Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180457

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1157 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 06z TAFs through 06z Wednesday Night)
Issued at 1148 PM CDT Tue Apr 17 2018

VFR conditions are expected to prevail through the overnight
hours. A warm front will lift northward tonight across the region.
MVFR cigs are likely to develop near UIN by Wednesday morning,
persisting into Wednesday afternoon and evening. Otherwise, a few
showers may impact UIN as well Wednesday morning into the early
afternoon, but latest thinking is best coverage may stay just
north of the terminal. A strong cold front will sweep through all
sites through the day on Wednesday, bringing gusty southerly winds
ahead of it, transitioning to west-northwesterly as the front
passes through. Winds will gust as high as 35 knots in places. The
wind gusts should slowly diminish Wednesday evening towards the
end of the TAF period.

Expect VFR flight conditions to prevail through the period. Winds
will increase overnight and turn to the southeast and then south
by Wednesday morning. There will likely be a period Wednesday
morning into the early afternoon where a strong crosswinds exists
on the large runways. Wind should turn further to the west-
northwest by early afternoon with gusts near 35 knots likely.
Otherwise, expect VFR cigs ahead of and behind the front on
Wednesday and wind gusts diminishing towards the end of the





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