Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 221730
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1130 AM MDT Thu Sep 22 2016

.SHORT TERM...(Today Through Friday Night)
Issued at 248 AM MDT Thu Sep 22 2016

08z surface analysis had well-defined east/northeast flow over
the CWA with upslope flow/stratus over most locations. Given
continued shallow lift and moisture availability, will continue
mention of patchy drizzle/fog through the morning hours. Concern
then turns toward upper low spinning over the northwest US.

This afternoon and evening, upper low moves into NV with lead
shortwave energy and surface low over WY bringing chance pops over
northeast WY, which will spread east this evening. Temperatures will
be below guidance on the SD plains where stratus hangs on the
longest and near guidance in the west where some breaks will develop.

Overnight and Friday morning, upper low moves into southeast WY with
surface low pushing into western NE. Another piece of energy moves
out with system with enhanced lift and theta-e advection. Chances
for thunderstorms will shift east of the Black Hills, but continued
upslope boundary layer flow will support fog and drizzle.

Friday afternoon and night, slight timing differences in model
guidance will have a large impact on sensible weather. Upper low
lifts into eastern MT with surface low rapidly riding north ahead of
upper low into central ND. Narrow tongue of around 1KJ/kg MUCAPE
develops due to low level theta-e ridge. Forcing should easily
overcome MUCIN. 0-6km bulk shear impressive, reaching 25-35m/s.
Wild card is stratus. Active fasting moving storms may result
with main threat strong to severe wind gusts. Positive buoyancy
will be fairly low and 0C heights will be relatively high, so
support for large hail is low. Temperatures will be problematic
with complex fast moving system, but just hit it down the middle.

.LONG TERM...(Saturday Through Wednesday)
Issued at 248 AM MDT Thu Sep 22 2016

By Saturday morning, nearly stacked low will be over northeastern
WY/eastern MT. Chances for precip will gradually decrease as the
CWA becomes dry-slotted, although wrap-around precip will
continue across northeastern WY, the Black Hills, and northwestern
SD. As the low pushes into ND, a strong cold front will cross the
region. Models show 850 mb winds of 40-50 kts, accompanied by
strong cold air advection. Gusty northwest winds are expected,
especially across northwestern SD. Highs will range from the 50s
across the northwestern CWA to the mid 70s across south central
SD.

Tight pressure gradient and continued CAA will keep strong winds
around for Sunday. Highs will reach only the 50s and 60s. Other than
some possible wraparound showers continuing across northwestern SD,
Sunday will be dry.

Beyond the weekend, models are in disagreement. The GFS shows the
upper low/trough slowly pushing eastward, with a broad ridge
building over the western CONUS. Meanwhile, the ECMWF develops a
secondary low near the four corners region, which may eventually
meander towards the northern plains. Kept forecast for next week dry
for now, although slightly cooler.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1130 AM 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. Showers will increase in coverage
over northeast WY this afternoon and early evening, and across
western SD mainly tonight. Isolated thunderstorms are also
expected.

&&

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

$$

SHORT TERM...Helgeson
LONG TERM...Pojorlie
AVIATION...26



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