Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 231725

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1125 AM MDT Fri Sep 23 2016

.SHORT TERM...(Today Through Saturday Night)
Issued at 239 AM MDT Fri Sep 23 2016

08z surface analysis had low pressure over northern CO and
southwest WY with developing warm front into eastern NE. Easterly
upslope flow persists over the CWA with areas of fog and drizzle.
Water vapour showed upper low over eastern NV, which is the main
forecast concern this morning.

This morning, lead shortwave over WY shifts north. 850-700mb theta-e
advection increases as warm front lifts into northern NE and surface
low shifts into southeast WY. Should see an uptick in -shra/-tsra
activity north of this boundary. Areas of fog and drizzle will
continue given easterly upslope flow, but conditions will start to
improve later this morning as T/Td spreads widen.

This afternoon and this evening, dynamic evolution of aforementioned
upper low expected. It will move from UT into WY as surface low
races from southeast WY into northwest SD, which will push warm
front through eastern half of CWA in the afternoon. Narrow tongue
of 1KJ/kg MLCAPE develops over central portions of the CWA. 0-6km
bulk shear rapidly increases to 25-35m/s late this afternoon and
early this evening. Low level hodographs look fairly impressive
along frontal boundaries. The big wild card with severe weather
potential is amount of clearing and solar insolation. If stratus
sticks around, a much tamer blob of storms will occur. Some
clearing could result in an active late afternoon and early
evening with fast moving storms and perhaps a tornado near surface
low. With temperatures ranging from the lower 60s over northwest
SD to the upper 80s over south central SD across front boundary,
forecast uncertainty is high. CAMs show this with hour-to-hour
changes in evolution of storms. Went with high pops over central
CWA where confidence is highest in precipitation.

Overnight into Saturday, upper and surface low stack up over eastern
MT and push into central ND. Deformation zone precipitation expected
over northwest portions of the CWA. Fast moving cold front will push
into CWA Saturday afternoon with gusty west to northwest winds. NAM
weakest with GFS showing 45-55kt 850mb wind and well-defined cold
air advection. Will likely need wind headlines when all is said and
done with perhaps high wind criteria being met in the far north.
Will highlight in HWO for now given model differences. Temperatures
will be near guidance.

.LONG TERM...(Sunday Through Thursday)
Issued at 239 AM MDT Fri Sep 23 2016

The storm system will depart the region Sunday into Sunday night.
Behind the system...clearing skies and gusty winds expected. NW
winds will increase Sunday morning and continue through the
afternoon at 25 to 35 mph. Strong high pressure/upper ridging will
move into the region through at least midweek. This will bring dry
weather and a warming trend to the area with temps reaching the
70s to lower 80s.


.AVIATION...For the 18Z TAFS Through 18Z Saturday
Issued At 1119 AM MDT Fri Sep 23 2016

Widespread IFR cigs/fog cont acrs much of the fcst area in late
mrng, with conditions improving durg the aftn as low press ovr
nern CO moves into wrn SD. Shwrs and stg tstms wl dvlp ovr nern WY
in the aftn, then move into wrn SD by early even. MVFR/IFR cigs wl
form behind the low and cont thru much of the night alg with shwrs.
Gusty NW winds wl incrs behind the low, then bcm westerly Sat mrng.




SHORT TERM...Helgeson
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