Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272032
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
332 PM CDT TUE SEP 27 2016

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 331 PM CDT Tue Sep 27 2016

The synoptic pattern this afternoon features a broad upper-level low
across the northern Great Lakes region.  A strong PV anomaly
rounding this large trough will help drive a cold front through the
region late tonight into Wednesday morning.

The latest hi-res guidance members have trended a bit more bullish
with moisture availability along the front tonight as it drops south
through the region.  Given the fairly stout forcing due to the
approaching frontal surface and mid/upper level PVA, do think we
will be able to squeeze out a few showers along the front late
tonight into Wednesday morning.  Therefore, have added some
slight/low-end chance pops for extreme eastern MO and all of west-
central and southwest IL.

The front will push through the whole area by mid-morning Wednesday.
Cold air advection and the cyclonic flow aloft will likely lead to
strato-cu persisting across eastern MO and western IL through much
of the day.  Some guidance is more optimistic than others in some of
this mixing out, but have trended toward the more pessimistic
solutions given cyclonic flow aloft.  If clouds do indeed hang around
much of the day as forecast, temperatures will struggle in the 60s.

KD

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 331 PM CDT Tue Sep 27 2016

The synoptic pattern at the start of the long term period will
feature a broad upper-level low sagging southeast into the Ohio
Valley.  The low will become cut off from the progressive westerlies
through the end of the week, which will keep it spinning just east
of the region until it finally lifts back to the northeast by late
in the upcoming weekend.

The latest deterministic and ensemble guidance has shifted a bit to
the west with the track of this upper-level cutoff low.  With this
westward shift, portions of western and southwestern IL will be more
under the influence of several vort maxes rounding the low.
Therefore, have upped cloud cover Thursday into Friday across
eastern MO and IL.  In addition, these vort maxes coupled with the
cold core aloft should help to force a few showers from time to time
Friday into Friday night, mainly across IL.  Have left pops out of
the forecast on Thursday for now, but if guidance continues to trend
westward, then pops may need to be added on Thursday across IL.

The gradient of thicker cloud cover to mainly clear skies will
likely be somewhere across the LSX CWA Thursday into Friday, so
temperatures are a bit tricky in this time period.  Tried to tighten
up the gradient a bit with this forecast package for Thursday and
Friday, with mid 60s over south-central IL and low 70s over central
MO.  Temperatures will then moderate a bit late this weekend into
early next week ahead of another trough diving into the
northwestern CONUS.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 157 PM CDT Tue Sep 27 2016

Surface ridge sliding off to the south as weak cold front slides
through region late tonight. VFR conditions expected with some
isolated showers possible for KUIN and metro area, but coverage is
hard to pin down, so kept tafs dry for now. Otherwise, west winds
to diminish towards sunset. Then winds to veer to the north and
pickup as cold front slides through.

SPECIFICS FOR KSTL:
Surface ridge sliding off to the south as weak cold front slides
through region late tonight. VFR conditions expected with some
isolated showers possible for metro area, but coverage is hard to
pin down, so kept taf dry for now. Otherwise, west winds to
diminish by 00z Wednesday. Then winds to veer to the north and
pickup as cold front slides through around 14z Wednesday.

Byrd

&&

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

$$

WFO LSX



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