Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 210520
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1120 PM MDT Mon Mar 20 2017

.SHORT TERM...(This Evening Through Tuesday Night)
Issued at 212 PM MDT Mon Mar 20 2017

20z surface analysis had frontal boundary from central WY into
southwest NE with ridge across the Dakotas into the Midwest.
Narrow band of lift noted per regional radar loop from far
southern MT into north central ND, which is under right entrance
region of 120kt jet zipping across south central Canada. Water
vapour showed a shortwave over southwest ID, and this will be the
main weather maker through Tuesday.

Tonight into Tuesday morning, ID shortwave moves through the CWA as
upper ridge builds over the Rockies. Lift under aforementioned jet
will intensify tonight enhancing shortwave as it approaches. Lower
troposphere responds with sharpening 850-650mb isentropic lift /
frontogenesis north of frontal boundary. Southeast upslope low level
flow will increase via surface high settling east of the CWA.
Precipitation should expand late this afternoon across western WY
into the CWA overnight. Thermal profiles indicate precipitation
should start as rain, but once diurnal cooling commences and dynamic
cooling sets in, -SN should occur. Guidance shows a decent swath of
1-2" snow from far northeast WY through central SD, but given
potential banded nature of -SN and latest high resolution model
forecast, suspect a 2-locally 4" band will occur from southeast MT
into the northern Black Hills. Areas of fog expected outside of
falling -SN given upslope fog pattern. Temperatures will be near
guidance.

Tuesday afternoon and night, first system whisks away but secondary
shortwave zips into the central plains as upper ridge continues to
build. Thermal ridge will nose over the low level cold air. Best
lift in the far south where a mix of -SN/-ZR will occur. -ZR amount
tentative at this point, but could be enough to cause issues
overnight. Temperatures will be near guidance.

.LONG TERM...(Wednesday Through Monday)
Issued at 212 PM MDT Mon Mar 20 2017

Lingering precipitation will end early Wednesday as upper ridge
builds into the area. Temperatures will warm nicely.

Active progressive flow on track Thursday into next weekend. A
parade of shortwave troughs and upper lows will move across the
central Rockies into the central and southern plains. Right now,
this would be too far south of the CWA to bring appreciable snows
and associated cold air to the CWA, but -RASN could occur.
Confidence is growing for this scenario, but certainly not high.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Tuesday Night)
Issued At 1115 PM MDT Mon Mar 20 2017

Widespread -RA changing to -SN will occur across the area
tonight, starting over WY and then spreading into western SD.
Conditions will deteriorate with widespread IFR CIGS/VSBYS due to
stratus/precipitation. Local LIFR conditions are expected with
the heaviest snow bands and near the Black Hills. Widespread IFR
conditions will continue through 18Z. VFR conditions will develop
over northeastern WY after 18Z while IFR conditions will continue
over much of western SD until around 23Z. Areas of MVFR CIGS will
continue over western SD through Tuesday night.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until noon MDT Tuesday for SDZ001-012-
     024-025-072.

WY...Winter Weather Advisory until noon MDT Tuesday for WYZ056-057-
     071.

&&

$$

SHORT TERM...Helgeson
LONG TERM...Helgeson
AVIATION...10


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