Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 131931

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
131 PM MDT Sun Aug 13 2017

.DISCUSSION...(This Evening Through Sunday)
Issued at 127 PM MDT Sun Aug 13 2017

The upper low spinning over eastern ND is helping to
produce some thunderstorms across eastern SD and western MN.
Meanwhile, weak ridging with an embedded wave is pushing through
MT/WY, resulting in a few showers and storms across central WY.
The next trough to affect the region is moving over the Pacific
Northwest. Water vapor indicates a swath of dry air over much of
the CWA. The area of low clouds across the western SD plains
continues to dissipate this afternoon. At the surface, low
pressure sits over central SD, and a cold front/stationary
boundary extends from it south into NE, and back north into WY and
MT. Winds across the area are around 10 kts and generally from
the northeast, and temps are warming into the 70s.

Ridge rider will enhance a few showers and thunderstorms this
afternoon and overnight along the aforementioned boundary. Marginal
instability may result in a few stronger storms, but best CAPE and
forcing will be to the south and east of the CWA. Conditions could
be favorable for some patchy fog across central SD tonight, but that
will also be more likely east of the CWA. Added mention across
south central SD for now.

First of several shortwaves associated with the Pac NW trough will
move through eastern MT Monday, resulting in some showers/storms
mainly across western and northern parts of the CWA. Southeasterly
winds will become breezy across the western SD plains ahead of the
surface low, and MLCAPE will increase to 1000-2000 J/kg during the
afternoon. A stronger secondary wave will approach the region
towards evening, and 0-6 km shear of 40+ kts will support the
development of severe thunderstorms. Showers/storms will continue
overnight Monday and through Tuesday as the wave slowly slides
through the northern plains. Heavy rain will be possible with any
storm that develops, especially Monday night into Tuesday, when
PWATs are expected to be 150-175% of normal.

Broad upper trough will slowly cross the central states midweek,
with chances for showers/storms at times. Models are still showing a
ridge towards the end of the week, bringing drier weather. Near to
below average temperatures will continue through the week.


.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1114 AM MDT Sun Aug 13 2017

Areas of MVFR and even IFR cigs across western SD east of the
Black Hills will slowly dissipate this afternoon...but there may
some redevelopment tonight. Some TSRA may develop over northeast
WY into far southern SD this afternoon and tonight with local MVFR




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