Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 042110
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
310 PM CST Sun Dec 4 2016

.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 18z TAFs through 18z Monday Afternoon)
Issued at 1203 PM CST Sun Dec 4 2016

Snow will be shifting east of UIN by about 20Z this afternoon
with rain shifting east of the St Louis metro area by about 20Z
as well. Could not rule out a very brief period of rain/snow mix
in the St Louis metro area before ending. IFR stratus cloud deck
may rise into the MVFR catagory before it clears out in COU early
this evening and at the rest of the taf sites late this evening.
The surface wind will become westerly late this afternoon after
passage of a weak surface trough, then light this evening. There
will be at least light fog late tonight/early Monday morning due
to a clearing sky, light surface wind, and saturated grounds from
recent rain/snowfall. The surface wind will increase from a
southeast direction by late Monday morning as a surface high
shifts east of our area.

SPECIFICS FOR KSTL: The rain will shift east of STL by about 20Z
this afternoon. Could not rule out a very brief period of
rain/snow mix before ending. IFR stratus cloud deck may rise into
the MVFR catagory before it clears out late this evening. The
surface wind will become westerly late this afternoon after
passage of a weak surface trough, then light this evening. There
will be at least light fog late tonight/early Monday morning due
to a clearing sky, light surface wind, and saturated grounds from
recent rain. The surface wind will increase from a southeast
direction by late Monday morning as a surface high shifts east of
our area.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     30  47  37  43 /   5   5  70  30
Quincy          29  45  31  38 /   5   5  30  10
Columbia        29  50  33  41 /   0   5  40  10
Jefferson City  29  50  34  42 /   0   5  50  10
Salem           32  47  39  44 /  10   5  90  50
Farmington      30  48  37  45 /   5  10  90  20

&&

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

$$

WFO LSX



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