Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 230923

Area Forecast Discussion
National Weather Service Sioux Falls SD
423 AM CDT Sat Sep 23 2017

.SHORT TERM...(Today and Tonight)
Issued at 423 AM CDT Sat Sep 23 2017

A few changes to previous forecast, mainly in regard to placement of
rain chances through tonight, though overall scenario for periods of
moderate to heavy rain affecting portions of the forecast area still
on track.

Starting the day with the surface boundary near/just east of I-29,
which is about a county/30+ miles east of earlier projections. This
has allowed for additional cooling into the I-29 corridor overnight,
but little if any effect on location of showers/storms, which are
focused closer to the 850mb boundary along the western edge of the
forecast area. This area projected to see the steepest mid-level
lapse rates through the morning, though still only marginal at just
over 7/km. Deep layer shear is also strongest in this area. Some of
that will be unusable due to the elevated nature of the storms, but
still seeing potential 30+kt effective shear. As such, cannot rule
out a marginal hail risk as the line of storms developing in central
Nebraska lift into our western counties through the morning, despite
surface temperatures in the 50s beneath the strong inversion.

Farther east, anticipate a sharp temperature/instability gradient
near the surface front, with highs ranging from near 60 along our
western CWA border/K9V9 area, to near 90 in parts of northwest Iowa.
Some uncertainty regarding exactly where the boundary will lie this
afternoon, though consensus keeps it close to a KYKN-KFSD-KMML line
through early afternoon, perhaps pushing slightly farther southeast
through the late afternoon in response to storms/outflow much like
we saw yesterday. Any storms that do form closer to the surface
boundary will have less overall shear as they are farther removed
from the strong mid-upper level winds. Mid-level lapse rates also
weaker and thus less likely to support hail growth. DCAPE values are
marginal at best in area where storm development is more favorable,
but may have to watch for isolated strong wind gusts as some model
forecast soundings/RH profiles depict fairly dry layer above 800mb.

Any surface based activity should wane quickly with loss of heating,
with overnight focus shifting back to the elevated boundary. Here is
where the aforementioned change comes in, as eastward progression of
the upper trough has slowed, and thus the key features in the heavy
rain potential tonight have shifted westward from previous thinking.
Consensus now places the 850mb front west of the forecast area by
06z, with the surface boundary lifting back into the James Valley as
a surface wave lifts out of northeast Colorado along the boundary.
This shift could keep the eastern 1/2 of the CWA dry during the late
evening/overnight, and even areas between the James Valley and I-29
may only see scattered activity at best. Have trended pops in this
direction, but would not be surprised to see a further westward
adjustment if the model trends hold in the next run.

.LONG TERM...(Sunday through Friday)
Issued at 423 AM CDT Sat Sep 23 2017

Start to see some eastward progression of the low level boundaries
Sunday, as the upper jet tops the ridge, and the surface low lifts
into northern Minnesota and drags the front eastward through the
day. This should gradually allow precipitation chances to expand
eastward through the day, though again largely focused near and
behind the surface front. Instability remains weak, in fact mid
level lapse rates are much weaker/closer to moist adiabatic, and
thus severe weather is not expected Sunday. In fact, coverage of
thunder will likely be more limited, and anticipate more numerous
showers with embedded storms. With the boundary still bisecting the
CWA, should again see a broad range of temperatures, with highs from
near 60 in our far west, to mid 80s southeast. Sunday night through
Monday night should see a gradual shift to the east, placing the
potential for moderate to locally heavy rain near or just to the
east of I-29 Sunday night, with rain/isolated thunder remaining
likely across the eastern CWA into Monday.

The slower progression, possibly in response to the blocking ridge
being locked in place by a westward trend in the track of Maria
through the western Atlantic, will result in the chance of showers
lingering in our east into Tuesday. However, a secondary upper jet
rounds the base of the trough and lifts into the Upper Midwest by
Tuesday afternoon, and this should help push the trough eastward
into the Great Lakes by Tuesday night/Wednesday. The latter half of
the week should feature generally dry conditions, with temperatures
on the cool side of normal with highs in the 60s and lows in the


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1045 PM CDT Fri Sep 22 2017

Scattered thunderstorms will be possible through the James River
Valley into east central SD during the overnight hours. There will
be a period of low level wind shear at KSUX later tonight as a low
level jet develops across the region. Shower and thunderstorm
chances will continue on Saturday roughly over the same areas,
with better coverage on Saturday afternoon.




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