000
FXUS63 KLSX 040403
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1003 PM CST Tue Dec 3 2013

.UPDATE:
Issued at 1001 PM CST Tue Dec 3 2013

Grids have been updated to reflect an earlier onset of fog and the
quicker northward push of stratus.

Kanofsky

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 340 PM CST Tue Dec 3 2013

Although the stratus clouds have been dissipating from west to east
this aftn, there is still another area of low level clouds over sern
MO advecting slowly nwd plus more stratus clouds well south of our
forecast area which may advect nwd into our region later this evng
and overnight.  The models depict very moist or saturated conditions
tgt in the sfc/boundary layer so am expecting stratus clouds to
advect or redevelop across much of our area again late tgt.  Despite
relatively strong swly low level winds tgt, with higher sfc dew
points continuing to advect nwd into our area there should be some
fog again late tgt.  Low level waa combined with a shallow layer of
moisture below 850 mb may also lead to patchy drizzle late tgt.
Temperatures will be warmer tgt than the previous ngt due to the
cloud cover, low level waa, sly sfc winds, and fairly high sfc dew
points.  The low temperatures tgt should be 15 to 20 degrees above
normal for early December.

GKS

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 340 PM CST Tue Dec 3 2013

(Wednesday - Friday)

Models have (finally) come into an excellent consensus on the timing
of the surface cold front, which will represent the leading edge of
Arctic air--an airmass that will then dominate much of the remainder
of the next seven days.  This timing was ultimately not much
different than what the GEM/EC had been saying for days:  passing
thru our area Wednesday afternoon and evening.  Falling afternoon
temps appear to now only be occurring for the far western areas of
our CWA--in central and northeast MO.

As the previous shift had pointed out, our weather for Thursday and
Friday is made up of a few components.  The main storm which will
steer the other key components up thru our area has been onshore
since Monday, but the actual disturbances that will pass thru and
create pcpn later this week are still over the Pacific Ocean at this
time and are difficult to sample well.  Thus we may continue to see
some shifting going on until this is rectified Thursday morning.

As it stands now, the disturbances are still on track for pcpn
Thursday and Friday, coming in two waves.  The first wave looks a
tad slower than before and is now expected to come in mainly
Thursday afternoon and continue into the evening.  As the cold air
builds and deepens from the NW, this is still expected to shunt the
second wave of pcpn for Friday a bit further SE.  For those that can
view it, the NAM output seems to be on everyone`s minds, but
comparing to the SREF (of which the NAM is a member), reveals this
to be very much an outlier and also differs pretty significantly
from the more mainstay GFS and EC.  The NAM already has backed off
by a lot for what it showed for Thursday and therefore not much
confidence can be placed in the 12z run at this time.

Pcpn-types are not much different from forecasts 12-24hrs ago, with
mainly snow to the N and W of STL metro with sleet and FZRA heading
further SE with initial onset Thursday afternoon, and snow gradually
winning the war over time heading thru Thursday night and Friday as
the cold air builds and the pcpn slowly gets pushed to the SE.

Used the EC as a compromise on max QPF axes and when it is all said
and done, a stripe of 2-4" of snow with some sleet can be expected
around the I-44 corridor in MO and continuing E-NE toward central-se
IL with lesser amounts to the NW and SE (from mixing with sleet and
FZRA).  Unfortunately, FZRA is expected across SE MO and into parts
of S IL Thursday afternoon thru Thursday night (it may edge into the
far SE STL metro area briefly late Thursday as well), where ice
accums of up to two tenths of an inch are possible.  By Friday, pcpn
should be snow everywhere where minor additional accums are
expected--mainly in SE MO and S IL.

No headlines with this package:  amounts point towards advisory
criteria but too far out to issue at this time with decent spread on
amounts continuing.

Cannot discount the new MOS for Wednesday max temps--with a slower
FROPA--should allow temps to surge well into the 60s and approach 70
in spots if we can see decent sun.  Stratus in the morning will
complicate that a bit, though.

Thursday on it will be tough for the temp to break freezing for
several days, and new snow cover will further make that difficult.

(Saturday - Next Tuesday)

Cold continues with reinforcing Arctic air intrusions.  Only other
highlight will be another system moving up from the SW.  The EC
continues to advertise a more northwesterly track than the GFS and
introduce more mixed pcpn.  For now, will maintain the predominantly
snow forecast with the overall pattern favoring cold and fallout
from late week systems may further influence that towards a colder
solution.

TES

&&

.AVIATION:  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 535 PM CST Tue Dec 3 2013

Flight conditions are expected to deteriorate tonight. The leading
edge of a large region of MVFR cigs stretched from south central
Il across southeast MO into AR early this evening and has already
progressed into KCPS. Low level flow within the cloud layer was
southerly around 20-25 kts, suggesting it will continue to advance
northward through advection this evening. I have brought MVFR cigs
into KSTL and KSUS earlier at 02Z but this may not be fast enough.
Elsewhere the low clouds will both advect and develop through
central MO and into northeast MO/west central Il during the later
half of the evening and overnight. Cig heights are expected to
lower through the overnight hours along with vsbys restrictions
due to fog/br, and may be actually be IFR. There should be
improvement in both the cig heights and vsbys on Wednesday as south
to southwesterly winds increase and mixing commences and as
surface winds veer to more westerly with time.

Specifics for KSTL:

Flight conditions are expected to deteriorate tonight. The leading
edge of a large region of MVFR cigs was located just north of KCPS
early this evening. Low level flow within the cloud layer suggests it
will continue to advance northward into the terminal by mid-evening,
roughly around 02Z but this may not be fast enough. Cig heights are
expected to lower through the overnight hours along with vsby
restrictions due to fog/br, and may be actually be IFR. There
should be improvement in both the cig heights and vsbys on
Wednesday as south to southwesterly winds increase and mixing
commences and as surface winds veer to more westerly with time. A
cold front will move through the terminal early Wednesday evening.

Glass

&&

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

$$

WFO LSX