Area Forecast Discussion
Issued by NWS Rapid City, SD

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933
FXUS63 KUNR 211730
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1130 AM MDT Mon Aug 21 2017

.UPDATE...
Issued at 1007 AM MDT Mon Aug 21 2017

Current surface analysis shows low pressure over south central SD,
with cool front south of the CWA. Upper level analysis shows
trough over south central Canada with west-northwest flow over the
Northern Plains. KUDX radar shows strong thunderstorms over
eastern portions of Mellette and Todd counties, as well as most of
Tripp County. A severe thunderstorm watch is in effect for these
areas through midday.

As far as cloud cover goes, plenty of it east of a line from near
Faith to Pine Ridge. The back edge of this cloud cover will only
slowly move/dissipate east-southeast over the next few hours, so
most of at least south central SD is unlikely to see the eclipse.
Further west, patchy low clouds and fog have burned off with
mostly sunny conditions. Some scattered high clouds are moving
southeast across northeast WY and parts of far western SD. This
could cause at least some minor obscuration of the eclipse in some
areas, but it does look thin enough in general to see through.
Temps are currently in the mid 60s to mid 70s with breezy
northerly winds developing across much of the plains.

Mostly minor adjustments made to the forecast this morning. Main
change was to raise pcpn chances across south central SD through
midday. Minor adjustments made to cloud cover and wind.

&&

.DISCUSSION...(Today Through Sunday)
Issued at 156 AM MDT Mon Aug 21 2017

A weak wave moving through zonal flow is helping to create some
scattered thunderstorms across northwestern SD. Satellite shows
some low clouds developing across Nebraska and portions of the
western SD plains. Surface high is over eastern ND/northern MN,
and with low pressure over eastern WY, winds across the CWA are
southeasterly around 10-15 kts. Temperatures are in the lower 60s.

Upper wave will slide through the northern plains today, and winds
across the CWA will become northerly as the surface trough shifts
east. Southeasterly flow and theta-e advection will continue across
south central SD through the morning. Hi-res models are showing
thunderstorms over south central SD late morning/early afternoon,
and with MLCAPE around 2000 J/kg and 0-6 km shear of 30-40 kts, some
strong to severe storms will be possible. Although morning fog/low
clouds across south central/southwestern SD should dissipate by the
time of the eclipse, skies across central SD are still expected to
be mostly cloudy. However, far western SD and especially
northeastern WY should see some clearing. Highs today will be in the
lower 80s.

An upper ridge will keep weather warm and dry through the middle of
the week. Chances for showers and thunderstorms will increase
towards the end of the week as a Canadian low pressure system
flattens the ridge. This system should shift east of the region by
the weekend, and another ridge is expected to build over the
western CONUS, potentially persisting through early next week.
Near average temperatures in the 80s are expected through the
week.

&&

.AVIATION...For the 18Z TAFS Through 18Z Tuesday
Issued At 1125 AM MDT Mon Aug 21 2017

Tstms are exiting Tripp Co. MVRF cigs remain over southern SD and
will improve by mid afternoon, leaving VFR conditions through the
forecast period. Gusty north winds this afternoon will decrease,
then switch to the south early Tuesday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

Update...26
DISCUSSION...Pojorlie
AVIATION...55



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