Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 272315

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

.SHORT TERM...(This Evening Through Tuesday Night)
Issued at 142 PM MDT Mon Mar 27 2017

Latest analysis shows and upper ridge over the central CONUS and
the Canadian prairie provinces with an upper trough over the
western CONUS and British Columbia. It is this upper trough that
will affect the weather over the forecast area over the next few
days. The upper trough will approach the forecast area tonight and
on Tuesday with an upper low developing over the Four Corners
region tonight and moving slowly into northern Texas on
Wednesday as it splits from the northern stream energy on Tuesday.

Tonight through Tuesday night, moist WAA aloft will increase over
the forecast area as the upper trough approaches from the west.
At the surface, an inverted trough will develop over eastern
Wyoming tonight moving into western South Dakota early on Tuesday.
The pressure gradient associated with the sfc trough will develop
into southeasterly upslope flow over the forecast area. 850-650mb
frontogenesis will strengthen as the jet streak moves through
aloft, increasing synoptic scale lift. Latest models are showing
the more widespread precipitation developing around 6 hours later
than in previous runs, so expect to see the precipitation begin to
increase in coverage late this evening into the early morning
hours. The precipitation will become most widespread during the
day on Tuesday then will move off to the southeast late Tuesday
night/early Wednesday morning. Beneficial rains are probable with
liquid equivalents 1"+ over and near the Black Hills, parts of
northeastern Wyoming and over far southwestern South Dakota.

Precipitation type and the timing for a rain/snow changeover over
the higher elevations of the Black Hills and possibly over the far
southwestern corner of Campbell county in northeastern Wyoming is
the main forecast concern tonight through Tuesday night. Wet bulb
zero values in the forecast soundings are marginal for snow, but
expected sustained precipitation rates are expected to support
several inches of snow over the higher Black Hills in a couple of
batches, one later tonight/early Tuesday and then later
Tuesday/Tuesday night. With the relatively high forecast wet bulb
zeros, snow over the higher elevations may change back to rain
during the diurnal heating cycle on Tuesday. Although not
currently indicated, if slightly drier air develops in the column,
substantial snow accumulations may occur above 5500 feet.

.LONG TERM...(Wednesday Through Monday)
Issued at 142 PM MDT Mon Mar 27 2017

Lingering precipitation will end on Wednesday as the precipitation
moves off to the southeast. Dry weather will occur on Thursday.
Another splitting upper trough will bring mainly rain from
Thursday night into early Saturday. Temperatures will be 2-12F
above average through the period.


.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 509 PM MDT Mon Mar 27 2017

Widespread IFR ceilings will develop across northeast Wyoming and
western South Dakota tonight and continue through Tuesday, with
LIFR ceilings around the Black Hills. IFR/LIFR visibility in fog
also expected late tonight and Tuesday for the higher terrain of
northeast Wyoming and western South Dakota.

Rain will spread into the area tonight and Tuesday with the best
chance of rain across northeast Wyoming into southwest and south
central South Dakota. Temperatures will be cold enough for a
rain/snow mix across portions of northeast Wyoming late tonight
and for snow across the higher elevations of the Black Hills late
tonight and Tuesday.




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