Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 291148
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
648 AM CDT Fri Aug 29 2014

.SHORT TERM:  (Through Sunday night)
Issued at 326 AM CDT Fri Aug 29 2014

The primary forecast concern over the next 7 days is precipitation
chances.

An upper trough axis is forecast to move across the area
today/tomorrow accompanied by SH/TS. A surface low moving into
the Great Lakes will drag a cold front into MO/IL on Sat, but the
temperature gradient is forecast to be fairly weak and this looks
like more of a gentle wind shift line rather than a true exchange
of air masses. Even so, models show a secondary vort max clipping
the northern CWA on Sat and the wind shift line might act as a
focus for SH/TS development. It doesn`t look like solid rain for
the whole weekend, but it`s tough to rule out scattered SH/TS in
almost any forecast period because isentropic ascent starts to
increase ahead of the next trough almost as soon as the Fri/Sat
trough moves away from the area. The most likely time period for
mostly dry weather across most of the LSX CWA appears to be from
late Sat night through late Sun morning.

Kanofsky

.LONG TERM: (Monday through Thursday)
Issued at 326 AM CDT Fri Aug 29 2014

Precipitation chances increase again on Mon/Mon night ahead of an
approaching longwave trough. With several embedded vorticity
maxima moving through the base of the broad trough, scattered
SH/TS will be possible across MO/IL through Tue as a cold front
sinks towards the MO/IA border. The best opportunity for
precipitation appears to be on Mon morning as the nocturnal LLJ
veers and becomes focused across the CWA. The temperature gradient
is forecast to be stronger with this front compared to the
previous one, but models still show that the synoptic front never
makes it very far into MO before it is lifted back northward as a
warm front. Broad upper troughing then gives way to quasizonal
flow by Tue night followed by weak upper ridging towards the end
of the week.

Temperatures look seasonally warm, although there is more
uncertainty with temps on days when precipitation is expected.
If recent model runs are correct with the expansion of the upper
high over the southeastern CONUS, then expect a warming trend for
late next week.

Kanofsky

&&

.AVIATION:  (For the 12z TAFs through 12z Saturday Morning)
Issued at 638 AM CDT Fri Aug 29 2014

Precipitation chances will be on the increase tonight as a
weakening short wave trof move across the area. Models all seem
to agree that the accompanying surface cold front will not make
it across the area. With upper level support weakening and no
strong forcing at the surface, other than outflows, that puts
some question into the timing and coverage of the precipitaton.
Have thus settled for prob30s for this evening and overnight.

Specifics for KSTL: Will maintain vcts for this afternoon,
although with the warm front further north, suspect the coverage
may be less. Question is with the approaching but weakening system
tonight. Models all want to bring in precipitation, but seem to
what to highlight areas to the north and south. A prob30 looks
good for this evening and overnight.

JPK

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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