Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 181723

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1123 AM MDT Sat Aug 18 2018

.DISCUSSION...(Today Through Friday)
Issued at 317 AM MDT Sat Aug 18 2018

Current upper level analysis highlights a fairly potent upper level
trough centered over Idaho. Ahead of this trough, showers and a
few thunderstorms have formed from Colorado to Montana. Surface
winds are southeasterly as return flow has commenced ahead of the

The approach of this trough and subsequent cyclogenesis will
dominate this weekend`s weather. Moisture return is ongoing ahead
of the trough and will contribute to anomalously high precipitable
water values today (1-1.75 inches). Steep lapse rates coupled
with surface dewpoints around 60 degrees will yield MLCAPE
500-1800 J/kg across the region. However, flow aloft will be
rather weak and resulting bulk shear of 25-35 knots may limit
overall severe weather threat today. Despite the lack of more
favorable shear, marginally severe wind and hail are possible
today, especially with any discrete storms.

Additionally, localized flooding will be possible in response to
weak steering flow, slow storm motion, and high precipitable water
values today and into Sunday.

The cold front will crash through the region this afternoon and
evening. Rapid pressure rises behind the front will allow for gusty
winds to develop this afternoon through Sunday. Rain and storms will
taper off Sunday with much of the forecast area drying out by Sunday
evening. Much cooler temperatures Sunday behind the front with mid
60s to low 70s for highs.

Another wave will clip the area Monday night and bring with it more
chances of showers and storms, mainly for northeastern Wyoming and
far western South Dakota. The latter half of the week will be warmer
and drier.


.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1120 AM MDT Sat Aug 18 2018

Showers and thunderstorms will continue to develop from west to
east across the area today, with widespread coverage later this
afternoon and evening. MVFR/local IFR conditions expected with the
thunderstorms. After the initial wave of showers/thunderstorms
cross the area, a brief dry period may be possible before
widespread showers develop again this evening/overnight. MVFR/IFR
ceilings will increase in coverage tonight, around 03Z across
northeast Wyoming and after 06Z across western South Dakota.




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