Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170028

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
628 PM CST Thu Nov 16 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 00z TAFs through 00z Friday Evening)
Issued at 551 PM CST Thu Nov 16 2017

Expect VFR flight conditions to prevail this evening...with
ceilings developing after midnight. 2 areas of concern tonight.
First, the stratus over southeast Illinois now seems likely to to
build back to the west- northwest to along and east of the
Mississippi River by 07-09Z. Guidance is suggesting IFR ceilings
in this area now so have introduced 700-900ft cigs in the STL
Metro TAFs and KUIN after 06Z. Not sure how long the IFR will last
into the morning, but most indications are that ceilings will
lift to MVFR by around 18Z. The second issue is the storm system
approaching from the west on Friday. This system will sweep into
the Plains on Friday morning with primarily MVFR ceilings and
light rain or drizzle overspreading the area during the day.


Expect VFR flight conditions at Lambert this evening, with stratus
overspreading the terminal after midnight. 07Z timing in the TAF
is not certain, but think the ceiling will move in between 06Z and
09Z. IFR will likely prevail into the daylight hours at Lambert
with some improvement during the mid and late morning. There may
be some intermittent light rain or drizzle as well ahead of a
storm system which will move through the region late Friday night.





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