Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 190501
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1101 PM MDT Sat Aug 18 2018

.DISCUSSION...(This Evening Through Saturday)
Issued at 258 PM MDT Sat Aug 18 2018

Current surface analysis shows lee trough over far western SD,
with weak low pressure over southwest ND. Upper level analysis
shows strong trough over MT, starting to close off as it moves
over the high plains. Skies are mainly cloudy over northeast WY
and far western SD, with increasing clouds to the east. KUDX radar
shows showers and storms from northeast WY into the Black HIlls,
with the strongest activity over the northern Black Hills area.
Temps range from the 60s and lower 70s over northeast WY and the
Black Hills to near 90 over south central SD. Winds are generally
west- northwest behind the surface trough, southerly to the east
of it.

Unsettled and wet weather can be expected through the weekend as the
trough moves across the region. The rainfall will generally come in
two phases. The first will be with the passing surface trough and
shortwave energy through early this evening. Another batch of
rainfall is then expected later tonight through much of Sunday with
the passing upper level system. Many areas will receive beneficial
rainfall over the weekend, a half inch or more, with some areas from
the northern Black Hills and northwest SD to central SD seeing 1 to
2 inches or more.

For late this afternoon and early evening, showers and thunderstorms
will continue to expand eastward across western SD, with some
decrease in coverage likely over northeast WY. Plenty of instability
around for at least isolated strong to marginally severe storms over
western and central SD, MUCAPE values of 1000 to over 2000 j/kg.
However, shear and winds aloft are relatively weak and will likely
result in most of the stronger storms not sustaining themselves for
very long. With precipitable water values above average, locally
heavy rainfall can be expected. Any stronger storms will generally
move east of the area mid to late evening, with lingering showers
and a few storms overnight. Lows tonight will be in the 50s and
lower 60s.

Sunday will be cool, cloudy, blustery, and showery for most areas as
the upper level system passes over the region. A few thunderstorms
are likely, especially to the south and east of the Black Hills,
with some heavier rainfall expected. Highs will only be in the
60s to mid 70s. Showers will end and skies will gradually clear
from northwest to southeast Sunday afternoon and night as the
system moves away.

Cooler conditions will linger into at least the first part of the
week, with highs mostly in the upper 60s and 70s on Monday and
Tuesday. A ridge is forecast to build over the western US during the
first half of the week. As it does, a weak disturbance could bring
isolated to widely scattered showers and thunderstorms Monday night
into Tuesday, especially to northeast WY and southwest SD. At this
point, the rest of the week looks mostly dry and at least seasonably
warm. The ridge to the west will likely weaken some as is moves
toward the plains late in the week.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1101 PM MDT Sat Aug 18 2018

TSRA/SHRA will continue to overspread the area through the
remainder of the night. Meanwhile, cigs will gradually
deteriorate to MVFR/IFR across the majority of the area.
Northwesterly wind gusts of 30-40 kt will return late overnight,
particularly over the SD plains, and continue through mid-
afternoon Sunday. Conditions will gradually improve from west to
east Sunday afternoon as high pressure builds into the region.

&&

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

$$

DISCUSSION...26
AVIATION...Sherburn



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