Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 131157

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
657 AM CDT Wed Sep 13 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 346 AM CDT Wed Sep 13 2017

Main concern deals with rain chances with the remnants of Irma.

Isolated showers continued over parts of eastern Missouri and
southwest Illinois early this morning. An additional band of
showers over south central Illinois is getting ready to rotate
back westward into southwest Illinois. This is all occurring as
the remnant low of Irma currently over western Tennessee moves
northeast during the day today. Scattered showers can be expected
to continue over parts of southeast/east central Missouri as well
as southwest/south central Illinois this morning. The rain will
end from west to east during the late morning and afternoon as the
upper low lifts northeast from Kentucky into Indiana this

Expect dry conditions tonight with clouds gradually clearing.
With these trends and a moist boundary layer, may see some areas
of fog develop late tonight and early tomorrow morning.

Will see another relatively wide difference in temperatures with
the coolest highs over south central Illinois and the warmest
highs across central and northeast Missouri where there will be
some sunshine and no rain. Lows tonight will be close to normal.


.LONG TERM...  (Thursday through Tuesday)
Issued at 346 AM CDT Wed Sep 13 2017

Quiet weather conditions are expected from Thursday through the
first half of the weekend. A cold front will then bring a chance
of SHRA/TSRA to the region from late Saturday night through early
next week.

A trough moving through the northwestern CONUS will induce a
cut-off low initially located off the CA coast to eject
northeastward and rejoin the prevailing flow ahead of the trough
as it approaches the Rockies. The combined influence of these
features will induce surface cyclogenesis across the plains on
Fri/Sat. The resulting low pressure system and its associated
fronts will influence both precipitation and temperature trends
across MO/IL from late this week through the weekend.

Although the best upper dynamics remain well to the west and
north of MO/IL, the trailing surface cold front associated with
the newly developed low pressure system will edge into at least
the northern CWA on Sun/Sun night. Compared to previous model
runs, the latest model guidance does not bring the cold front
nearly as far southward before the boundary lifts back
northeastward as a warm front. However, the more northward
model solutions also mean that there would be an additional day
with warm/humid conditions contributing to instability.

Temperatures should quickly rebound into the mid/upper 80s to near
90 degrees for Thu through Sat. The warmest day is likely to be
Fri due to a favorable southwesterly surface wind direction as
well as the lack of widespread precipitation. Temperatures for
Sunday through Tue will depend on how far southward the cold front
progresses. Highs in the mid/upper 80s look reasonable attm but
may need to be adjusted a few degrees in either direction depending
on the cold front.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 655 AM CDT Wed Sep 13 2017

Large area of MVFR ceilings over southeast and east central Missouri
as well as southwest and south central Illinois is affecting the
St. Louis area terminals this morning. Expect ceilings between
1500-3000FT AGL through the morning hours. Isolated showers may
also affect the St. Louis area terminals this morning before they
move off to the east. KCOU and KUIN are expected to be dry and VFR
this morning at this point. Then mainly dry and VFR conditions
are expected this afternoon and this evening before patchy fog
develops overnight with some MVFR visibilities.

SPECIFICS FOR KSTL: Ceilings between 2000-3000ft AGL are expected
this morning and should improve to VFR by afternoon. Isolated
showers will be in the area through early afternoon, though
coverage does not necessitate putting any rain in the TAF at this
time. Then mainly dry and VFR conditions are expected the rest of
the period. The only exception may be the risk of some patchy fog
that will be in the area late tonight and early Thursday.





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