Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161157

657 AM CDT Tue Sep 16 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 330 AM CDT Tue Sep 16 2014

Predawn low cloud imagery indicates that southward progress of
clearing line (near a Sante Fe-Barry line at 0745z) is agonizingly
slow, probably in part due to low level flow taking on a bit of an
eastward component.  Extrapolation of the back edge of the clouds
suggests clearing reaching a KVIH-KSTL-KMTO line by around 15z, with
forecast soundings then indicating the clearing accelerating across
the southeast half of the CWA during the late morning and early
afternoon due to combination of the southward advection of drier air
as well as fairly strong mid-September sunshine.  Will also continue
a mention of early morning patchy fog for those areas that are
expected to be mostly clear by 11z, as t/td spreads in northern
areas are fairly low.

Highs should be in the 60s over most of the FA.


.LONG TERM:  (Tonight through Monday)
Issued at 330 AM CDT Tue Sep 16 2014

Showers and storms may move into central MO late tonight due to
low-mid level warm air advection ahead of a weak northwest flow
shortwave, and as return flow on the backside of the surface ridge
shifting east of the region brings low level moisture and
instability back to southwest MO.  The potential for convection will
continue Wednesday and Wednesday night, mainly across central and
southeast MO along and north of a weak warm front.  A strong surface
ridge will build south southeastward through the Great Lakes region
Wednesday night and Thursday which should bring slightly drier and
more stable air southwestward into our region, likely ending the
threat for any convection even across the southwestern portion of
our forecast area Thursday and Thursday night.  Warmer temperatures
can be expected on Friday as an upper level ridge shifts eastward
into MO and southerly surface/low level flow strengthens over the
region.  Strong shortwaves will break down this upper level ridge
and send a cold front southeastward through our forecast area
Saturday night and Sunday morning.  This should lead to the best
chance of rain during this forecast period, beginning Saturday
afternoon and ending Sunday morning. Colder air will move into the
region behind this front, plus the ECMWF model also has deep upper
level troffing moving eastward through the northern Plains and
Great Lakes region Sunday and Sunday night resulting in below
normal temperatures to start the next work week. The GFS model is
a little slower with the upper level trough and hence also slower
bringing the coolest air into our region.



.AVIATION:  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 638 AM CDT Tue Sep 16 2014

Over the past several hours the n edge of stratus deck has become
increasingly ragged, and at 1115z roughly stretched from just
south of KSPI to near KMHL with a secondary large hole developing
in the cloud deck over sw IL just east of STL metro area. Believe
these trends indicate that the drier air is beginning to make
inroads into the lower level moisture. The cloud deck will be
south of KUIN this morning, although some fog is still possible
with low t/td spreads. Low clouds have also exited KCOU which will
also make it suspectable to fog; however am also seeing a
westward expansion of the low cloud deck just e of KMYJ, so
believe that back edge of stratus may clip KCOU in the 14-15z
time frame. For STL metro sites have followed trends of obs in and
north of area, with a mix of MVFR ceilings/vsbys and occasional
IFR cigs for several more hours, followed by the low cloud deck
scattering out by late morning. By afternoon, clouds throughout
the FA should be limited to scattered CU/SC aoa 3kft.

Specifics for KSTL: Although hole has developed in cloud deck e of
KSTL, northerly low level trajectories should mean prevailing MVFR
ceilings aob 1500 ft through 15z, with lower stratus occasionally
producing vsbys 3-5sm and cigs 700-900ft. Believe there should be
a rapid improvement 16-17z, with scattered clouds aoa 3kft for the
afternoon hours.





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