Area Forecast Discussion
Issued by NWS Rapid City, SD

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097
FXUS63 KUNR 222046
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
246 PM MDT Thu Sep 22 2016

.SHORT TERM...(This Evening Through Friday Night)
Issued at 224 PM MDT Thu Sep 22 2016

Upper ridge has shifted slightly eastward this afternoon as the
upper low continues to deepen over the Four Corners area,
generating fast southwesterly flow aloft over the forecast area.
At the surface a large area of high pressure is centered over
Manitoba with low pressure over Utah, Colorado and southwestern
Wyoming. A sfc trough extends eastward from the low across
eastern Colorado and into southeastern Nebraska. The resulting sfc
pressure gradient is creating moist easterly flow over the region,
especially over western South Dakota where very close T/Td spreads
exist this afternoon.

Tonight into Friday the upper low will eject quickly northward
moving into western Wyoming on Friday afternoon. Fast moving
disturbances in the upper flow will combine with abundant moisture
to produce showers and a few thunderstorms. Widespread fog and
some drizzle is also expected overnight through Friday morning as
temperatures cool this evening and the moist easterly upslope
boundary layer flow continues.

Friday afternoon and night, slight timing differences remain
between the models which would create noticeable differences in
the weather. The upper low will continue to track northeastward,
into northeastern Wyoming/southeastern Montana late Friday night
with the surface low rapidly moving northward as well. This could
produce a dry slot over western SD. If the dry slot develops,
increased diurnal heating would create a narrow area of 1KJ/kg
MUCAPE which combined with 25-35m/s bulk shear could create some
strong to severe fast moving thunderstorms. Temperatures will
likely remain relatively cool over much of the area on Friday
however much warmer temperatures are possible if the dry slot
develops over parts of southwestern and especially south central
SD.

.LONG TERM...(Saturday Through Thursday)
Issued at 224 PM MDT Thu Sep 22 2016

By Saturday morning, nearly stacked low will be over
northeastern WY/southeastern MT. As the low moves further northeast
during the day, chances for precip will gradually decrease as the
CWA becomes dry-slotted, although wrap around showers are possible
across northeastern WY, the Black Hills, and northwestern SD into
Saturday evening. As the low moves northeast of the region, a tight
pressure gradient will develop across the area. The tight gradient,
along with cold advection, will result in strong northwest winds
across portions of the area. The strongest winds will develop to the
north and east of the Black Hills from late Saturday into Sunday.
Highs on Saturday will range from the 50s over northeast WY and the
Black Hills to the 70s over south central SD, with 50s to mid 60s
across the forecast area on Sunday.

Both GFS and EC show the system now moving away from the region
Sunday night and Monday, though GFS keeps the system stalled out
over portions of the upper midwest through early next week. In
either case, dry weather is expected through the rest of the
extended period. After a cool and breezy start to the week, a
warming trend will take place Tuesday through Thursday. Temperatures
look to be above average Wednesday and Thursday, with highs in the
70s across the plains.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 224 PM MDT Thu Sep 22 2016

MVFR to IFR conditions will persist across much of
the forecast area this afternoon and tonight, especially over
northeast WY, the Black Hills, and southern SD. Areas of LIFR
conditions, fog, and drizzle will develop overnight. Chances for
showers will increase over northeast WY this afternoon and early
evening, and across western SD mainly tonight. Isolated
thunderstorms are also possible.

&&

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

$$

SHORT TERM...10
LONG TERM...26
AVIATION...26



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