Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170606

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1206 AM CST Fri Nov 17 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 1146 AM CST Thu Nov 16 2017

The synoptic pattern early this afternoon features a trough sliding
off the northeast CONUS, with yet another trough moving into the
Northwest. This regime has placed the Midwest within shortwave
ridging aloft, a pattern which will transition to quasi-zonal by the
end of the period.

Dry and mainly clear conditions will persist through the remainder
of this afternoon into early this evening. However, some of the hi-
res guidance particularly the HRRR/NARRE/RAP all of which are
handling the current stratus fairly well, bring the moisture and
potential stratus westward this evening into the overnight hours as
the 925mb flow turns more easterly. Therefore, have upped cloud
cover across Illinois tonight into Saturday morning. Future
forecasts may need to bring this cloud cover into eastern Missouri
if trends hold. Cannot rule out some patchy fog in these areas as
well, but think increasing low-level winds will tend to favor a
stratus deck. Overnight lows will dip into the mid 30s to low 40s,
with temperatures actually increasing late tonight across central

Moisture transport will increase late tonight and into the day on
Friday as a low-level jet strengthens over the area. Forecast
soundings depict moisture will be fairly shallow through the day
tomorrow and concentrated mainly in the low-levels, thus think it
will be more of a drizzle scenario with fairly low rainfall amounts.
It will be fairly breezy despite limited mixing, with gusts around
25 mph at times. High temperatures will climb into the mid/upper 50s
and low 60s.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 224 PM CST Thu Nov 16 2017

Lingering drizzle early Friday evening will give way to more
steady rainfall overnight as additional mid-level energy rotates
in from the northwest. This will force a cold front across the
region with several model soundings advertising the potential of
weak elevated instability out ahead of the boundary as it pushes
through, and for now, have maintained an isolated thunder mention
mainly between 06z - 15z Saturday. As cooler air advances over
the region Saturday morning, the potential also exists for a
brief period of a RA/SN mix across northern portions of the fcst
area before precip largely exits to the east later in the day. If
any snow does occur, little to no snow accumulation is expected
based on very marginal surface temps.

Drier weather will return by Saturday afternoon and evening with
the dry trend expected to persist through the remainder of the
extended fcst as amplified energy associated with next inbound
West Coast storm system remains south of our area. Temps through
at least the first half of the work week look to remain at or
above normal levels for this time of year with low to mid 50s
possible both Monday and Tuesday, with a temporary cool down
expected by Wednesday following the passage of a cold front late
Tuesday. With a pronounced surface ridge extending across the
central and western Gulf however, limited moisture return is
expected ahead of the front which should result in a dry frontal



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1145 PM CST Thu Nov 16 2017

An area of IFR ceilings over southwest Illinois will continue to
drift northwest tonight...and likely expand in areal coverage
through daybreak to cover much of the area along and east of the
Mississippi River. Feel pretty confident that the movement will
continue...less so in how much expansion will occur.
However...several models have been very consistent this evening in
expanding the coverage of the ceilings. A few sprinkles or some
drizzle may develop toward morning and continue into Friday
morning. MVFR ceilings over central Missouri will continue to
spread eastward tonight as well...eventually falling to MVFR
during the late morning or early afternoon. Meanwhile...the IFR
ceilings over the eastern portion of the forecast area will lift
and move east by mid-morning...just as MVFR clouds move up to the
Mississippi River. MVFR will likely prevail thereafter.


An area of IFR ceilings over southwest Illinois is moving toward
the terminal. Think it is likely to get into the vicinity of KSTL
by 10-11Z. VFR conditions will likely prevail until then. The IFR
flight conditions should improve by mid-morning. There may be a
few sprinkles or some drizzle at any time through the day, but
likely not enough precip to impact the visibility.





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