Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231710

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1210 PM CDT Wed Aug 23 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 318 AM CDT Wed Aug 23 2017

If you like abundant sunshine, low humidity, and below average
temperatures in late August, then this will be pretty spectacular

A prominent upper trof across eastern NOAM will continue to deepen
over the next 24 hours. This will maintain a large surface high
pressure system across the region, whose center will move slowly
from the central Plains into the MS Valley today and into the
western Great Lakes region tonight. Relatively cloud-free skies are
expected across central MO, with a little diurnal cu from eastern MO
into IL. Highs will range from the upper 70s to lower 80s, or about
6-8 degrees below average.

Tonight will start off with clear skies and then we will see some
mid level WAA clouds in the northwest flow aloft during the
overnight hours. Still a great night and it might feel a bit chilly
once the sun goes down. Lows will be in the 50s, with most locations
close to 10 degrees below normal.


.LONG TERM...  (Thursday through Tuesday)
Issued at 318 AM CDT Wed Aug 23 2017

(Thursday - Saturday Night)

Main story will be the continued seasonably cool and dry weather
heading into this weekend. High pressure will be quasi-stationary
over the Great Lakes with northwest flow aloft helping to promote
the cool and dry weather the bi-state area will experience. After
an initial period of a clear or mostly clear sky, an increase in
mid/high level cloudiness is expected beginning Friday as a weak
disturbance aloft transverses the area. No precipitation is
expected however due to the weak forcing for ascent aloft and very
dry low levels of the atmosphere.

High temperatures in the upper 70s to low 80s are expected each day,
as although 850-hPa temperatures modify through the period, the
increase in cloud cover will help keep temperatures from moderating.
Lows will be mainly in the 50s, with the exception of portions of
the St. Louis metropolitan area which will likely dip into the low
60s each night. The coolest night appears to be Thursday night as
this is the night that should have the least amount of cloud cover
allowing for good radiational cooling.

(Sunday - Tuesday)

Forecast uncertainty increases substantially late this weekend
heading into early next week. Deterministic guidance seems to be
struggling with the mid/upper level low moving southward toward the
mid-Mississippi Valley, and in particular, the track of Harvey and
its potential interaction with the deepening trough across the
central CONUS.

Most likely outcome currently is for deepening trough to provide a
chance at showers and potentially a few thunderstorms mainly early
next week. Cold pocket of temperatures aloft suggests activity may
be focused diurnally on Monday and Tuesday afternoons. As for the
remnants of Harvey, tons of forecast uncertainty, but flow aloft
highly suggests a track into the Tennessee Valley and up towards the
interior of the northeast US (assuming it eventually gets caught up
in the westerlies). Only impact across our region in that
forecast scenario would likely be an increase in thick cirrus.

Temperatures are likely to moderate a bit at night due to increasing
cloud cover, while highs should stay predominantly in the upper 70s
to low 80s. Certainly looks like a cool end to meteorological summer.



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1209 PM CDT Wed Aug 23 2017

VFR conditions are expected through the period as a surface ridge
provides fairly light winds and mostly clear skies. Can`t rule out
some river fog near SUS/CPS early Thursday morning, but don`t have
high enough confidence in its occurence to include at this time.





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