Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211811
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
111 PM CDT Sat Oct 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 336 AM CDT Sat Oct 21 2017

Main focus through tonight will continue to be on an approaching
cold front from the west which will bring a round of showers and
thunderstorms to the area, some of which may be strong to severe.
Timing wise, it appears the front and convection along it will
likely be a few hours slower than previously anticipated, similar to
the last frontal passage last week. Believe the experimental HRRR is
a couple of hours too fast while the NAM`s progression of the
frontal convection likely just a bit too slow. Went between these
two extremes with convection nearing far NW CWA border just after
0300 UTC, which is pretty much in line with both the ARW/NMM models.
Still appears to be a rather small temporal window for severe
thunderstorms, mainly focused across far western sections of the
forecast area. Slowly waning instability should lessen the severe
threat late tonight for areas further to the east. Damaging
straight line wind gusts appear to be the primary threat with a
linear mode expected as deep-layer shear vectors are nearly
parallel to frontal boundary. Also cannot rule out an isolated
tornadic threat due to very strong low-level shear and helicity.

Temperatures today should be mild once again ahead of the cold
front. Expect highs to reach into the upper 70s to near 80 degrees.
Leaned slightly toward the cooler MET guidance for today given
expected cloud cover and the slightly cooler readings observed
yesterday than forecast. Lows tonight will vary from west to east as
portions of central and northeastern Missouri will drop back into
the low to mid 50s behind the front where areas further to the east
will only get back down into the low 60s.

Gosselin

.LONG TERM...  (Sunday through Friday)
Issued at 336 AM CDT Sat Oct 21 2017

The synoptic pattern at the start of the long term period will
feature an amplified regime across the central/eastern CONUS, with a
deep trough digging into the Plains/Midwest and a downstream ridge
located over the eastern seaboard. This trough will slide east
through the period, becoming reinforced over the region by a strong
PV anomaly diving out of Canada by the middle of next week.

The aforementioned trough will push through the region on Sunday.
Guidance continues to struggle with its exact evolution (GFS cutting
off an upper-level low, the Euro more progressive), but the general
trend is for a slightly quicker progression of precipitation out of
the area. Widespread showers and a few rumbles of thunder Sunday
morning/early afternoon should largely taper off by late Sunday
night, although a few showers will still be possible Monday mainly
across southeast MO and IL. The passage of the trough and associated
surface cold front will allow cooler temperatures to move into the
region on Sunday, as highs will only climb into the 60s.

A real shot of fall will move into the region on Tuesday.  A strong
PV anomaly giving out of Canada will help to sharpen an upper-level
trough over the Midwest, driving a surface cold front through the
region. Tuesday is shaping up to be a fairly raw fall day with cloud
cover and perhaps some showers across Illinois Tuesday afternoon.
Gusty northwest winds will help usher in much cooler temperatures as
highs only make it into the low/mid 50s. It is possible even these
values may be too warm and highs may not make it out of the 40s in
some locations.

The upper-level flow will flatten out a bit by the middle of next
week ahead of yet another trough digging into the western CONUS.
Guidance solutions diverge quite a bit with this trough, thus an
ensemble approach was taken at this time which brings another chance
of showers back into the region by next Friday.

KD
&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1253 PM CDT Sat Oct 21 2017

Showers ahead of boundary to remain fairly isolated, so kept tafs
dry til late this evening as main cold front approaches region. As
for winds, to persist from the south and gust to near 25kts at
times. Have showers moving in between 04z and 07z and persist
through rest of forecast period. As for thunderstorm chances,
models showing instability waning as system approaches, so for now
just kept vicinity thunderstorm mention til daybreak for taf
sites. South winds to veer to the west to northwest behind cold
front between 10z-11z for KCOU and KUIN and around 16z for STL
metro tafs.

SPECIFICS FOR KSTL:
Showers ahead of boundary to remain fairly isolated, so kept taf
dry. As for winds, to persist from the south. Have showers moving
in by 07z and persist through rest of forecast period. As for
thunderstorm chances, models showing instability waning as system
approaches, so for now just kept vicinity thunderstorm mention til
13z Sunday. South winds to veer to the west to northwest behind
cold front by 16z for STL.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$
WFO LSX



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