cdfnt continues to lurk just W of the CWA, but will be pushing
thru the CWA this afternoon thru tonight. Believe some dz will
accompany this fnt as it pushes thru overnight. Latest mdl guidance
suggests some light ra may be briefly possible tonight as a wave of
WAA moves just S of the area.
Have trended aob the coolest MOS guidance for temps tonight given
temps over wrn MO are already in the mid 20s.
.LONG TERM: (Thursday through Next Wednesday)
Issued at 333 PM CST Wed Dec 4 2013
(Thursday through Saturday)
Focus thru this period will be the approaching system, which is
expected to bring the first snow of the season to the southern third
to half of the CWA.
The 12z mdls are in remarkably good agreement thru this period with
respect to mass fields. Thermal profiles are not far off either, but
there are some important differences. There are also some big
differences regarding precip placement.
For the past couple of days, the NAM has been the farthest N with
precip, until last night. The NAM has now trended further S and SE
with the precip and it continues to be the furthest SE. The
GFS/ECMWF have taken the middle of the road solns with the GEM an
outlier to the N and NW. Have generally followed a consensus of the
GFS/ECMWF which has good support with the local WRF trending away
from the NAM soln and twd the GFS.
This soln suggests precip starting with either RA or FZRA late Thurs
morning across far srn portions of the CWA with precip spreading NE
and gradually working nwd during the afternoon. As CAA continues
thru the day, precip will gradually change over from FZRA to IP and
eventually to all SN by 06z Fri. Further N, roughly a Cuba, MO to
Cahokia, IL to Vandalia, IL line, is expected to remain all SN thru
the event. However, this is on the nrn fringes of the precip shield
and will likely still see lower amounts due to that.
Precip is expected to be in two waves. The first wave is expected to
enter the CWA Thurs late morning and continue thru Thurs evening.
After a lull lasting thru about mid Fri morning, another round of
precip is expected, but p-type shud be all SN.
Have increased SN amounts further S where higher rates are expected
with higher SLRs mainly Fri with the second wave. This SN will be on
top of what IP accumulates during the precip change Thurs night.
Have not added this to the forecast yet, but mdls suggest a few
hours of thunder will be possible across the srn couple tier of
counties Thurs afternoon, possibly into early evening while IP is
expected. This could lead to higher sleet/snow totals than is
currently depicted, but this will largely be a nowcast feature.
With general mdl consensus shifting precip further S, have lowered
amounts across the nrn half of the CWA, but kept lower POPs going
for now. With some uncertainty and due to the prev forecast, have
kept POPs higher, further nwd to keep some consistency. If trends
continue, future shifts will be able to continue to push POPs and
amounts further S.
Have issued a Winter Storm Watch for srn and sern portions of the
(Sunday through Wednesday)
Few changes have been made to the extended for this package. A very
cold Arctic air mass will remain firmly entrenched over the region
through the extended period, with current guidance indicating that
much of the central CONUS will remain below freezing through at
least late next week. GFS and ECMWF guidance continue to indicate
another round of wintry precipitation expanding northeastward across
the area beginning late Saturday night and tapering off on Sunday
night as the western CONUS trough begins to finally move eastward.
Despite the cold temperatures, soundings and critical thicknesses
from both the ECMWF and GFS continue to indicate the potential for
mixed precipitation mainly over the southern tier of the CWFA.
While the GFS is bit more robust regarding the northward extent of
mixed precipitation (as far north as the I-70 corridor in Missouri
and Illinois), have elected to maintain the previous mention of a
mix of sleet and snow over the southern CWFA during the day Sunday,
as confidence for mixed precipitation is greater further south.
.AVIATION: (For the 06z TAFs through 06z Thursday Night)
Issued at 1118 PM CST Wed Dec 4 2013
MVFR cigs will prevail through much of the overnight period with
gradual clearing of the lower clouds from west to east between
12-15z. Extensive mid-high clouds will be present in the wake of
the clearing lower clouds and some patchy light precipitation is
certainly not of the question on Thursday morning. The main thrust
of wintry precipitation however should be in the mid-late afternoon
from southwest MO into southeast/east central MO in the form of a
snow and sleet mix for the St. Louis Metro TAFS. The longevity of this
snow/sleet mix should only be 3-4 hours centered in the 22z to 02z
time frame and thereafter VFR conditions will prevail.
Specifics for KSTL:
MVFR cigs will prevail overnight into mid morning, followed by VFR
conditions consisting of extensive mid-high clouds until late
afternoon. There could be some patchy light precipitation on Thursday
morning, however the main thrust of wintry precipitation in the
form of a snow/sleet mix will impact KLSX beginning around 22Z.
The longevity of this snow/sleet mix should only be 3-4 hours
centered in the 22z to 02z time frame and thereafter VFR conditions
MO...WINTER STORM WATCH from Thursday afternoon through Friday
afternoon FOR Crawford MO-Iron MO-Jefferson MO-Madison MO-
Reynolds MO-St. Francois MO-Ste. Genevieve MO-Washington MO.
IL...WINTER STORM WATCH from Thursday afternoon through Friday
afternoon FOR Clinton IL-Fayette IL-Marion IL-Monroe IL-
Randolph IL-St. Clair IL-Washington IL.