Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232045

Area Forecast Discussion
National Weather Service Saint Louis MO
345 PM CDT FRI SEP 23 2016

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 343 PM CDT Fri Sep 23 2016

The synoptic pattern this afternoon continues to feature a longwave
trough across the western CONUS, with downstream ridging found
across much of the Central/Southern Plains into the Midwest.  This
regime will persist through the short term period.

Tonight will be similar to past nights, with some localized valley
fog and lows dipping into the mid and upper 60s.

On Saturday, the upper-level ridging that has been in place for the
past several days will slowly shift off to the east-southeast.
Winds at the surface will turn more to the SSE, which will help
dewpoints climb into the upper 60s to even low 70s.  Given the
increase in surface moisture, surface-based instability will build in
the afternoon hours. However, forecast soundings continue to exhibit
a substantial capping inversion around 600mb through the daylight
hours suggesting strong mid-level subsidence persisting.  Given the
lack of any appreciable surface convergence and the presence of this
mid-level inversion, will continue with a dry forecast although a
rogue shower/storm is not completely out of the question tomorrow
afternoon. Looks for highs once again topping out in the upper 80s
to near 90.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 343 PM CDT Fri Sep 23 2016

Convection should move into northeast and central MO Saturday night
as a deepening upper level trough moves eastward through the
northern Plains and the persistent upper level ridge over our area
begins to shift further southeast.  This upper level ridge along
with surface ridging extending from the Great Lakes and Ohio Valley
regions southwest into southeastern MO and southern IL will likely
keep areas from STL south and east dry through Saturday night.  The
showers/storms should eventually shift southeastward through most of
the forecast area on Sunday as a cold front drops southeastward
through our region and the upper level ridge continues to get
suppressed further south. This front should move southeast of our
forecast area by late Sunday night. The ECMWF model is not quite as
deep with the upper level trough compared to the NAM and GFS models.
Its QPF appears a little too light, especially for Sunday night, and
prefer the NAM and GFS model solutions which are similar.  The
precipitation should shift southeast of our forecast area already by
Monday morning. Cooler temperatures along with lower
humidities/surface dew points can be expected for late Sunday night
through Monday night due to cold air advection behind the cold front
and lower mid-upper level heights and eventual northwest upper level
flow as the upper level trough deepens amd closes off over the Great
Lakes region.  The 850 mb temperature should drop down to around 7-8
degrees C in the STL area by 00Z Tuesday.  Should finally have below
normal temperatures Monday and Monday night across our area which
will make it feel like Fall. The dry pattern will continue through
the remainder of the extended forecast as a large surface ridge
moves slowly eastward through the region.  Will have moderating
temperatures back to more normal values by Thursday as an upper
level ridge shifts eastward into the region, while the surface ridge
shifts well east of our area with a return to south or
southeast surface winds on the backside of the surface ridge.



.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1210 PM CDT Fri Sep 23 2016

VFR conditions are expected through much of the TAF period. Main
exception will be at SUS/CPS, where some fog is once again
expected toward sunrise Saturday. Given a very similar setup to
the past couple of nights, have gone ahead and introduced IFR
vsbys. Otherwise, expect just some passing high clouds and light
southerly winds through the period.

VFR conditions expected through the period with just a few passing
high clouds and light southerly winds, becoming more SSE through
the day on Saturday.





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