Area Forecast Discussion
Issued by NWS St. Louis, MO

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588
FXUS63 KLSX 180202
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
802 PM CST Sun Dec 17 2017

.UPDATE...
Issued at 802 PM CST Sun Dec 17 2017

It is trending more and more that we are going to see clouds hang
around for much of Monday for areas around STL metro and points to
the S and E. Clearing is still anticipated for areas N and W of
STL metro during Monday afternoon, with overcast skies in the
interim.

With this in mind, and a steady SW flow expected, temps probably
aren`t going to drop much tonight despite the long December
night. Bumped min temps above MOS values and closer to
HRRR/RAP/GLAMP values instead, which should have a firm handle on
what should be stagnant conditions.

Persistent clouds into Monday, especially for the STL metro area
and areas to the SE will keep temps down, especially considering
the weak December sun. Trended max temps on Monday closer to MET
MOS values which also hang on to clouds longer than the MAV MOS.
This should result in a lot of lower to mid 50s for maxes for the
forecast area.

Drizzle still looks tough to get a handle on for overnight with
difficulty especially in identifying persistent weak low level
convergence. However, recent observations suggest some drizzle has
already occurred, especially in the NE MO and W IL, with VSBY
reductions noted. Will continue patchy drizzle forecast for most
areas for now, but fortunately temps much too warm to be concerned
about any freezing.

TES

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 301 PM CST Sun Dec 17 2017

Looks like another mild night tonight with the warm front stalled
well north of the area along the Missouri/Iowa border. Light
south to southwest wind will prevail with an overcast sky and
potentially some patchy drizzle. These conditions do not support a
tremendous amount of cooling tonight, so diurnal temperature drop
looks to be only 8-12 degrees...perhaps even less. Stuck pretty
close to the consensus MOS forecast with temperatures ranging from
the upper 30s to low 40s. There may be a bit of drizzle
overnight...forecast time cross-sections show some weak lift in
the lowest 200-250mb along with saturated conditions. Guidance is
not producing any QPF and MOS has single digit PoPs. I therefore
put in patchy drizzle with measurable PoPs below 15%.

Clouds are expected to stick around at least through most of the
morning on Monday. Southwest flow should eventually erode them
from west to east during the afternoon. The southwest flow will
help to push temperatures into the upper 50s across much of the
region. Some 60s are even possible if the sky clears out faster
than anticipated.

Carney

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 301 PM CST Sun Dec 17 2017

The upper air pattern will offer very few opportunities for
widespread precipitation until the end of the week. A few
disturbances will move through NW flow across the upper Midwest,
but these features will be too far north and east to affect
precipitation chances across the LSX CWA. The only appreciable
chance for rainfall between Monday night and Thursday night will
be when the remnants of a cutoff low pass through AR/LA on
Tue/Wed, but even these rain chances will be limited to the far
S/SE CWA.

Models are in better agreement through approximately 00z Fri
regarding the upper air pattern leading up to the busy holiday
travel period. The GFS/ECMWF/GEM all show a positively tilted
trough with a strong PV anomaly in its base located over the Four
Corners region, placing the LSX CWA within southwesterly flow
aloft and downstream of the trough axis. Although there is
limited agreement thereafter, all 3 models also show another
disturbance reaching the PacNW by 00z Sat and then moving into the
western CONUS.

The aforementioned limited agreement through 00z Fri extends to
broad depictions of surface features including as a warm frontal
passage sometime on Thu, surface cyclogenesis over the plains, a
strengthening baroclinic zone associated with the developing
surface low, and its subsequent passage as a cold front on Fri or
Fri night. Although there is broad agreement between models in the
most general sense, there are considerable differences regarding
fine-scale details and critical elements such as the orientation
of the cold front and how quickly it moves across the area.
Nevertheless, this system-- as currently forecast-- is expected
to bring widespread precipitation to the area from late Thu night
through early Fri evening. The most likely time period for
precipitation based on this forecast is during the day on Friday,
and the most likely precipitation type will be rain. Some snow
could mix in with the rain on late Thu night and/or on Fri evening
when the column is cold enough to support snow, but there is low
confidence at this time (especially for Fri night) due to
consistently poor model agreement regarding the position and speed
of the cold front.

Temperatures will be above average to well above average for most
of the week. A weak cold front will sag southward into MO/IL on
Tue night, but overnight lows on Tue night and daytime highs on
Wed will still be above average even within the slightly cooler
post-frontal air mass. Temperatures will warm into the upper 50s
to near 60 degrees again on Thu after a warm front lifts through
the region ahead of an approaching low pressure system. It is
difficult to glean any insight into Friday`s temperature trends
because of the aforementioned model disagreements regarding the
position and timing of the cold front on Fri/Fri night. An earlier
fropa would support colder temperatures areawide on Fri, but a
slower fropa would support another day of above-normal
temperatures across at least the eastern or southeastern half of
the CWA. Models are in decent agreement that a much colder air
mass will settle into the central CONUS next weekend and early
next week.

Kanofsky

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 625 PM CST Sun Dec 17 2017

MVFR CIGs are expected to prevail for much of the period well
into Monday for the TAF sites. A few areas of IFR CIGs exist
around UIN and COU and have a short period mainly this evening for
that, but this should avoid the STL metro sites. Some drizzle is
also expected overnight. Have delayed the clearing a bit but will
finally mean sites can return to VFR Monday afternoon.
Otherwise, SW surface winds will continue thru the period. The
late day clearing of clouds could complicate things for potential
fog Monday night depending on when it happens.

SPECIFICS FOR KSTL: MVFR conditions to prevail until Monday
afternoon with clearing expected. Patchy drizzle also expected
overnight and as confidence builds, may need to add VSBY
reductions which is more typical.

TES

&&

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

$$

WFO LSX



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