Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 302009
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
309 PM CDT FRI SEP 30 2016

.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 18z TAFs through 18z Saturday Afternoon)
Issued at 1237 PM CDT Fri Sep 30 2016

With closed upper low just to our east, will continue to see sc
deck rotate around it. So VFR/MVFR cigs to persist today, lowering
to IFR tonight, especially for taf sites along Mississippi River.
Cigs to lift back to MVFR by mid morning on Saturday. Otherwise,
some scattered showers during the afternoon hours today for STL
metro area and vicinity showers at KUIN. KCOU could see some
sprinkles but not enough coverage worth mentioning in their taf.
As for winds, to persist from the north then back to the northwest
by late tonight. Wind speeds to diminish by sunset.

SPECIFICS FOR KSTL:
With closed upper low just to our east, will continue to see sc
deck rotate around it. So VFR/MVFR cigs to persist today, lowering
to IFR by 07z Saturday. Cigs to lift back to MVFR by 15z Saturday.
Otherwise, some scattered showers during the afternoon hours today
for STL metro area, so kept tempo mention from 18z to 22z Friday.
As for winds, to persist from the north between 10 and 15 kts then
back to the northwest by 07z Saturday. Wind speeds to diminish by
03z Saturday.

Byrd

&&

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

$$

WFO LSX



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