FXUS63 KLSX 172146

Area Forecast Discussion
National Weather Service Saint Louis MO
346 PM CST Sat Nov 17 2018

.SHORT TERM... (Through Late Sunday Afternoon)
Issued at 330 PM CST Sat Nov 17 2018

The cold front continues to progress southward through the region,
and at mid-afternoon stretched near a Effingham-Cahokia-Lake of
the Ozarks line. The cold front will continue to move southward
the remainder of the afternoon into tonight with surface high
pressure building into the area and surface winds becoming
northeasterly. Temperatures tonight and into Sunday morning in the
post-frontal air mass do not appear to be as cold as previously
thought. The temperature advection from the northeast component of
the surface wind, which will become a bit more northerly later
tonight, will keep the freezing and below temps largely confined
to the northwest third or so of the CWA. In fact my freezing line
is south of the RAP 2m temperature forecast. As far as precipitation,
the driving mechanism will largely be low-mid level frontogenetic
forcing with a slight contribution of weak large scale ascent
from a passing disturbance in the westerly flow aloft. This is
what is responsible for the west-east band of snow from northern
IL across southern IA into southern Nebraska at this time. This
forcing will move into the area tonight with the aforementioned
frontogenetic forcing aligned across the northern CWA, supporting
a band of rain changing to snow. The marginal temperatures,
forcing, and progged qpf suggest that snow accumulations up to
inch are possible across northeast MO into west central IL tonight
into early Sunday morning, and the best accumulation potential
will be on elevated surfaces. Further south, the precipitation
tonight will be more scattered or spotty in nature associated with
weaker frontogenetic forcing and even weaker large scale forcing
for ascent. The precipitation further south also appears to be
more of a rain or snow situation with more limited southward
extent of freezing temperatures tonight and gradually increasing
moisture in the ice crystal growth zone. All forcing weakens
rather quickly by daybreak Sunday and it remains this way until
early afternoon, suggesting any precipitation will be spotty and
light. During the afternoon a new wave of predominantly rain will
spread from southwest MO into southeast MO, and then across
southeast MO and southern IL in the evening. This precipitation is
associated with a positively tilted migratory short wave and the
entrance region of an upper level jet streak. Thermal profiles
suggest this should be all rain although on the far northern edge
of the precipitation shield there could be a few wet snowflakes
mixing in the evening. This precipitation should exit to the east
by midnight.


.LONG TERM... (Sunday Night through Next Saturday)
Issued at 330 PM CST Sat Nov 17 2018

Most of the week ahead looks uneventful, dry, and tranquil until
Friday into early Saturday. Northwest flow aloft will dominate
through Tuesday resulting in below average temperatures with
another reinforcing cold shot on Monday afternoon/night. By
Wednesday we see a pattern change with heights aloft on the rise
and a WAA regime becoming established, and this will result in
temperatures rising to near average and maybe above average
Wednesday and beyond. On Friday an approaching upper trof and
associated cold front will bring what looks to be a widespread
rain event.



.AVIATION... (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1157 AM CST Sat Nov 17 2018

The cold front has pushed to near a KCOU to KTAZ line at midday
with winds becoming north-northeast in its wake. Lagging the front
by 20-30 miles is an extensive area of IFR ceilings/flight
conditions which are prevalent at KUIN. The cold front will
continue to push south the remainder of today with IFR flight
conditions spreading into KCOU and the St. Louis terminals this
afternoon. After the stratus/IFR moves into the terminals it
should prevail through 18z Sunday. The best chance for
precipitation will be across northeast MO and west central IL
impacting KUIN. Initially this afternoon it should be rain tending
to a rain/snow mix and then snow from late afternoon into tonight.
Further south the threat of precipitation is lesser and coverage
will be more spotty or scattered. KCOU could see some light rain
or a rain/snow mix overnight into early Sunday, while the St.
Louis area looks largely to remain all rain at this time.


The cold front should move through the terminal around 20-21z
accompanied by a northeast wind shift. Stratus resulting in IFR
flight conditions will move into the terminal several hours after
fropa with current thinking 22-23z. Once conditions become IFR
they should remain IFR until Sunday afternoon when improvement to
MVFR is expected. We could see some spotty precipitation impact
the terminal overnight into Sunday morning. Present thinking is
temperatures will remain above freezing and this should be in the
form of light rain or drizzle.





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