000
FXUS63 KLSX 031214
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
614 AM CST Tue Dec 3 2013

.SHORT TERM:  (Through Late This Afternoon)
Issued at 359 AM CST Tue Dec 3 2013

Main concern through mid morning is dense fog.  I have expanded the
dense fog advisory to include all of the area except the far east
until 10 am.   Visibilities have been varying most of the night,
however there has been a general decline the past two hours.  These
sites include SET and a few in the metro east, those in Southwest
Illinois including Sparta and Carbondale and southeast Missouri.
There is also a large area of fog over southwest and south central
Missouri that is expanding rapidly northeastward.  Not sure how
widespread the fog will be this morning, however there will be
enough areas of dense fog to warrant an advisory.

Surface winds will begin to increase later this morning as pressure
gradient tights.  Still expect a significant warm up this afternoon
with the strong warm air advection this afternoon.  Kept with going
high temperatures which are supported by mixing down 900mb
temperatures.

Britt

.LONG TERM:  (Tonight through Monday)
Issued at 359 AM CST Tue Dec 3 2013

Primary concern this morning remains the pending wintry weather for
Thursday and Friday so will focus most of this discussion on that
period.  The ingredients are coming together...and model guidance is
coming into better agreement visa-vis the initial fropa Wednesday.
Leading up to Thursday:  a strong shortwave rotating south along the
northwest coast of the U.S. will dive down across the west coast
into the Desert Southwest.  This will amplify the flow allowing very
cold air from southwest Canada to move down the east side of the
Rockies into the High Plains.  Lee side cyclogenesis will occur over
the Plains tonight and a strong low will ride up the resulting
baroclinic zone between the arctic air over the northern Plains and
the warmer air to the south.  The resulting cold front will barrel
through the area Wednesday.  Used a blend of guidance to attempt to
nail down the temperature trends Wednesday, and the end results are
a slightly slower and warmer solution than the previous forecast.
Still think there might be some rain showers along and ahead of the
front, but any precip will be light and isolated due to a strong cap.

As the longwave trof deepens over the southwestern U.S. on Wednesday
it will pick up a cutoff low now sitting about 1100 miles southwest
of San Diego.  The vorticity from this cutoff will ride up over the
Baja Peninsula early Wednesday evening and cross Mexico overnight,
emerging across west Texas around 12Z Thursday.  This is the energy
which will produce our wintry precip Thursday.  This wave of
vorticity will not be sampled by our upper air network until
Thursday morning...so my confidence in details of Thursday`s
forecast is low.  Much will depend on the track of the wave as it
moves rapidly northeast across the area and how deep the cold air
will be when precipitation is occurring.  Used a combination of
partial thickness forecasts and model soundings to forecast precip
type, leaning toward the NAM which typically handles shallow arctic
airmasses better.  Generally speaking, the predominant precip type
should be snow north of I-70, along and 30 miles south of I-70 a mix
of sleet and snow should occur, with sleet and freezing rain further
south across the Eastern Ozarks.  The precipitation will slowly
trend toward more sleet and snow through Thursday into Thursday
night as cold air deepens.  The accumulation forecast is complicated
by the mixed precip and the speed with which the wave will be
moving.  There remains little or no low level thermodynamic forcing,
this looks to be all mid and upper level synoptic lift.  Think the
precip will generally be light, beginning in the late morning or
early afternoon and taper off over most of the area by mid to late
evening.  Precip may linger longer over the eastern Ozarks.  1-3
inches of a mix of sleet and snow looks like good first
estimate along and north of I-44 with around 0.1 inch of
freezing rain topped off by a little sleet and snow over the eastern
Ozarks.  Once again...let me stress that this is this is low
confidence since we won`t be sampling the wave until it crosses into
Texas Thursday morning.

Another wave of vorticity will eject northeast from the base of the
longwave trof on Friday.  This will be the second wave of wintry
precip we will have to contend with before the weekend.  Precip type
should be less of an issue as cold air will be deep enough for all
snow across all but far southeast Missouri.  The NAM is coming in
with a potential for more mixed precip in those areas with a warm
nose in the forecast soundings between 850mb and 750mb.  For the
moment, have kept an all snow forecast going, since the NAM is just
starting to get out to that time range.  In addition to the upper
level support, there may be a lower level thermodynamic component to
Friday`s precip.  There is a stronger low level reflection of the
shortwave which moves across Arkansas into the Tennessee Valley
Friday.  Both the NAM and GFS show substantial low level moisture
convergence ahead of this low...and this will definitely enhance
lift and precipitation efficiency.  The GFS is further south keeping
this thermodynamic component out of the area...but the NAM pushes it
up into southeast Missouri and southern Illinois.  For now, have
stayed the course and kept chance to likely pops and, as mentioned
before, all snow.

Remainder of the forecast looks cold.  Temperatures should be well
below normal with arctic air locked over the region.  There is still
a chance for light precip (most likely snow) as low level warm
advection ramps up in response to the longwave trof finally
de-amplifies and moves east into the Great Plains.  Boundary layer
air will be so dry due to the arctic airmass locked in over the
region that any additional snowfall amounts should be light.


Carney

&&

.AVIATION:  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 534 AM CST Tue Dec 3 2013

Areas of VLIFR and LIFR conditions have been prevalent through the
night, and expect them to continue through 15-16Z. This is when
the low level inversion will begin to erode and surface pressure
gradient increases. Then expect mainly VFR conditions until around
06Z tonight when computer model guidance suggest that MVFR or IFR
ceilings and visibilities will redevelop after 06Z tonight.

Specifics for KSTL: Expect current LIFR to last through at least
15Z with VFR conditions developing by 16-17Z. Then expect VFR
through most of the day before MVFR or IFR conditions redevelop
after 06Z tonight.

Britt

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     64  50  58  29 /  10  10  20  10
Quincy          62  46  49  24 /  10  10  20  10
Columbia        65  45  46  24 /  10  10  20  10
Jefferson City  67  46  49  26 /  10  10  20  10
Salem           63  51  61  33 /  10  10  30  20
Farmington      64  52  60  31 /  10  10  30  10

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...DENSE FOG ADVISORY until 10 AM CST this morning FOR Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Shelby MO-St. Charles
     MO-St. Francois MO-St. Louis City MO-St. Louis MO-Ste.
     Genevieve MO-Warren MO-Washington MO.

IL...DENSE FOG ADVISORY until 10 AM CST this morning FOR Adams IL-
     Bond IL-Brown IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Monroe IL-Montgomery IL-Pike IL-Randolph IL-St.
     Clair IL.

&&

$$

WFO LSX