Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 140516
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1116 PM MDT Wed Sep 13 2017

.DISCUSSION...(This Evening Through Wednesday)
Issued at 238 PM MDT Wed Sep 13 2017

Upper level analysis shows upstream trof over western Canada, an
upper low off the southern California coast, and weak ridging over
the northern high plains. Water vapor imagery shows ample high and
mid level moisture streaming into the Pacific Northwest and Upper
Rockies associated with and ahead of the approaching upper level
trof. At the surface, a weak frontal boundary extends from northern
Minnesota through northern South Dakota into southeastern Montana,
with a more pronounced cold front extending from southeastern
Saskatchewan through central and southwestern Montana.

For this afternoon and evening, thunderstorms will continue to
develop and move eastward, especially across northwestern South
Dakota along the weak frontal boundary. Heavy rainfall will be the
main threat from the storms with precipitable water values of 125%
to 150% of normal. Gusty winds and small hail also a possibility,
but overall the severe threat is low.

Beginning tonight and continuing into the weekend, moist and colder
air will move into the region as the upper trof approaches and
swings through the northern plains. The first surge of colder air
will push southward across the forecast area overnight. Chances for
precipitation will increase significantly, especially on Thursday
and Friday. Precipitable water values Thursday night and Friday
across the forecast area look to be 1.0 to 1.2" or 175% to 200% of
average. The chance for showers and thunderstorms will continue into
next week, but warmer temperatures will return for Monday and
Tuesday.

The system over the next few days is just the start to an overall
weather pattern shift for the area that should last through the end
of the month. It appears as if the typical fall/late September
pattern begins to sets up. Zonal to southwest flow aloft will allow
moisture and energy to stream into the area from the Pacific
Northwest. This will bring some chances for precipitation, some windy
days, and widely fluctuating day to day temperatures for the area
during the next couple of weeks.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1113 PM MDT Wed Sep 13 2017

Areas MVFR FU will continue through the period. Isolated/scattered
TSRA over northwest SD will end by 12z. Additional TSRA with local
MVFR conditions will develop in the afternoon over northeast WY
and then spread east into western SD Thursday night. MVFR CIGS
will move into northeast WY after 06z Thursday night.

&&

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

$$

DISCUSSION...MLS
AVIATION...Helgeson



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