Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142015
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
315 PM CDT Sat Oct 14 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 314 PM CDT Sat Oct 14 2017

A few showers and thunderstorms were moving through parts of
northeast MO and west central IL this afternoon, just south of a
warm front and ahead of weak southwest flow shortwaves.  Relatively
strong and gusty south-southwesterly surface and low level winds
south of the warm front have lead to unseasonably warm temperatures
this afternoon, topping out around 90 degrees in the St Louis metro
area.  An upper level trough over the northern Plains will move
eastward into the Great Lakes region Sunday morning and send a
strong cold front southeastward through our forecast area tonight. A
line of storms is expected along this front which will sweep
southeastward through our area later this evening and overnight.
Some of the storms may be severe this evening across northeast into
central MO and west central IL with the most likely threat being
damaging winds.  The wind shear will be favorable for severe storms
due to a strong southwesterly low level jet and a strong mid level
wind max in the base of the upper level trough. The instability
early this evening will also be favorable for mid October with
surface CAPE values of 1500 to 2000 J/Kg across this area.  This
line of storms should gradually weaken late tonight due to weakening
instability as the storms move into east central and southeast MO
and southwest IL.  Lingering showers should continue across
southeast MO and southwest IL Sunday morning, followed by clearing
skies. Strong and gusty northwesterly surface winds will bring much
cooler and drier air into northeast and central MO and west central
IL late tonight and the rest of our area Sunday.  It will feel much
different and more like Fall Sunday with below normal highs forecast
plus windy conditions will make it feel colder.

GKS

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 314 PM CDT Sat Oct 14 2017

The synoptic pattern at the start of the long term period will
feature a trough sliding across much of the Great Lakes and Midwest
regions.  This will promote northwesterly upper-level flow over the
region through the middle of next week, before the flow turns more
zonal and eventually southwesterly by the middle/end of next week.

It appears to be a fairly quiet long term period, as an area of
sprawling high pressure moves into the central part of the country.
Much cooler temperatures are expected Sunday night into Monday. In
fact, given the expected mainly clear skies and light winds, some
areas may dip into the upper 30s Monday morning. This may promote
some patchy frost in a few of the typically cool areas, but a
widespread frost looks unlikely at this time.

Temperatures will slowly warm through next week as the surface ridge
slowly meanders east and the surface flow turns southerly.
Temperatures will warm through the 70s each day, perhaps approaching
the 80s by Friday of next week. It will be dry through much of the
period, although a cold front may approach the area by Saturday
bringing a chance for some showers. However, these is high
uncertainty regarding this front thus went with an ensemble approach
for now.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1142 AM CDT Sat Oct 14 2017

A cold front will drop southeastward through the taf sites
tonight. There may be scattered showers or thunderstorms in UIN
this afternoon ahead of the front. A line of thunderstorms will
move southeastward along the front through UIN and COU this
evening and through the St Louis metro area late tonight. There
may be strong winds with the storms in UIN and COU. A strong
southwesterly low level jet will lead to LLWS conditions in the
taf sites late this evening and overnight. The showers and storms
will shift southeast of UIN and COU by early Sunday morning, and
southeast of the St Louis metro area by late morning. There will
be a band of MVFR cigs which will move through the taf sites late
tonight and early Sunday morning along and just behind the cold
front. Gusty south-southwest surface winds this afternoon will
veer around to a northwesterly direction late tonight after fropa
and will be strong and gusty through Sunday morning due to a tight
surface pressure gradient.

SPECIFICS FOR KSTL: A cold front will drop southeastward through
STL late tonight. A line of thunderstorms will move southeastward
along the front through STL late tonight, beginning around 07Z
Sunday. A strong southwesterly low level jet will lead to LLWS
conditions in the STL area late this evening and overnight.
Lingering showers will shift southeast of the St Louis metro area
by late morning. There will be a band of MVFR cigs which will
move through the STL area late tonight and early Sunday morning
along and just behind the cold front. Gusty south-southwest
surface winds this afternoon and this evening will veer around to
a northwesterly direction late tonight and early Sunday morning
after fropa and will be strong and gusty through Sunday morning
due to a tight surface pressure gradient.

GKS

&&

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

$$

WFO LSX



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