Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251149

Area Forecast Discussion
National Weather Service Sioux Falls SD
649 AM CDT Mon Sep 25 2017

.SHORT TERM...(Today and Tonight)
Issued at 335 AM CDT Mon Sep 25 2017

A cool and wet day in store for much of the area today, persisting
into tonight, as remain situated between slow-moving upper trough
over the Rockies, and strong upper ridge east of the Mississippi.
Entire forecast area should remain on the cool side of the surface
boundary through tonight, with the nearly stationary 850mb boundary
near/southeast of KYKN-KFSD-KMML line continuing to focus rain and
embedded thunderstorms over the southeast 1/2-2/3 of the CWA. See a
weakening in 300K-310K isentropic lift around 18Z behind the morning
wave, so could see a decrease in areal coverage of showers around
midday. However, another decent wave moves in from the southwest in
the late afternoon/evening, which should expand coverage back to the
west again. Elevated instability likewise wanes a bit through the
midday period, but some weak instability returns to the southeast
CWA through the afternoon/evening, so will keep an isolated thunder
mention there. Expect we will still see pockets of heavier rain, as
PWAT values across the southeast half of the CWA remain extremely
high (1.5+ inches) for this time of year through tonight. The weak
instability would also support more isolated showers with more
intense rainfall rates. With the widespread clouds/rain and cool air
mass, highs today will be 10-15 degrees below normal in most areas,
with readings only topping out in the mid-upper 50s for all but our
far southeast.

Although a bit slower than previous runs, still seeing a good punch
of mid-upper level drying move into the forecast area as the upper
trough shifts east late tonight. This along with broad downglide on
the isentropic surfaces and weak cold advection in the lower levels,
should bring a gradual decrease in precipitation through the late
night hours. Low clouds and a modest northwest breeze will keep
temperatures from falling off too much, but still looking at lows in
the mid 40s-lower 50s.

.LONG TERM...(Tuesday through Sunday)
Issued at 335 AM CDT Mon Sep 25 2017

Could see some light rain linger across our eastern zones into
Tuesday morning before drying aloft shunts everything east of the
area. Should see some increasing sunshine by afternoon, though 850mb
temperatures in the single digits will still yield sub-normal highs
in the upper 50s-lower 60s.

The broad upper trough slides eastward through the Great Lakes
through the mid-late week period, while a ridge builds over the
western CONUS. Anticipate some moderation in temperatures Wednesday
and Thursday, but models in generally good agreement in dropping a
wave/associated cool front southward through the Upper Midwest
Thursday night/Friday. Moisture somewhat lacking as the wave moves
through, so do not anticipate any precipitation in our area at this
time. The front will knock temperatures back for Friday, though how
far back is somewhat in question, as models differ on the strength
of the cold push. Given this uncertainty, did not alter blended
guidance, though could see highs a few degrees cooler than current
forecast if the more aggressive models pan out.

Next weekend sees the upper trough retreat eastward, allowing upper
ridge to build toward the Northern Plains/Mississippi Valley. This
should bring warmer temperatures back into the region, along with
spotty precipitation chances as southerly flow increases Saturday
night/Sunday ahead of another upper trough. System looks to be more
progressive, so do not see a repeat of this past weekend as far as
rainfall amounts. However, GFS does depict PWAT values increasing to
over 1 inch, so modest amounts are possible.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 641 AM CDT Mon Sep 25 2017

The frontal boundary is still with us and that means another messy
aviation forecast. While some occasional breaks in the clouds may
occur, stratus will be the predominant condition across the
region over the next 24 hours. This will be combined with showers
and attendant visibility reductions. Expect MVFR and IFR
restrictions at all terminals, particularly FSD and SUX. Some
sporadic thunder is possible as well, but coverage looks to be
very limited, so have not included it in any of the TAFs.




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