Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172022

Area Forecast Discussion
National Weather Service Saint Louis MO
322 PM CDT Tue Apr 17 2018

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 321 PM CDT Tue Apr 17 2018

Next storm system is expected to eject due eastward across the
south-central Plains tonight. Main impact will be an increase in
cloudiness across the bi-state area, but portions of northeast
Missouri and west-central Illinois may be on the southern fringe of
the light rain late tonight into early Wednesday.

Surface low is forecast to move east of the CWA by early afternoon
on Wednesday. Main concern behind in its wake will be strong
westerly winds. Sustained wind speeds on the order of 15-25 knots
with gusts of 30-35+ knots appear likely, and wind advisory criteria
(26+ knots sustained, 39+ knot gusts) may be threatened, especially
across portions of southwest Illinois during the afternoon hours.

Temperatures will remain on the milder side (well, for the month of
April 2018 anyway) tonight and Wednesday. Forecast lows tonight are
forecast to be in the 40s and 50s or about 15-20 degrees warmer than
that observed this morning. Leaned toward warmer MAV guidance for
minimum temperatures tonight due to expected increase in cloud
cover. Highs on Wednesday are much trickier and will depend on exact
timing and track of surface low. There should be a tight gradient
between areas across northeastern Missouri and southwest Illinois,
with highs ranging from the mid 40s to low 70s from northwest to
southeast across the CWFA.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 321 PM CDT Tue Apr 17 2018

Unseasonably cold conditions will return Wednesday night with
northwesterly upper level flow behind the upper level trough moving
through the Great Lakes region, while a strong surface ridge in the
northern Plains builds southeastward into our area.  Lows will be
around 10 degrees below normal.  The strong and gusty northwesterly
surface winds will gradually diminish through the night, but will
still likely be too strong for much frost formation, plus there will
likely be low level cloud cover over most of the forecast area.
Frost is much more likely Thursday night due to good radiational
cooling near the surface ridge axis with a mostly clear sky and
light surface wind. Warmer temperatures can be expected by Friday
afternoon due to upper level ridging over the central US, and
surface winds becoming east-southeasterly as the surface ridge
shifts east of the region. The models continue to trend slower and
further south with the upper level low and associated surface low
which will pass well south of our forecast area later this weekend.
It now appears that any measurable showers will be confined to
southeast MO Saturday night through Sunday evening.  More
significant showers or storms should remain south of our forecast
area. Temperatures by Monday and Tuesday should be close to seasonal



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1240 PM CDT Tue Apr 17 2018

Surface high pressure will move off to the east this afternoon
with winds turning to the southeast ahead of the next storm
system. This low will track across the terminals very late tonight
into early Wednesday. Precipitation is expected to stay north of
the area but a period of MVFR ceilings are likely along and ahead
of the attendant warm front late tonight and early Wednesday
morning. Main story behind the system will be the very strong
westerly winds. Sustained wind speeds of 15-20 knots with gusts of
30-35 knots are likely toward the end of the valid TAF period.

Another potential concern will be low-level wind shear tonight as
southwesterly low-level jet increases to 40-45 knots. Strongest
southwesterly winds stay south of the terminals so held off
mention for now but will pass on to evening shift to take another


Initially light/variable winds will become southeasterly this
afternoon as surface high slides into the lower Ohio Valley. Next
storm system will approach the terminal late tonight and bring a
round of MVFR ceilings to the area. Surface low will move east of
the terminal by mid-late Wednesday morning, with winds switching
to the west and increasing in speed. Sustained wind speeds of
15-20 knots with gusts of 30-35+ knots appears likely during the
afternoon hours.





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