Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 121925
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
125 PM MDT Sat Aug 12 2017

.DISCUSSION...(This Evening Through Saturday)
Issued at 124 PM MDT Sat Aug 12 2017

Upper level analysis shows strong wave rotating
through eastern MT/western ND. Surface trough stretches across
western SD. Storms have started to develop across south central SD,
as well as a few scattered showers across the northern Hills and
parts of the western SD plains. Winds are around 10 kts from varying
directions, and temperatures are rising into the 70s.

Breezy southeasterly winds ahead of the surface low will contribute
to increased instability as warm, moist air moves into the region.
Best MLCAPE around 2000 J/kg with minimal CIN is expected across
central SD this afternoon. CAMs are fairly consistent in showing
storms developing across northwestern SD by mid-afternoon as the
wave pushes into that area, as well as another area of storms
forming across south central into eastern SD along a line of
enhanced Q-vector convergence ahead of the surface trough.
Northwestern SD storms may strengthen as they move southeastward and
approach the more unstable environment. Supercells with large hail
and damaging wind will be supported by 0-6 km shear of 35-45 kts,
and PWATs of 125-150% of normal indicate the potential for heavy
rain. Storms are expected to exit the CWA around midnight.

A few models are showing some fog developing late tonight into
Sunday morning across northwestern SD to the northeastern foothills.
This is supported by overnight RH values near 100% and light
upsloping winds, so have added mention of fog in grids.

A mainly dry day is expected Sunday as a weak upper ridge briefly
builds over the region, although an even weaker wave passing through
the ridge may produce some storms mainly across the southwestern 1/3
of the CWA. Decent shear and some marginal instability may result in
a few stronger storms. Highs Sunday will be in the mid 70s to mid
80s.

Ridge axis will cross the northern plains Monday, and temps will
warm slightly. A series of waves associated with a Canadian low
pressure system will begin approaching the area in southwest flow,
the first of which is expected to spark another round of showers and
thunderstorms Monday afternoon and evening. Decent instability and
shear will support strong to severe storms again. Chances for
showers/storms will continue through midweek, although at this point
it appears the threat for severe weather will gradually diminish.
Both the GFS and ECMWF agree on a broad ridge building over the
western CONUS towards the end of the week, bringing drier weather.
Temperatures are expected to be near average through the week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1123 AM MDT Sat Aug 12 2017

Scattered afternoon/evening TSRA expected, mainly over western and
central SD. Local IFR conditions, gusty winds, and isolated
severe storms possible. Some IFR cigs/vsbys may redevelop later
tonight across northwest SD. Across northeast WY, VFR conditions
expected through the period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...Pojorlie
AVIATION...Johnson



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