Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 211957
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
157 PM MDT SAT MAY 21 2016

.SHORT TERM...(This Evening Through Sunday Night)
Issued at 151 PM MDT Sat May 21 2016

19z surface analysis had surface low over north central WY with
frontal boundary north of low into eastern MT and south of low
into central CO. Strong southerly flow ahead of low with stratus
and stratocumulus clouds over most of the SD CWA. Solar insolation
is working on CIN over northeast WY with partial erosion over
central WY evidenced by SPC analysis and CU starting to pop near
low and boundary. CIN quite strong over SD for surface based
convection. 110kt jet streak working into WY which will increase
the 0-6km bulk shear to 20m/s over northeast WY. If thunderstorms
can develop with around 2KJ/kg MLCAPE available, a couple of
supercells are likely across northeast WY through this evening
with large hail and damaging winds. Isolated tornado threat in
place given subjective pattern recognition with position of
surface low, low level southeast flow, and rich boundary layer
moisture. Activity will shift east overnight and become elevated
reducing severe threat substantially over western SD overnight.
Temperatures will be near or slightly above guidance.

Sunday, aforementioned jet streak spins up another upper low over
southern AB with decent shortwave rotating it around it into WY and
western SD Sunday afternoon. Frontal boundary will push east as low
pushes northeast and drags feature southeast. 2-3KJ/kg SBCAPE
forecast ahead of boundary with 20-30m/s 0-6km bulk shear. A recipe
for severe thunderstorms, potentially significant, as long as
boundary and synoptic forcing don`t push too far east too fast.
Temperatures will be near guidance.

Sunday night, thunderstorm activity shifts east and drier air
pushes in behind front. Temperatures will be seasonal.

.LONG TERM...(Monday Through Saturday)
Issued at 151 PM MDT Sat May 21 2016

Mean upper trough will change little next week extending from
southwest Canada into the northwest US. Southwest flow aloft will
prevail over the CWA with periodic shortwaves and a couple of
transient upper lows passing through. Hard to pinpoint timing at
this point, so PoPs generally broadbrushed. Monday looks to be the
only nearly dry day. The weather does look unsettled with daily
chances for storms Tuesday through next weekend. Temperatures will
be seasonal.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 151 PM MDT Sat May 21 2016

Some MVFR ceilings may linger across portions of the western SD
plains this afternoon, with more widespread MVFR/IFR conditions
this evening and overnight as low clouds/fog/showers move back in.
Gusty southerly winds are also expected this afternoon, especially
across the western SD plains, with speeds of 25-40 mph and gusts
near 50 mph. Thunderstorms are expected to develop over
northeastern WY this afternoon and evening, with MVFR conditions
and gusty winds possible with any showers and storms.

&&

.UNR Watches/Warnings/Advisories...
SD...Wind Advisory until 9 PM MDT this evening for SDZ001-002-012>014-
     073.

WY...None.
&&

$$

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


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