Scientific Forecaster Discussion
fxus63 kunr 220432
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1032 PM MDT Wed Aug 21 2019
Discussion...(this evening through wednesday)
issued at 206 PM MDT Wed Aug 21 2019
Current surface analysis shows frontal boundary to the south and
west of the forecast area, with high pressure centered over ND.
Upper level analysis shows low pressure over Ontario, with a ridge
over The Rockies. Upper trough is located off the West Coast.
Skies are clear to partly cloudy with temps in the 70s to around
80 degrees. Kudx radar is quiet at this time, but regional radars
show some storms across the Big Horn mts and the higher terrain of
The forecast looks relatively quiet in the near term, but a period
of more unsettled weather is expected from late Thursday through the
weekend as the trough along the West Coast slowly moves east across
the western US and plains. Temperatures will mostly be below average
through the period, especially early next week.
For late today and this evening, isolated showers and thunderstorms
are expected across northeast Wyoming and possibly into far southwest South Dakota
as weak energy aloft crosses over the ridge. The best cape values
late today will be located along and just east of the frontal
boundary from eastern Wyoming to the NE Panhandle. Some of the convective
models are showing a stronger storm or two clipping Campbell County
Wyoming early this evening, so this looks to be where the better
potential will be for a storm or two. Otherwise, any activity will
dissipate by midnight, with temps dropping into the upper 50s and
lower 60s in most areas by morning.
Most of the day on Thursday looks nice as ridge moves across the
Dakotas. However, south to southeast winds will become quite breezy
across much of the area, especially northeast Wyoming to northwest South Dakota.
Highs will be in the upper 70s and lower 80s across much of the
area, a bit warmer over the northeast Wyoming plains. As the upper trough
moves into the Great Basin region, southwest flow aloft will develop
over The Rockies into the High Plains. Isolated to scattered storms
will likely develop across northeast Wyoming late in the day and early
evening, possibly making it into at least far western South Dakota during the
evening/overnight hours. Shear looks to be relatively weak late
Thursday, but increasing instability along and ahead of the frontal
boundary will lead to the possibility for isolated strong to severe
storms across these areas.
The better and more widespread chances for storms look to be Friday
afternoon through Saturday night, and possibly Sunday as well, as
the upper trough slowly crosses the region. Isolated strong storms
look possible late Friday from northeast Wyoming to the Black Hills area,
with the better chances shifting further east to western and central
South Dakota on Saturday. A cold front will eventually move southeast across
the region over the weekend, with Saturday looking to be the warmer
day in most areas.
Upper trough will intensify east of the region late in the weekend
and early next week, with drier and cooler northwest flow likely
dominating the first half of next week. Temps look to be below
average, with highs in the upper 60s and 70s.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1029 PM MDT Wed Aug 21 2019
Continued moist southeast upslope flow may support MVFR to IFR cigs
overnight and Thursday morning on the South Dakota plains, otherwise VFR
conditions expected through Thursday. Thunderstorm chances will
increase Thursday evening across northeast Wyoming and far western South Dakota.