Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 191637
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1037 AM CST Wed Nov 19 2014
Issued at 1035 AM CST Wed Nov 19 2014
The warmer weather is quickly coming to an end as the next cold
front is moving across the CWA this morning. Temperatures topped
out in the 40s in the St. Louis area (first time since Veterans
Day). Expect temperatures to slowly fall this afternoon with an
increase in cloud cover and cold advection on a brisk and gusty
northwest wind. Can`t rule out a few flurries across the northern
zones given upstream observations in Iowa.
.SHORT TERM: (Through Late This Afternoon)
Issued at 306 AM CST Wed Nov 19 2014
A deep upper level TROF continues to dominate eastern North America,
with resultant NW flow aloft for our region. A pair of disturbances
of note exist within this flow. The first was over southeast MN and
is helping to generate an area of snow for parts of the upper MS
valley. Another disturbance was rather well sheared and stretched
from the mid-MO valley and back into the northern High Plains and
was not associated with any pcpn but did correlate well with clouds.
At the surface, a warm front was nosing into central MO and
resulting in rising temps into the lo 40s, with readings remaining
in the 20s for most areas to the east of it. Much further to the
northwest, another cold front was entering northwest IA and was
surging southeastward. The latest satellite loop shows a lot of mid
level cloud over our region from the approaching warm front, with a
more significant area of low clouds further to the NW near the cold
front. Our region was pcpn-free.
The warm front is expected to push into STL metro by 12z and then
thru the rest of the forecast area by 15z providing a brief temp
surge upward before the cold front arrives a few hours later. The
cold front is also expected to be accompanied by a push of low
clouds as well. This will mean morning max temps for most locations
with steady to slowly falling temps during the afternoon with
decently strong CAA behind the front. Prefer colder MET MOS for the
northern forecast area, such as UIN, due to doubts on warm air surge
reaching there in the first place, and COU where warm air be snuffed
out early. Prefer the warmer MAV MOS for around STL metro and
further S and E where warm air surge will at least make it into to
have an impact on max temps in the first place.
Low amounts of moisture, only weak cyclonic flow, and lack of upper
level support with only real disturbance rolling thru will be
heavily sheared should result in a dry day with even flurries
struggling to appear.
Despite the very cold start this morning, daytime max temps should
be warmer than persistence for the MO side of the river thanks to
decent levels of sunshine and surface flow becoming more W-SW,
promoting deeper mixing layers and strongly favoring the higher MAV
MOS. Where the clouds are more numerous in IL, persistence a good
forecast guess where MOS blend preferred.
.LONG TERM: (Tonight through Tuesday)
Issued at 306 AM CST Wed Nov 19 2014
(Tonight through Friday)
Not many changes from the prev forecast thru this period. Main
change has been mdl solns slowing the retreating sfc ridge on Thurs
night into Fri and also slowing the onset of precip.
Trended twd cooler guidance tonight due to fropa today. Since most,
if not all, of the snow cover shud melt today, have trended twd the
warmer guidance beyond except as mentioned below.
Mdls are in very good agreement wrt mass fields thru this period.
Timing of the leading s/w that may bring the first round of precip
to the region late Thurs night. However, as mentioned yesterday,
this system is still off the wrn U.S. coast and still will not be
well sampled by 12z this morning.
One area where mdl solns begin to greatly differ is the thermal
profile, esp within the boundary layer. For now, have trended temps
twd the GFS soln with the ECMWF as a warmer soln and the NAM/local
WRF as a colder soln. Based on fcst soundings, it appears that at
least some of the QPF the mdls suggest is due to low level moisture
return and not actual precip. Precip for Fri night and beyond
appears to be associated with the leading s/w and is supported well
by low level forcing. Still, this portion of the event is expected
to be low precip.
With the colder solns, precip may begin as IP, then gradually
transition to FZRA as the wrmfnt arrives. However, with trending
twd the GFS soln, have kept p-type as all FZRA for now.
(Saturday through Tuesday)
Not much change from yesterday. 19/00z guidance has come into better
agreement and therefore increased PoPs for Sat thru Sun. Continued
warm trend thru the weekend then gradually cooling beginning next
week. With an upper trof returning by Mon, continued cooler trend
thru the rest of the forecast.
.AVIATION: (For the 12z TAFs through 12z Thursday Morning)
Issued at 545 AM CST Wed Nov 19 2014
Gusty SW winds will continue to veer W-NW behind a cold front
passage during the morning hours. The frontal passage will then
soon be followed by intrusion of MVFR or low VFR CIGs which are
expected to linger for much of the remainder of the daylight
hours. There is also some chance they may extend well into the
evening as well, but for now only followed a slightly more
optimistic tone in the forecast by scattering out the clouds
around sunset. Winds to then diminish by sunset.
Specifics for KSTL: VFR thru the period. Gusty SW winds will
become W-NW behind a cold frontal passage around 16z. CIGs are
then expected to lower significantly, but should remain just above
MVFR category at around 3500ft. There is a reasonable chance that
clouds do not scatter out and this should be able to be evaluated
better for the 18z issuance. Winds will diminish by sunset.