Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160441

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1141 PM CDT Fri Sep 15 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 309 PM CDT Fri Sep 15 2017

High pressure dominates the area ahead of low pressure system
currently centered over NE. Southerly surface flow will push temps
this afternoon into the upper 80 to low 90s. A tightening pressure
gradient ahead of low will increase the winds this afternoon
especially in the NW forecast area where 15mph are expected.
Overnight lows will be a few degrees warmer than last night due to
increasing low-level moisture and persistent southerly flow. Some
high clouds return on Saturday but temps will once again reach the
upper 80s as the southerly flow persists.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 309 PM CDT Fri Sep 15 2017

Above average temperatures should persist through the end of next
week. Specifics in the precipitation chances are a bit more uncertain.

An upper ridge axis will be centered through the lower-mid MS Valley
early Saturday evening. This ridge will get suppressed slightly
southward on Saturday night as a short wave tracks e/ne through the
mid-upper MS Valley. The short wave will also help push a front into
northwest/north-central MO by early Sunday morning, slightly
slower than previous model runs. Steep mid level lapse rates, and
increasing low level moisture/instability along and ahead of the
advancing cold front will result in an environment supportive of
thunderstorms from northeast/central MO into west central IL.
Activity that moves into this area will likely be showers/storms
that initiate futher west/upstream early Saturday evening. The
front will be slow to advance south/southeast on Sunday owing to
only weak surface high pressure to the north and the exiting short
wave. The sagging front and any prefrontal boundaries however will
support a chance of scattered storms on Sunday, both residual
morning activity and then new afternoon development. Things get a
bit more fuzzy Sunday night into the first part of next week.
There is some debate amongst the models on the southward progession
of the front, and I would lean towards the lesser southward
movement shown in the GFS and ECMWF which would have the front
sagging to near I-70 by Monday morning. An additional short wave
is forecast to track through MO in the westerly flow aloft on
Sunday night, supporting a threat of showers and storms and also
further suppressing the upper ridge southward. Yet another short
wave is forecast to traverse the area on Monday, albiet slight
further north. There is actually quite a bit of model generated
precipitation from all the guidance on Monday, some across
southern MO well south of the anticipated front. I certainly can`t
rule such southern exent to the threat of showers and storms away
from the front, especially if any residual boundaries from the
overnight activity reside in this area.

Tuesday and Wednesday will perhaps be the days with a minimum in
coverage and chance of showers and storms. The front should have
retreated well to the north and an extensive warm moist unstable
warm sector will be present, which is generally support of only diurnally
driven pulse activity. Beyond Wednesday there is growing uncertainty
and confidence lowers as the GFS and ECMWF diverge on the progression
of the western longwave trof, preceding short waves, and attendant
surface boundaries.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1140 PM CDT Fri Sep 15 2017

VFR conditions, dry wx, and southerly surface winds will prevail
at the TAF sites thru the valid period. Patchy fog at SUS or CPS
is looking less probable given current dewpoint depressions and
will continue to leave any fog mention out. Pcpn probs begin to
increase from the west late in the valid period, but are not high
enough yet by 06z/Sat for a mention.





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