Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250410

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1110 PM CDT SAT SEP 24 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 316 PM CDT Sat Sep 24 2016

Getting a little more diurnal cumulus clouds today than previous
days with slightly more favorable low level moisture and
instability. Widely scattered showers/storms have also recently
developed this afternoon which will likely impact parts of northeast
and central MO.  This cumulus field along with the showers/storms
should dissipate early this evening due to the loss of daytime
heating/surface instability. Showers and storms should move back
into northeast MO late tonight ahead of an approaching upper level
trough and associated cold front. Unseasonably warm and muggy
conditions can be expected tonight with high surface dew points for
late September. Lows tonight will be about 15 degrees above normal
for late September. There may also be patchy fog late tonight across
parts of southeast MO and southwest IL where the least cloud cover
and lightest surface winds are forecast.


.LONG TERM...  (Sunday through Next Saturday)
Issued at 316 PM CDT Sat Sep 24 2016

The synoptic pattern at the start of the long term period will
feature a longwave trough across portions of the Central Plains,
with downstream ridging across the Midwest.  This trough will split
into two branches during the day on Sunday, with one part becoming a
cutoff low across the Desert Southwest, and another more progressive
branch digging into the Northeast by early next week.  The latter
will help push a strong cold front through the region on Sunday,
which will bring much cooler and drier weather to the region for
much of next week.

Main focus of the period will be the frontal passage Sunday evening.
Ahead of the front, temperatures will rise into the upper 80s to
near 90 once again across eastern MO and western IL.  However, cloud
cover will quickly begin to filter in from the west through the
daylight hours ahead of the upper-level trough and surface cold
front.  How quickly these clouds fill in will likely affect our
convective coverage Sunday afternoon/evening.  If we are sunny
through much of the morning, then convective coverage will likely be
a bit higher than the current 60% chances.  However if clouds move
in quicker and limit destabilization, then precipitation coverage
along/ahead of the front will likely remain more scattered in
nature. Given the cooler mid-level temps remaining well off to the
northwest with the core of the trough, mid-level lapse rates will be
weak. Additionally, the better kinematics will be displaced behind
the frontal surface, so severe weather is not anticipated.  Main
threat will be locally heavy rainfall.

The front will push through Sunday evening, bringing the coolest
temperatures thus far in this young meteorological fall season.
Highs Monday through Wednesday will be confined to the lower 70s
(some areas may not even get out of the 60s Monday afternoon).
Overnight lows will dip into the upper 40s and lower 50s!

A slight moderating trend is expected late next week as the Canadian
surface ridge shifts off to the east.  Look for highs to recover
back into the mid/upper 70s with overnight lows in the low/mid 50s.
Dry weather looks to persist through the upcoming workweek.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1058 PM CDT Sat Sep 24 2016

Still need to watch for some radiational fog overnight at KSUS and KCPS,
however confidence is only modest so added a TEMPO group for MVFR
visibilites. Think the terminals will remain void of any precipitation
overnight into early Sunday morning as activity currently in
western MO is expected to weaken and dissipate.

A cold front will move across central and eastern Missouri from
late Sunday morning into the afternoon, passing through KSTL near
00Z. Expecting a lull in precipitation during the first part of
the day with development of scattered showers and thunderstorms
ahead of the front Sunday afternoon, expanding in coverage into
the evening as the front moves through St. Louis into southwest
IL. It also appears a swath of scattered showers may linger in
the wake of the front and this is most likely to bring any
precipitation to KUIN and KCOU on Sunday afternoon. The greatest
probability of showers and thunderstorms and terminal impacts will
be in the St. Louis region centered near 00z.


VFR flight conditions should prevail tonight and into at least
mid-morning on Sunday. Scattered showers and thunderstorms will
develop along/ahead of an advancing cold front on Sunday
afternoon expanding in coverage into the evening as the front
moves through St. Louis. The greatest probability of showers and
thunderstorms and terminal impacts will centered near the 00Z/26
time frame as the front moves through the area.





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