Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 162120
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
320 PM CST Tue Dec 16 2014

.SHORT TERM: (Through Friday Afternoon)
Issued at 320 PM CST Tue Dec 16 2014

(Tonight)

Deep cyclonic flow and cold air advection in the wake of yesterday`s
storm system kept diurnal temperature rises minimal today. Solid
shield of stratus yielded an occasional period of drizzle or
flurry, but nothing measurable was observed.

Cloud trends for tonight are uncertain. An extensive area of stratus
was located upstream, and northwest winds near 925mb remain
favorable for continued advection of these clouds into our area. On
the other hand, a hole was located in the cloud deck over western
IA, and if this area does not fill in, then skies should start to
clear over the Columbia and St. Louis areas between 23-02z. RAP
plots of increasing condensation pressure deficits between 975-900mb
tonight along with BUFKIT soundings which depict drying aloft both
support the idea of dissipating stratus/stratocu this evening,
although mid/high clouds will be increasing later tonight regardless
of what happens with the stratus/stratocu. However, BUFKIT profiles
also show a fair amount of subsidence above the low cloud layer,
which may act to trap moisture below the inversion and keep the
clouds in place. Partial clearing appears more likely over central
MO than elsewhere. Winds should remain around 10 kts for most of the
night.

CVKING/Kanofsky

(Wednesday - Friday)

Deep trof of low pressure currently coming onshore along the west
coast with our eyes tracking a pair of circulations that will bring
two chances of precipitation to the area the next seven days.

The first circulation will come ashore along the Baja tonight and
lift northeast across the southwest CONUS and southern Plains on
Wednesday. Warm advection will ramp up across the area Wednesday
night with light snow expected to rapidly develop and move into the
area from the southwest. Models continue to "moisten" the forecast
with higher QPF noted. Have increased POP and expected snowfall
amounts with up to two inches now possible across central Missouri
and the eastern Ozarks and an inch into the western suburbs of the
St. Louis Metro Area. Unfortunately it appears that the snow will be
hitting the St. Louis area either right before or at rush hour
Thursday morning. Luckily, ground temperatures are relatively warm
and air temperatures should be in the upper 20s or lower 30s. The
hope is that impact on travel will be low with this event. Snow will
taper off during the morning hours on Thursday as the shortwave
lifts east of the area. Lingering low level moisture and rising
surface temperatures suggest drizzle will be possible in the
afternoon hours.

The second and more potent circulation of the two will dig even
further south across the Baja before lifting east/northeast across
the deep south on Friday. Meanwhile, a northern stream shortwave
will also be diving southeast across the Plains. Unfortunately for
the snow lovers these two systems don`t appear to fully phase until
they are east of the area. Some snow flurries or light snow will be
possible across the far southern CWA Friday afternoon.

CVKING

.LONG TERM: (Friday Night through Next Tuesday)
Issued at 320 PM CST Tue Dec 16 2014

Continuing with the late week system, models push the southern
system, which will be the main driver of pcpn, well to our south
with some interaction with a northern system still anticipated to
help bring perhaps some light pcpn well to the north into a good
part of our region for Friday night. PoPs remain low because of
concerns of measurability, with many locations perhaps tracing out
instead. Pcpn-types favor snow.

The northern system itself will not quite get thru the forecast area
until later on Saturday and maintained slight chances for that
reason.

Southerly flow at the low levels will then be seen for Sunday and
Monday, helping temps to at or above normal levels once again.

Heading towards the middle of next week, a longwave RIDGE will
attempt to re-establish itself over western North America in
imitation of the dominant pattern in November, with a strong
shortwave disturbance dropping down in NW flow and a developing
upper level TROF over the central CONUS.  Look for increasing PoPs
for this period with temps beginning to drop back to below normal
once again.

Christmas could be pretty quiet across our region, taking a sneak
peak at the long range models.  But it looks to only be a timing
issue, with one system leaving and another lurking about a day away,
so this could change very quickly given all of the typical timing
issues the models have this far out.

TES

&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1112 AM CST Tue Dec 16 2014

The primary forecast question for this set of TAFs is whether the
stratus will end during the next 24 hours. On one hand, there`s an
extensive area of stratus located upstream as well as northwest
winds which will continue advecting the clouds into MO/IL. On the
other hand, there`s also a hole in the aforementioned cloud bank,
and RAP forecasts (CPD) as well as BUFKIT soundings show
additional drying, which suggests that existing clouds should
start to dissipate and that it will be difficult for new clouds to
develop even within cyclonic flow. On the other other hand, the
cig/vis trend viewer in AVNFPS is evenly split on whether clouds
usually persist through 12z or become less likely after 04-06z,
depending on what conditions look like at 21z and 00z. In short,
there is a surprisingly high amount of uncertainty with the cig
forecast. This set of TAFs splits the difference and keeps MVFR
ceilings in place through 06-08z, but it`s equally likely that
ceilings could improve earlier than forecast or persist later than
forecast.

Specifics for KSTL: It`s equally likely that MVFR stratus could
persist beyond 12z or depart earlier around 06z. There might also
be a brief break between 23z-02z, depending on what happens with
the hole in the clouds over IA which was noted on satellite
imagery at TAF issuance. Regardless of what happens with the MVFR
stratus, increasing mid and high level clouds are expected after
12z.

Kanofsky

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     27  36  30  36 /   0   0  60  60
Quincy          23  34  24  32 /   0   0  20  50
Columbia        23  36  29  36 /   0   5  80  60
Jefferson City  23  36  29  37 /   0   5  80  60
Salem           27  36  30  36 /   0   0  30  60
Farmington      26  38  29  36 /   0   5  80  60

&&

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

$$

WFO LSX



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