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Area forecast discussion
National Weather Service Sioux Falls South Dakota
628 am CDT Fri Aug 23 2019

Short term...(today and tonight)
issued at 334 am CDT Fri Aug 23 2019

Surface high pressure over the Great Lakes, along with an upper
ridge axis moving very slowly east through the upper Mississippi
Valley, will maintain control over much of our forecast area into
Saturday. Eastward movement of systems continues to slow from run to
run, and low level flow remains a bit more east-of-south through the
day Saturday, keeping increased low level moisture and associated
instability largely west of the James River as the lead wave lifts
through central South Dakota on Saturday. Seeing good consensus among the
models with this scenario, and thus have further backed precip
chances off to the west on Saturday, limiting to areas along and
west of the James River. Instability axis remains focused well west
of the area over the High Plains, with most models struggling to
produce a few hundred j/kg in our far west. With that, appears even
thunder coverage may be somewhat limited if any showers work into
our western counties.

Little change in low level temperatures over the next 36 hours, with
Saturday perhaps slightly cooler than today. In general, looking at
highs in the mid 70s-lower 80s, a few degrees cooler than normal. On
the other hand, modest southerly breeze should keep lows tonight on
the warmer side, ranging from mid 50s east to mid 60s west.

Long term...(saturday through thursday)
issued at 334 am CDT Fri Aug 23 2019

Upper jet begins to dig into the western side of the upper trough
across the northern rockies through Saturday night, helping to push
the trough slowly east through the latter half of the weekend while
the prominent upper ridge shifts slowly through New England toward
the Canadian Maritimes. Currently, this timing places best chance of
widespread rainfall across our area Sunday night, as the upper jet
and surface front begin to work into our western counties. However,
with the strong upper ridge to the east, could continue to see a
slowing trend with this system through Monday.

Ahead of the front, low level flow becomes more southerly on Sunday,
which should result in higher humidity gradually expanding eastward.
Instability likewise increases, though forecast soundings depict a
pretty Stout cap with warm layer above 850-800mb, so despite lead
wave swinging through the area, may be difficult to get more than
isolated-scattered storms through the day.

Main wave with the upper jet nosing in will result in increasing
deep layer shear by evening across our western counties, which may
support an isolated strong to severe elevated storm. But agree with
Storm Prediction Center day 3 outlook that threat is only marginal at this time. Locally
heavy rainfall may be just as much of a concern, as an axis of deep
moisture with ensemble mean precipitable water values near the 90th
percentile of climatology works across the region.

Current timing of system will slowly shift rainfall east of the
forecast area through the day Monday, though as mentioned above,
this may linger a bit longer if the system progression continues to
slow down.

Beyond Monday, broad upper trough remains anchored over the Canadian
prairies, keeping below normal temperatures and spotty rain chances
across the region much of the upcoming week. Some models indicate a
secondary push of cold air working southward into the region as a
stronger wave dives southward into the northern plains late next
week, but some uncertainty in this solution as others pointing to
the upper trough weakening and lifting northeast more quickly. Will
stick with blended temperatures for now, but if the former solutions
pan out, many areas could struggle to reach 70 degrees as we head
into the Labor Day Holiday weekend.


Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 625 am CDT Fri Aug 23 2019

Low-end VFR to MVFR stratus/stratocu expected near and west of the
James River Valley through much of the taf period. MVFR ceilings
could occasionally brush khon, but think majority of this taf
period will see ceilings above 3kft above ground level. This area also most
likely to see occasional south-southeast gusts to 20-25kt late
this morning through sunset.

VFR conditions will prevail east of the James River Valley
through this evening, with a potential for MVFR ceilings working
eastward toward the I-29 corridor late in this taf period.


Fsd watches/warnings/advisories...
South Dakota...none.

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