Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 211607
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1007 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 12Z TAFS Through 12Z Tuesday Morning)
Issued At 512 AM MDT Mon Aug 21 2017

Isolated thunderstorms from KD07-KPHP will shift east/southeast
this morning. Area of IFR/LIFR stratus and fog located along the
Nebraska border will linger through the morning, with MVFR
ceilings in the vicinity of the Black Hills. Ceilings will improve
by late this morning, with VFR conditions expected this afternoon
into tonight.

&&

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

$$

Update...26
DISCUSSION...Pojorlie
AVIATION...7



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