FXUS63 KUNR 171724

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1124 AM MDT Tue Jul 17 2018

Issued at 815 AM MDT Tue Jul 17 2018

Latest NAM/RUC and CAMs suggest widespread thunderstorms late this
afternoon/evening. Given slow storm motion, have expanded Flash
Flood Watch for late this afternoon/tonight.


.DISCUSSION...(Today Through Monday)
Issued at 312 AM MDT Tue Jul 17 2018

Latest upper analysis shows an upper ridge over the western CONUS
with a couple of disturbances moving through the upper ridge and
affecting the forecast area. The first disturbance is a relatively
compact shortwave/vort max located over Nebraska that is
currently producing some showers and thunderstorms over parts of
south central SD. That feature will continue to move off to the
southeast, ending the showers and thunderstorms there a little
later this morning. To the west of the forecast area is a
shortwave trough that will increase in strength as it affects
the forecast area this afternoon, tonight and on Wednesday.

Southeasterly upslope low-level flow will push plenty of
moisture into the region today. Dewpoints are already in the mid-
50s to lower 60s over the forecast area, with the calculated
precipitable water increasing to 1.60"+ in some areas this
afternoon. The abundance of moisture combined with the potential
for slow moving storms indicate that heavy rainfall is possible.
At this time, most of the models agree that the area with the
highest potential for heavy rain will be in a NW-SE oriented band
across Weston county in northeastern Wyoming and then into Custer
and Fall River counties in southwestern South Dakota. Due to the
steep terrain and the potential for heavy rain this evening, have
issued a Flash Flood Watch for those areas tonight. Some severe
thunderstorms are also possible over northeastern Wyoming, the
Black Hills area and southwestern South Dakota this afternoon and
evening, with large hail and damaging wind gusts the main severe

Cyclonic upper flow will remain over the region on Wednesday,
which combined with diurnal heating and abundant low level
moisture, will again lead to chances of showers and thunderstorms
with some severe thunderstorms possible during the afternoon and
evening, mainly on the plains east of the Black Hills and into
south central South Dakota.

Dry and warmer weather is then expected for Thursday, Friday and
Saturday as the upper ridge rebuilds over the forecast area. An
upper level low crossing the Canadian prairie provinces will
flatten the upper ridge and bring another chance of showers and
thunderstorms on Sunday and Monday.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1121 AM MDT Tue Jul 17 2018

TSRA with MVFR/local IFR conditions will develop late in the
afternoon across northeastern WY and the Black Hills, then spread
east onto the SD plains tonight. Strong to severe storms will be
possible. Additionally, heavy rainfall will be possible with any
storms. Storms will shift into central SD Wednesday morning.


Issued at 312 AM MDT Tue Jul 17 2018

Due to the potential for slow moving thunderstorms and areas of
heavy rain, have issued a Flash Flood Watch for the southern
Black Hills area for tonight, including Weston, Fall River and
much of Custer counties.


SD...Flash Flood Watch from 3 PM MDT this afternoon through this
evening for SDZ024-028.

Flash Flood Watch from 5 PM MDT this afternoon through late
tonight for SDZ012-025>027-029-030-041-072-074.

WY...Flash Flood Watch from 3 PM MDT this afternoon through this
evening for WYZ054>058-071.



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