Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 290836

336 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

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.


.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.



.AVIATION:  (For the 06z TAFs through 06z Friday Night)
Issued at 1105 PM CDT Thu Aug 28 2014

The GFS model appears overdone with its precipitation forecast
late tonight and will keep tafs dry. There may be a little fog
late tonight/early Friday morning, mainly in the St Louis metro
area, and especially where there was recent rainfall early this
evening. The MOS guidance does not have as low of vsbys as it has
forecast the past few nights. It appears that the surface wind in
UIN and COU will be a little stronger than the previous couple of
nights. Sely surface wind late tonight will become sly on Friday
with scattered storms expected again during the late afternoon and
evening hours due to unstable conditions ahead of an approaching
upper level disturbance.

Specifics for KSTL: A light sely surface wind late tonight will
become sly on Friday and increase to 9-11 kts in the afternoon.
Scattered diurnal cumulus clouds should develop late Friday
morning and afternoon with scattered storms in the late afternoon
and evening.





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