Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171126
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
626 AM CDT Tue Oct 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 338 AM CDT Tue Oct 17 2017

Early this morning, a RIDGE of surface high pressure extended from
northeastern Texas into the Ohio Valley with near calm winds for
much of our region or a very light southerly flow.  Northwest flow
prevailed aloft, with a weak upper level disturbance over eastern
Nebraska.  The atmospheric column is bone dry, and the disturbance
is only able to generate an area of high cirrus clouds out ahead of
it into our region.  Needless to say, temperatures have cooled the
best in southern MO and IL, where winds are lighter and skies clear
the longest, with 30s rather common there, and 40s elsewhere.

The surface RIDGE axis of high pressure will remain nearly
stationary to our south as a cold front enters the Plains later
tonight.  This will maintain an initially light southerly flow over
our region this morning, gradually strengthening this afternoon and
continuing tonight in response to the tightening pressure gradient
from the approaching front.  The only clouds expected thru tonight
will be the area of cirrus currently moving overhead, and these
should exit by early afternoon with clear skies thereafter.

With the southerly flow, temperatures are expected to return to
above average values this afternoon, with the higher MOS values
favored, in the lower 70s for most locations.  Temps tonight should
be higher than persistence with the warming trend underway, and much
of the forecast area will be in the 40s.

TES

.LONG TERM...  (Wednesday through Monday)
Issued at 338 AM CDT Tue Oct 17 2017

The forecast remains tranquil for Wednesday through at least Friday
under the influence of a sprawling surface high centered over the
eastern U.S.  South-southwest low level flow will prevail over the
Mid-Mississippi valley on the western side of the high.  Heights
will begin to increase aloft toward the latter half of the week in
the wake of a fast-moving shortwave which will pass over the area on
Wednesday. Between the increasing heights and southwest flow, the
gradual warming trend will continue through the end of the week.
GFS and ECMWF push 850mb temperatures into the mid teens on Thursday
and Friday.  While forecast soundings don`t mix that high, it should
be good enough for upper 70s to low 80s for Thursday through
Saturday...especially on Saturday ahead of the next system.

Medium range guidance is in pretty good agreement with respect to
timing and and precip associated with the next strong trof to affect
the region.  The GFS and Canadian models are a few hours faster than
the ECMWF and tend to favor the slower solutions in the medium
range.  In this case the differences are pretty minor so think the
blended solution works out fine.  The cold front associated with
this system should move through the region Saturday night into
Sunday morning with precipitation along and ahead of the front. GFS
is forecasting limited instability at this time...but it`s still
pretty early to have high confidence in CAPE forecasts. The synoptic
setup is favorable for thunderstorms though so will keep storms
going in the forecast.  Guidance has rain likely ending after
frontal passage either Sunday afternoon or evening. Strong cold
advection in northwest flow behind the front should bring another
shot of more Autumn-like temperatures to the forecast
area...although current indications are that temperatures will
still be several degrees above normal.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 626 AM CDT Tue Oct 17 2017

VFR conditions, dry weather, and light southerly surface winds
will prevail at the TAF sites thru much of the valid period. While
min temps late tonight may still support values for steam fog
potential, prevailing flow from the south and slightly warmer
temps than the previous night will make it less likely. Have left
out for the upcoming night with the idea that it will be revisited
and looked at by subsequent issuances.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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