Area Forecast Discussion
Issued by NWS Rapid City, SD

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239
FXUS63 KUNR 232326
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
526 PM MDT Sat Sep 23 2017

.DISCUSSION...(This Evening Through Saturday)
Issued at 252 PM MDT Sat Sep 23 2017

Current surface analysis shows cold front now well south and east
of the region, with very cool air in place. Upper level analysis
shows large scale trough over the western US, with fast south-
southwest flow over the Rockies into the Northern Plains. Skies
are cloudy with scattered to numerous showers expanding in
coverage across much of the area. Some thunderstorms with heavier
rainfall are also tracking across south central SD. Temps are
mostly in the 40s over northeast WY and far western SD, with upper
40s to mid 50s to the east of the Black Hills.

Wet weather is to continue through the rest of the weekend, with
well below average temperatures, as the upper trough only slowly
move east-northeast. A series of disturbances will continue to bring
showers at times. The next and strongest disturbance is moving into
the area now, with showers increasing in coverage and intensity. The
heaviest rainfall will generally be to the east of the Black Hills
through Sunday, especially toward south central SD, where 1 to 3
inches of total rainfall is likely for the weekend, with locally
higher amounts certainly possible. Further west, a half inch to
around an inch is likely in many areas, though some amounts less
than that are likely over parts of northeast WY on the western edge
of the main shower bands. Highs will stay mostly in the mid 40s to
mid 50s Sunday and Monday. Temperatures will not drop much tonight
across the area with saturated airmass in place, but could still get
cold enough for some wet snow to mix in over the highest elevations
of the Black Hills later tonight/Sunday morning. Could see a mix or
changeover to snow spread lower across the Black Hills and into
parts of northeast WY Sunday night into Monday, though pcpn is
expected to be relatively light and more spotty by that time.

Main trough and bulk of upper level energy will begin to lift north
of the area on Monday, with showers shrinking in coverage and
eventually ending for the most part by Monday night. Southern
portion of the large trough will break off, settle further south
over the southwest US and become a cutoff low, while drier
northwesterly flow develops over our region for the rest of the
week. Temperatures will recover some mid to late week, but still
stay a bit below average, mostly 60s by late week. Could see a more
significant warmup toward the latter part of the weekend if ridge
builds further east at that time. Forecast still looks to be mainly
dry Tuesday through the end of the week, with a ridge building over
the Pacific NW and the Rockies.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 520 PM MDT Sat Sep 23 2017

Widespread IFR conditions with LCL LIFR conditions with areas of
fog and SHRA will occur across northeastern Wyoming and western
and central South Dakota tonight into early Sunday afteroon.
Isolated thunderstorms are also expected. Conditions will improve
to MVFR cigs with -SHRA over parts of western SD later on Sunday
afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...26
AVIATION...10



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