Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201136

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
636 AM CDT Fri Oct 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 305 AM CDT Fri Oct 20 2017

A weak shortwave trough currently near the Oklahoma/Texas border is
forecast to move northeastward into central Missouri very late this
afternoon. Not expecting a whole lot of impact in terms of sensible
weather other than an increase in mid/high level cloud cover. Cannot
totally rule out a few sprinkles across portions of central Missouri
associated with this feature, but dry low levels of the atmosphere
precludes a better chance for precipitation reaching the ground.
High temperatures this afternoon should be similar to that of
yesterday given very little to no change in ambient airmass. Expect
highs in the upper 70s to low 80s across the bi-state region, though
there is a possibility that portions of central Missouri could be a
couple of degrees cooler than forecast if cloud cover thickens early
enough this afternoon.

A very mild night is expected tonight as south/southeast winds
will be stronger associated with tighter pressure gradient. Cloud
cover will also be on the increase overnight which will also help
keep low readings on the mild side. Temperatures are only forecast
to dip into the upper 50s to low 60s. Seeing a better chance for
showers late tonight, mainly across portions of central and
southeastern Missouri as another shortwave transverses the area in
conjunction with broad low-level warm/moist advection. However,
forcing for ascent does not look too focused nor strong, so
thinking isolated coverage is the most likely scenario for late


.LONG TERM...  (Saturday through Thursday)
Issued at 305 AM CDT Fri Oct 20 2017

The synoptic pattern at the start of the long term period will
feature a fairly amplified flow regime over the CONUS, with a deep
trough digging into the Intermountain West and a downstream ridge
anchored over the eastern seaboard. This trough will slide into the
area this weekend, becoming reinforced by another strong trough
diving out of central Canada by early next week.  These systems will
result in a fairly active period Saturday night into Sunday, before
much cooler conditions arrive by the middle of next week.

The main focus of the period continues to center on Saturday
night/early Sunday as a strong cold front associated with the
aforementioned upper trough plows through the region.  The latest
guidance has trended a tad slower with the frontal passage, but the
front still looks to push through the area between 06Z-18Z Sunday.
Ahead of the front, a decent instability axis characterized by
MLCAPEs of 1000-2000 J/kg should build across portions of eastern KS
and western MO.  Given sufficient deep-layer shear oriented parallel
to the synoptic cold front, organized strong/severe storms should
develop and become linear by Saturday evening.  As they approach
central Missouri towards 06Z Sunday morning, they will encounter an
increasingly unfavorable thermodynamic environment.  Therefore,
storms are expected to begin weakening as they enter the western LSX
CWA. That being said, there does appear to be a brief window where
strong/severe storms will be possible during the early overnight
hours, where MLCAPEs of 400-700 J/kg will be possible on the eastern
edge of the instability gradient across central MO.  The main threat
would be locally damaging wind gusts. The storms should weaken
considerably by the time they reach eastern MO and western IL
(including the St. Louis metro).

After a warm day on Saturday (highs in the upper 70s/low 80s), they
will be cooler for Sunday behind the front.  The 00Z suite of
guidance continues to struggle with the development of a cutoff low
over the southern CONUS.  The ECMWF continues to remain more
progressive with it, while the GFS is a bit slower.  This has
ramifications on precip potential mainly across southwest IL Sunday
night into Monday.  For now, will keep low pops in that area to
account for the uncertainty.

A strong PV anomaly diving southeast out of Canada will help to
reinforce/amplify the trough over the eastern CONUS by early next
week, bringing a shot of much cooler air to the Midwest.
Temperatures Tuesday and Wednesday likely won`t make it out of the
50s, with lows near 40. In fact, these highs may even be a bit too
warm given the expected cloud cover within the cyclonic flow aloft
(and maybe even some light shower activity).



.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 627 AM CDT Fri Oct 20 2017

Some fog is possible early this morning at KSUS and KCPS.
Otherwise, south/southeast winds of around 10mph along with
dry/VFR conditions likely through the period. Low-level wind shear
appears likely overnight tonight for KCOU/KUIN.


Dry/VFR through the period with south/southeast winds around



Saint Louis     81  61  80  63 /   0  10   5  80
Quincy          77  60  79  57 /   0  10  10  90
Columbia        76  62  79  56 /   5  20  10  90
Jefferson City  79  62  81  57 /  10  20  10  90
Salem           79  54  78  62 /   0   0   5  80
Farmington      80  59  78  62 /   0  10  10  80




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