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FXUS63 KUNR 102108

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
208 PM MST Tue Dec 10 2019

.DISCUSSION...(This Evening Through Tuesday)
Issued at 208 PM MST Tue Dec 10 2019

Water vapor imagery this afternoon shows nern WY/wrn SD on the wrn 
periphery of a broad upper trof centered over the Upper Midwest. An 
associated upper jet streak is above the area, identified by cloud 
cover and a few embedded snow showers in its left exit region across 
nwrn SD/swrn ND/sern MT. Shortwave ridging is approaching the nrn 
Rockies, with another shortwave trof near the Pac NW. 

Limited snow chances this evening into tonight will be focused in 
two areas: 1) the nrn/cntrl Black Hills, where Froude number/RH 
profiles are marginally favorable for upslope snow through 
approximately 03z, and 2) the nwrn SD plains, given increasing 
forcing overnight owing to the passage of a backdoor cold front and 
secondary upper jet streak. That said, suspect that any 
accumulations will be very modest given lackluster thermo profiles 
in forecast soundings.  

As initial high pressure shifts ewd tonight, a second high builds 
into the ern Dakotas through Wed AM behind the backdoor cold front. 
This will bring a period of upslope flow to the wrn SD plains and 
nrn/ern foothills beginning late tonight, which will likely lead to 
the development of low clouds and potentially some freezing fog in 
these areas. Expect this potential will be aided by snowmelt and 
flow over lingering snowpack across the plains. 

Another shot for light snow over nwrn SD is expected Wed evening and 
night as a second weak jet streak/shortwave crosses the nrn Plains. 
However, little to no impacts expected. Low-level thermal ridging 
builds across the area on Thursday, with high confidence in above 
normal temps through early Friday. 

Next chance for precip and blast of cold air arrives late Friday 
into Saturday. Confidence is low in hazards for now, but certainly 
looks like most areas will see measurable precipitation and at least 
breezy conditions on Friday. Best potential for appreciable snow 
accumulation looks to be over the nrn Black Hills for now, given a 
favorable upslope setup.

Below normal temps are then expected through the weekend; however, 
the thermal trof axis will settle in to our east, sparing us from 
the coldest air. Although the deterministic GFS advertises another 
blast of cold air near the end of the forecast period, most GEFS/EPS 
members suggest temps will moderate by the middle of next week, 
suggesting that odds favor near to above normal temps over the 
latter half of next week.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1026 AM MST Tue Dec 10 2019

Areas of MVFR CIGS in occasional light snow or flurries will occur
over parts of northeastern Wyoming, the Black Hills and
northwestern South Dakota today into tonight. Rather widespread
MVFR to IFR CIGS expected to develop on the plains of western SD
after 09Z, including the KRAP terminal. The KGCC terminal is
expected to remain VFR through the period.



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