Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 311212

712 AM CDT Sun Aug 31 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 330 AM CDT Sun Aug 31 2014

Today will be the pick day of the remaining holiday weekend. Some
patchy fog will be possible during the early morning hours across
parts of northeast and central Missouri. There could also be a
spot shower across parts of southwest and south central IL through
around 15z in association with weak convergence at H85 and pooling
moisture. Otherwise the day will be rather seasonable with warm
temperatures and a threat of isolated diurnally driven thunderstorms
across southeast and east central MO and southwest IL.


.LONG TERM:  (Tonight through Saturday)
Issued at 330 AM CDT Sun Aug 31 2014

A rather active weather regime unfolds later tonight as the broad
progressive longwave trof currently centered in the Great Basin
moves into the central U.S. The combination of a lead fast-moving
shortwave and intensifying and veering southwesterly LLJ is
expected to help produce a progressive MCS, which should stretch
from NW IL across northern MO at daybreak on Labor Day. Showers
and thunderstorms should persist through at least mid morning
across northern MO into west central IL with the southwesterly LLJ
aiding development on the western flank and the spread into
central MO. The convective evolution from late morning into the
early afternoon is a little unclear. Overall the system should go
through a decaying stage with motion to the east, but there could
be continuous convective flare-ups on the western flank. By mid-
late afternoon the combination of the southward moving cold
front/residual boundaries and strongly buoyant air south of the
boundaries should result in new convective development. Improving
forcing from late afternoon into the evening via another shortwave
and the veering LLJ is expected to produce upscale growth into one
or more MCSs. Deep layer shear is rather impressive at 30-40+ kts
supporting an organized severe weather threat.

Any ongoing convection on Tuesday morning should be focused
across southern MO and southern IL in advance of the southward
moving cold front with convective cold pools and outflows well in
advance of the front. Tuesday afternoon and night the main threat
of showers and thunderstorms appears to be confined to southern MO
and southern IL in the vicinity of the stalling front. The front
will then retreat northward on Wednesday with heights aloft
beginning to rise in the wake of the progressive upper trof.
Heights will continue to rise through late Thursday as the upper
ridge once again establishes itself across the southern half of
the Nation, with summer heat in full swing. There is uncertainty
in the period Fri-Sat. Another cold front is forecast to move into
the through the area in this time frame with the ECMWF flattening
the northern extent of the upper ridge quicker and about 18h
faster with the cold front.



.AVIATION:  (For the 12z TAFs through 12z Monday Morning)
Issued at 616 AM CDT Sun Aug 31 2014

Any lingering light fog should mix out this morning. Some diurnal
cumulus is possible, especially at KSUS/KCPS. Mid and high clouds
will increase later in the forecast period ahead of an approaching
cold front. A thunderstorm complex should be approaching KUIN some
time after 06z but the timing is not exact. Just beyond the end of
the 24-hr TAF period, showers and thunderstorms are expected to
affect most of the terminals.

Specifics for KSTL: VFR for at least the first 0-12 hours and
possibly through most of the 30-hr period. There may be some
diurnal cumulus clouds today followed by increasing mid/high
clouds later tonight. Thunderstorm chances increase considerably
after 12z due to an approaching cold front.





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