Scientific Forecaster Discussion
fxus63 kunr 200523
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1123 PM MDT Thu Sep 19 2019
Discussion...(this evening through thursday)
issued at 104 PM MDT Thu Sep 19 2019
Water vapor shows large western Continental U.S. Trof with southwest flow
aloft through the northern plains. At the surface, high over the
upper Midwest and trof over eastern Montana/Wyoming producing gusty
southerly winds across the region. Low level moisture transport
has already pushed dewpoints to near 70f in south central South Dakota.
Lead short wave combined with strengthening low level jet/Theta-E
advection will bring a gradual increase in chances for storms
tonight. Latest cams suggesting the best potential will be over
northeast Wyoming and far western South Dakota.
Thunderstorm chances will continue areawide on Friday within
disturbed southwest flow to the east of upper trof crossing The
Rockies. Best chances for widespread activity will be late Friday
afternoon into Friday night as strongest impulse rotates through
the region. Best axis of cape/shear will be east of the Black
Hills along/near the cold front where some severe storms are
Main upper low pushes across ND on Saturday, with gusty northwest
winds and lingering rain showers across the area. Could see near
advisory winds across northwest South Dakota. Lows Saturday night under
clearing skies well down into the 40s, with some 30s from
northeast Wyoming into southwest South Dakota and the Black Hills.
Dry and warmer conditions expected early next week with upper
ridge overhead. Splitting trof midweek will keep US mainly dry
with slightly cooler temps.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1123 PM MDT Thu Sep 19 2019
Isolated/scattered rain showers/thunderstorms and rain will continue to develop across the area
tonight, which could bring localized MVFR conditions. Small hail
will also be possible in the strongest storms. A potent S/SW low-
level jet of 50-60 kt will produce low-level wind shear in the
0-3 kft above ground level layer over the South Dakota plains from now through 09-10z.
Additional rain showers/thunderstorms and rain are expected Friday afternoon and evening,
which could produce brief, localized IFR conditions and the threat
for large hail/damaging winds. Confidence in timing is too low to
include in the krap taf for now, but will likely begin in the
21-01z time frame. Behind the storms, MVFR cigs may develop
tomorrow night as a cooler air mass overspreads the region.