Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 270520

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1120 PM MDT Sat May 26 2018

.DISCUSSION...(This Evening Through Saturday)
Issued at 137 PM MDT Sat May 26 2018

Western CONUS mean troughing will bring persistent SW flow
through much of the period. This will support an active pattern as
several impulses eject NE into the Northern Plains. Upper level
ridge over the area today suppressing convection and allowing for
very warm temperatures. Subtle speed max will advect through the
region tonight. Pos theta-e adv associated with this feature might
support a cluster of northward moving elevated convection, given
elevated instability in place. Retained 20 pop mention for this.
Expected convective complex over MT/ND associated with the
aforementioned impulse will convectively force a weak cool front
into the region tonight, with a period of breezy NW winds expected
across NW SD. This trough will stall over the area Sun with a
decent upper trough pushing into the region. Ample LL moisture
will advect into the region per SE flow, supporting CAPE of
1.5-2.5 KJ/KG over NE WY and Western SD. Increasing shear per
advecting jet steak should support a decent chance of evening TS
with some severe storms certainly possible given
moisture/CAPE/shear numbers. Strong inversion in place will likely
limit any convection to perhaps the Black Hills at first, with
chances increasing in the evening as an MCS pushes NE out of
eastern WY. Another thing to watch will be the threat for heavy
rain. Several hires models in addition to the NAM and GFS prong
very heavy rainfall with tomorrow`s expected convective complex,
with over 2 inches possible. Storm motion is pretty slow at 10-15
knots, with MCS forcing possibly leading to training in some areas
for period. The heaviest rain attm looks to fall around the BH
area. However,the exact location of heavy rain (as well as svr wx)
will hinge on where the sfc boundary stalls out, which remains
uncertain attm. Felt at least a mention of heavy rain in this area
was a good first step and will let the night shift look at things
to determine if a flash flood watch maybe prudent for Sunday

Sfc boundary will linger over the area Mon with sufficient flow
aloft and ll moisture certainly in place to support more diurnal
shra/TS, some possible severe once again. Bulk of upper trough
will advect through the region Tues, with another good chance of
convection once again, esp from the Black Hills east. Drier
weather is expected mid next week with weak ridging in place
before another round of western CONUS troughing develops and
supports unsettled flow once again. Look for a brief cool down
early next week, with a warming trend mid week and on.


.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1116 PM MDT Sat May 26 2018

Mainly VFR conditions will prevail through Sunday afternoon.
Isolated thunderstorms are possible tonight, especially over
northeast WY into far northwest SD. Low level wind shear is
expected on the SD plains due to a low level jet. Higher coverage
TSRA will develop late Sunday afternoon and persist into Sunday
night - in/near storms expect IFR conditions.




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