Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160839

339 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 06z TAFs through 06z Tuesday Night)
Issued at 1123 PM CDT Mon Sep 15 2014

MVFR Ceilings will continue to clear slowly from the north. Where
the sky becomes clear tonight, expect fog develop. Unsure how
dense the fog will be, but low end MVFR/IFR is certainly possible.
Further south across central and east central Missouri as well as
southwest Illinois and southeast Missouri, expect MVFR cigs to
prevail for the rest of the night...and likely lowering to IFR
ceilings in places along and south of the I-70 corridor. Low end
MVFR and IFR cigs will prevail until sunrise when ceilings should
lift and scatter by mid-late morning.

Specifics for KSTL:

Expect MVFR ceilings to lower to IFR before sunrise Tuesday
morning. Latest thinking is that the ceilings won`t be as low as
previously forecast, but still AOB 900FT. The timing of the
lowering is still uncertain as well, but feel it should be down to
IFR by 09Z-10Z. Ceilings should lift and scatter by mid-late
morning Tuesday with VFR conditions to prevail after.





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