Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261805
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1205 PM CST Wed Nov 26 2014

.UPDATE:
Issued at 1150 AM CST Wed Nov 26 2014

Clipper system continues to impact our forecast area at this time.

At 17z...a low pressure center was located in north-central MO and
was moving slowly to the southeast. A warm front extended to the
south of this low pressure area into central MO, where temps were
rising into the low 40s. The cold front associated with this
system was located from this low pressure area into the western KC
metro area. Temps ahead of the warm front remain stubbornly in the
low-mid 30s for most areas. An area of precipitation extended in a
band, mostly, from the cool side of the warm front in southeast MO
thru the STL metro area and to the north side of the low pressure
area in central IA. Most of this precipitation was snow.

The low pressure in northern MO was in the process of occluding
with all anticipation that a new triple point will develop over TN
this evening. In the meantime, the cold front is expected to catch
up to the warm front and pinch the warm sector off while the newly
developing cold front edges into the MS river area, including STL
metro, by nightfall.

So...look for temps to not really go anywhere today for areas near
the MS river and in IL where the warm air will never arrive. Over
in central and parts of southeast MO, temps will rise into the low
40s this afternoon.

The precipitation band will continue to slowly push east this
afternoon but at some point...likely over southwestern IL...stop
as the whole thing inflects and centers around the new LOW
developing in TN this evening. Scattered rain...and snow...showers
will be seen this afternoon for central MO...with these showers
shifting to the MS river area for this evening.

Surface temps will be critical for pcpn-types given the vertical
temp profiles, but much of the pcpn should remain all snow with
some better chances of rain where the pcpn scatters out on the
otherside of the warm front and near the new cold front. A wet
inch of snow is possible mostly for the UIN area, but also
possible for areas further south on grassy surfaces. Minor
additional accums possible into this evening before everything
dissipates and/or shifts southeast overnight.

TES

&&

.SHORT TERM: (Today and Tonight)
Issued at 310 AM CST Wed Nov 26 2014

Clipper system over Nebraska/Iowa border will continue to drop
southeast today, bringing the area some light snow/rain. Short
range guidance including the HRRR, RAP, NAM and GFS are in good
agreement that light precip will overspread the area through the
morning hours, reaching the Mississippi River between 16-18Z.
Primary lifting mechanisms continue to look like low level
moisture convergence, frontogenesis, and broad scale lift
generated by the parent shortwave. Precipitation will initially
have to fight some pretty dry low level air as it moves into the
area as dewpoints are in the upper teens and low 20s. Evaporative
cooling as well as favorable vertical profiles would indicate that
initially most if not all of the precipitation will fall as
snow...especially over central and northeast Missouri. Insolation
through later in the morning and afternoon, combined with
southerly flow along and east of the Mississippi river (which will
be east of the low track) will probably cause the snow to mix with
rain across a good portion of the area for at least part of the
afternoon. Between the mix, above freezing temperatures, and warm
ground conditions (2 inch soil temperatures range from 32 to 40
degrees across the area), expect only minor accumulations of and
inch or less today. The tail end of the clipper will continue to
affect the area this evening with additional minor accumulations
possible...generally along and east of the Mississippi in
southeast Missouri and southwest Illinois. The lingering
precipitation should end before 06Z this evening.

Carney

.LONG TERM:  (Thursday through Tuesday)
Issued at 410 AM CST Wed Nov 26 2014

Quiet period of weather for Thanksgiving Day through Saturday as the
pattern transitions from high amplitude to more zonal.  After a cold
day on Thanksgiving with highs struggling into the upper 20s to mid
30s, the trof currently over the eastern CONUS will shift offshore
by early Friday morning which will start a moderating trend.  Low
level southwest flow will bring temperatures up to near normal in
the low 50s, and temperatures will warm further on Saturday into the
upper 50s and low 60s.  Continuing to keep a dry forecast on
Saturday in spite of the GFS and ECMWF which continue to spit out
light QPF.  Still think this is due to the rapidly moistening
boundary layer and the model`s tendency to make precipitation when
this occurs.  Sunday`s highs are still in question as a 1035mb high
dips down into the Northern Plains and Upper Midwest.  The resulting
cold front will be moving into northern sections of our area during
the day which makes high temperature forecasts for Sunday quite
difficult.  Should be mild south of the front with highs in the 50s
and 60s...possibly warmer.  North of the front it will be sharply
colder.  As there will be frontal-scale lift as well as weak low
level moisture convergence, have kept low chance PoPs in the
forecast south of the front in the warm sector.  High will shift
eastward Monday and Tuesday.  Medium range guidance agrees that
Monday should be cool and dry under the influence of the high, with
the potential for temperatures rising back above normal for Tuesday
as southerly flow returns to the area.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1155 AM CST Wed Nov 26 2014

Low CIGs will prevail with the onset of the pcpn for MS river
sites and continue for COU for much of the valid TAF period given
expected flow at cloud level and where the low clouds are now and
how it will develop with this system. This will translate to IFR
and low-end MVFR CIGs. May need to expand IFR CIGs over a larger
area based on trends heading into early this evening. These clouds
should then break up during Thursday (Thanksgiving) morning. Wind
trends were updated with expected positions of the front with
FROPA around 00z but wind speeds not expected to pick up much near
the MS river until later tonight due to frontal proximity.

Specifics for KSTL: Currently have MVFR CIGs and these should at
least continue into Thursday morning. IFR a possibility later
tonight and will bear watching but prefer to see things develop to
our NW first before committing. Warm front FROPA around 00z, which
at that time will be close to when the cold front catches up
anyway, where winds shift from S to W. Otherwise, heaviest snow
during the early-mid afternoon with more scattered SHSN late
afternoon and evening.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX




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