Area Forecast Discussion
Issued by NWS St. Louis, MO

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874
FXUS63 KLSX 010316
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1016 PM CDT FRI SEP 30 2016

.UPDATE...
Issued at 843 PM CDT Fri Sep 30 2016

Persistent pattern continues this evening with clouds along with
scattered showers rotating through IL and the eastern half of MO
on the backside of slow moving upper level and surface low over
southern IN. Looking at the latest radar trends it appears that
scattered showers, mainly on the light side will continue for the
rest of the evening along and east of the Mississippi River. The
latest HRRR model runs diminishes this activity by midnight, but
still could not rule out isolated light showers or patchy drizzle
late tonight across west central and southwest IL. Low level
cloud deck should keep temperatures from getting too cool tonight
with lows similar to the previous night and slightly above normal
for the beginning of October.

GKS

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 300 PM CDT Fri Sep 30 2016

Upper low will begin to slowly lift to the north northeast late
tonight. In the meantime, isolated/scattered showers to continue
rotating counter clockwise across forecast area with best chances
along and east of Mississippi River through Saturday. Some isolated
thunderstorms not out of the question for Saturday afternoon, mainly
for our far eastern counties, so kept in grids in this area. With
the clouds lingering over the region, lows tonight will be a few
degrees warmer. However, highs on Saturday to remain below normal,
in the upper 60s to low 70s. As for winds, to lighten up and back to
the northwest by midday on Saturday as upper low moves away from
region.

Byrd

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 300 PM CDT Fri Sep 30 2016

The pesky upper low will be on the move during the first part of
this period, lifting into the lower Great Lakes by early Sunday
morning and the northeastern U.S. by early Monday. As it departs
we will see a lessing influence manifested by weakening low level
cyclonic flow and eventually the dominance of lower tropospheric
high pressure by Sunday.

I have kept some slight chance pops on Saturday evening to the
east of the MS River where there could be lingering diurnal
activity from the day. I have concerns that we may settle into a
period from Saturday night into early Monday with extensive clouds
trapped within the low level ridge. And if locations do clear out
during the day, then fog/stratus will have a tendency to redevelop
at night. SREF probabilities suggest this to a degree, and I think
the light QPF evident in some of the deterministic models for
Sunday is a reflection of the low level moisture and clouds. I
have boosted clouds quite a bit during the Saturday night-Monday
morning time frame, especially Saturday night-Sunday where greater
confidence lies. Accordingly I have also lower high temps a tad.

The deterministic and ensemble model soutions are in relatively good
agreement with the large scale pattern change this weekend into
next week as a progressive longwave trof moves out of the western
U.S. and into the Nation`s midsection in the Wed-Thurs time
frame. This pattern change will result in a retreat of the low-
level ridge Monday-Tuesday, slight height rises aloft followed by
increasing southwest flow aloft, and evolution of a low level WAA
regime. In terms on sensible weather there should be moderation of
temperatures Monday into Tuesday, with a cold front moving into
and through the area somewhere in the Wednesday-Thursday period.
There are differences in timing/speed of the attendant cold front
due to presence or lack of a trailing shortwave trof Thurs/Thursday
night. The ECMWF is faster and wetter late Wed/early Thurs period,
while GFS with the trailing shortwave scenario is slower with main
round of precipitation Thurs/Thurs night.

Seasonably cool and drier weather should then dominate Friday into
next weekend as the upper trof dampens out and passes to the east
with west-northwest flow aloft dominating the pattern aloft.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 958 PM CDT Fri Sep 30 2016

Low level clouds along with scattered showers continue to rotate
through eastern MO and IL on the backside of the slow moving upper
level and surface low over southern IN. Most of the showers should
be east of the taf sites late tonight, although could not rule out
a brief light shower or a little drizzle in UIN and the St Louis
metro area. The cloud ceiling will gradually lower overnight into
the MVFR catagory in COU and into the IFR catagory in UIN and the
St Louis metro area. The cloud ceiling should gradually rise into
the VFR catagory by late Saturday afternoon with daytime heating
and mixing and deepening of the boundary layer. May be a few
showers Saturday afternoon in UIN and the St Louis metro area.
North-northwesterly surface winds will continue through the
period.

SPECIFICS FOR KSTL: Low level clouds along with scattered showers
continue to rotate through eastern MO and IL on the backside of
the slow moving upper level and surface low over southern IN. Most
of the showers should be east of the taf sites late tonight,
although could not rule out a brief light shower or a little
drizzle in the STL area. The cloud ceiling will gradually lower
overnight into the IFR catagory in STL. The cloud ceiling should
gradually rise into the VFR catagory by late Saturday afternoon
with daytime heating and mixing and deepening of the boundary
layer. May be a few showers Saturday afternoon in STL. North-
northwesterly surface winds will continue through the period.

GKS

&&

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

$$

WFO LSX



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