Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 271726
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1126 AM MDT Mon Mar 27 2017

.UPDATE...
Issued at 1050 AM MDT Mon Mar 27 2017

Satellite shows the valley fog rapid dissipating so have updated
the forecast to adjust sky cover and remove areas of fog.

&&

.SHORT TERM...(Today Through Tuesday Night)
Issued at 226 AM MDT Mon Mar 27 2017

08z surface analysis had weak high over the Dakotas with lee
trough from western MT into northeast CO. Water vapour loop had
departing upper trough east of the CWA taking precipitation along
with it. Some fog across the CWA given low T/Td spreads - will
address with short term forecast updates. Next upper trough now
entering the west coast. Upper trough will be the main weather
maker in the short term.

Today, upper ridge builds over the CWA in response to upper trough
moving into the Rockies. Surface high shifts east with return flow
developing. Early morning fog should burn off quickly. Temperatures
will be 5-10F above normal.

Tonight through Tuesday night, upper trough moves into the plains
states with energy splitting as it emerges from the Rockies. Main
energy will create upper low over the four corners area 12z Tuesday
and then move east into the southern plains by 12z Wednesday.
Northern branch of system will move across the US/Canadian border.
Over the CWA, inverted trough expected to cross. Well-defined
southeast upslope flow will develop under left exit region of jet
ahead of upper low. 850-650mb frontogenesis nicely defined with good
linkage to support aloft. Should see precipitation break out this
evening across WY and spread over the CWA overnight. Precipitation
will peak Tuesday and then slowly move east Tuesday night.
Beneficial rains probable with liquid equivalents 1"+ over and near
the Black Hills. Precipitation type concerning for late tonight
through Tuesday night over the higher terrain. Forecast soundings
marginal for snow, but expected sustained precipitation rates will
support several inches of snow over the Black Hills in a couple of
batches -- one later tonight/early Tuesday and then later
Tuesday/Tuesday night. If precipitation doesn`t change back to
liquid during the diurnal heating cycle Tuesday, substantial snow
amounts may occur above 5000 feet. Given expected storm total snow
amounts over the Black Hills of 3-6" split over two periods, will
let later shifts assess need for headlines.

.LONG TERM...(Wednesday Through Sunday)
Issued at 226 AM MDT Mon Mar 27 2017

Lingering precipitation ends Wednesday as aforementioned storm
system moves away. Quiet weather Thursday. Another splitting
upper trough moves in for Friday and Saturday with PoPs warranted
for mainly rain. Temperatures will be 2-12F above normal through
the period.

&&

.AVIATION...For the 18Z TAFS Through 18Z Tuesday
Issued At 1116 AM MDT Mon Mar 27 2017

Localized IFR fog wl linger east of Blk Hls thru early aftn. A
storm system will bring moisture into the area overnight, with
increasing clds and precipitation. Easterly wnds along Blk Hls
will bring lower clds arnd midnight, while IFR cigs wl dvlp ovr
nern WY as sfc trof passes. Precip wl be rain in most areas, but
snow wl mix in nern WY and higher elevations.

&&

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

$$

Update...10.0
SHORT TERM...Helgeson
LONG TERM...Helgeson
AVIATION...55


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