Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 310848

348 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 bouyant 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
movning 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 06z TAFs through 06z Sunday Night)
Issued at 1037 PM CDT Sat Aug 30 2014

Most of the showers/storms have dissipated this evening, although
there was patchy clouds around 5000 feet in height along with mid-
high level clouds streaming northeastward through the St Louis
metro area. As the mid-upper level cloudiness shifts slowly
southeast of the taf sites late tonight and the surface wind
becomes light the MOS guidance is hinting at the formation of
stratus clouds and fog late tonight/early Sunday morning. The
stratus clouds and fog should dissipate by late Sunday morning,
although there will likely be scattered diurnal cumulus clouds in
the St Louis metro area during the late morning and afternoon.
S-swly surface winds can be expected on Sunday and Sunday evening.

Specifics for KSTL: Much of the mid-high level cloudiness may
shift southeast of STL late tonight with light fog developing
towards morning as the s-swly surface wind becomes light. The NAM
MOS guidance was also hinting at the possible formation of stratus
clouds as well. The fog and any stratus clouds should dissipate by
late Sunday morning, although there will likely be enough residual
low level moisture for the formation of scattered diurnal cumulus
clouds late Sunday morning and afternoon. The surface wind will
increase again to around 8 kts in the afternoon from a s-swly





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