Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 162232

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
432 PM MDT Thu Aug 16 2018

.DISCUSSION...(This Evening Through Thursday)
Issued at 303 PM MDT Thu Aug 16 2018

Current surface analysis shows weak high pressure over the area,
with a cold front across far south central Canada. Upper level
analysis shows low pressure over northwest IA, slowly moving away
from the region. Upper level ridge is in place over the Rockies,
with a trough along the Pacific Northwest coast. Skies are mostly
sunny, though hazy, across much of the area. Temps are in the
upper 70s and 80s with light winds.

Ridge will track east across the area tonight into Friday, with dry
and warm conditions continuing. Some thicker smoke from fires to the
west and northwest of the region will likely spread into the area
tonight and Friday and could reduce visibilities in spots. Highs on
Friday will be in the 80s and lower 90s. A thunderstorm or two could
try to develop over mainly southwest SD late in the day or early

The weekend looks to be much more unsettled as the trough along the
west coast moves east across the Rockies Friday night and into the
Northern Plains later Saturday into Sunday. Showers and
thunderstorms will increase from west to east on Saturday,
especially in the afternoon and evening. Moisture and instability
will increase Saturday ahead of a lee trough that is forecast to
move across northeast WY early Saturday and into western SD in the
afternoon. Steep lapse rates later in the day will help to produce
MLCAPE values of 1500 to near 3000 j/kg from the Black Hills
eastward. Shear does not look to be very impressive with relatively
weak winds aloft, but some strong to at least marginally severe
storms are certainly possible across much of the area in the
afternoon and evening. The better threat would likely be across
southwest SD where timing and instability is expected to be most
favorable. The other concern would be the threat for heavy rainfall,
especially near and east of the Black Hills, as precipitable water
values are forecast to be above average and storm motion looks to be
relatively slow.

Showers and storms will continue across much of the area later
Saturday night into Sunday. The heaviest rainfall later Saturday
night into Sunday will generally be toward central SD. The threat
for stronger storms should shift mostly south and east of the
forecast area on Sunday. A cold front passage late Saturday night
and Sunday morning will bring cooler air and breezy conditions
behind it for Sunday. Highs on Sunday will be in the upper 60s and

The system will exit the area late Sunday into early Monday, with
showers ending from west to east. A ridge building over the western
US will bring drier and eventually warmer conditions for much of
next week. The best chances for rainfall look to be late Monday into
Monday night over northeast WY and southwest SD as a weak
disturbance potentially clips the area.


.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 432 PM MDT Thu Aug 16 2018

VFR conditions will continue through the TAF period. Mid-to-upper
level smoke will increase from west to east on Friday. As this
mixes closer to the surface Friday morning and afternoon, some
reductions to vsbys will be possible, particularly over
northeastern WY. However, for now, vsbys are expected to remain
at least 6SM.




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