Area Forecast Discussion
Issued by NWS St. Louis, MO

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547
FXUS63 KLSX 050345
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
945 PM CST Sun Dec 4 2016

.UPDATE...
Issued at 938 PM CST Sun Dec 4 2016

Stratus has been clearing eastward at a good pace and present
indications are the back edge should move east of the MS River by
05-06Z,and clearing east of the CWA by about 09z. Surface winds
will be light and gradually back to more south or southwesterly
with time in association with the retreat of a weak surface ridge
across the region and into the western OH Valley by 12Z. With
clear skies overnight and the already low T/Td spreads, the
question is how much fog we will see develop and how low the
visibilities may drop. Some patchy fog has begun to develop in
central MO and I think we will continue to see fog develop across
the entire area through the night and into the early morning
hours on Monday. There will likely be some areas of dense fog,
however I`m not sure how widespread. At this time I will just beef
up the wording, however at some point overnight once the scenario
becomes clearer a dense fog advisory may be warranted. The fog
should then dissipate by 15-16z Monday morning.

Glass

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 309 PM CST Sun Dec 4 2016

Lingering light rain across parts of southwest IL will dissipate or
shift east of our forecast area by late afternoon.  The stratus
cloud deck was beginning to clear out of western MO and was thinning
over our area due to increased subsidence behind the upper level
trough shifting east of the region, and as surface ridging builds
into the area from the southern Plains.  Expecting the development
of fog tonight into early Monday morning due to the clearing sky,
surface wind becoming light, and a saturated ground from the recent
rain/snowfall and melting snow.  Low temperatures tonight will be
colder than the previous night, but at or slightly above normal for
early December.  Highs on Monday will be warmer with mainly just
some high level clouds to limit solar insolation and surface/low
level winds becoming southeasterly/southerly as a surface/850 mb
ridge shifts east of the area.  The NAM model tries to bring rain
back into southeast MO already by Monday afternoon ahead of an
approaching southern stream upper level trough over the southern
Plains.  This may be a little too fast as the GFS and ECMWF models
do not bring rain into our area until Monday evening.

GKS

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 309 PM CST Sun Dec 4 2016

It will be an active long term pattern as the current split flow
regime merges into more of an active northern stream pattern by the
middle of the week through the upcoming weekend.  This will allow
three systems of note to affect the region through next weekend.

The first system will arrive at the beginning of the period.  A
trough associated with the southern stream branch of the upper-level
jet will eject out into the Southern Plains.  This will result in a
surface low developing across the western Gulf of Mexico, pushing
northeast into Kentucky by Tuesday afternoon.  This will place
eastern MO and western IL on the cool, northwest side of the surface
low.  Forecast cross-sections continue to show decent mid-level
deformation and frontogenesis on the northwest side of the surface
cyclone in the right entrance region of the anticyclonically curved
jet streak, which will lead to a solid area of precipitation.
However, despite the surface low track, forecast soundings continue
to suggest the low levels will remain too warm for much in the way
of wintry precipitation.  Therefore, this looks to be mainly a rain
event with perhaps a few snowflakes mixing in on the backside across
central and northeast MO into western Illinois.

The second system in the long term period looks to be a bit more
wintry.  A surface front will have pushed well south of the region
by Wednesday.  A PV anomaly diving southeast will approach Wednesday
afternoon into Wednesday night.  Ascent from this PV lobe coupled
with a tightening mid-level thermal gradient will lead to a
frontogenetical band of precipitation sliding from the Central Plains
into the Midwest.  Thermal profiles suggest it will be cold enough
for mainly snow with this event, which could lead to some light
accumulations.  Still some uncertainty in how much QPF this system
will be able to squeeze out, but confidence is increasing that we
will see at least light accumulations, especially across central MO
and points further west where the f-gen circulation appears to be
stronger.

Behind this system, much colder air will filter into the central
CONUS.  Wind chills will likely be near 0 at times Wednesday night
into Friday with temperatures in the upper single digits and teens
for lows.

Quite a bit of uncertainty still exists with the weekend system.
Latest deterministic guidance from the GFS/Euro has trended
basically dry for this weekend, a far cry from their solutions 24
hours ago.  The ensemble members within the GEFS, however, show
quite a bit of spread.  Therefore, will lower pops for this weekend
but will not remove them completely.

KD

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 544 PM CST Sun Dec 4 2016

Lots of uncertainty in some of the specific details for the TAFS
tonight. I followed the general scenario of the stratus which is
currently covering IL and the eastern half of MO gradually
clearing east tonight. The issue is what happens in the wake of
the clearing with weak surface ridging sliding across the area and
attendant light surface winds. T/Td spreads are already low and
virtually all of the guidance suggests that fog will develop,
potentially as early as later this evening, but especially
overnight. The disparity is how the intensity of the fog and how
visibilities may drop. Some of the guidance is tanking
visibilities to below 1SM from northeast/central MO into west
central IL which would include both KCOU and KUIN. My confidence
is too low at this point to forecast LIFR conditions and at this
point opted for IFR flight conditions in that area, with MVFR
flight conditions/visibilities for the St. Louis region. Present
thinking is any fog should dissipate by mid Monday morning with
VFR thereafter.

SPECIFICS FOR KSTL:

Current thinking is stratus will persist through mid evening with
clearing around 04Z. At some point overnight I expect fog to
develop but there is alot of uncertainty in how the low the visibility
may drop. I have opted for MVFR flight conditions/visibility developing
overnight. Present thinking is any fog should dissipate by mid
Monday morning with VFR thereafter.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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