Area Forecast Discussion
Issued by NWS Rapid City, SD

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525
FXUS63 KUNR 262334
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
534 PM MDT Fri May 26 2017

.DISCUSSION...(This Evening Through Friday)
Issued at 202 PM MDT Fri May 26 2017

20z surface analysis had weakening high pressure over the CWA
with trough from ID into northeast CO, where surface low was
beginning to deepen ahead of shortwave/upper low noted over the
WY/ID border. Effect of this feature main concern.

Tonight through Saturday evening, shortwave/upper low opens up and
weakens as it moves east across WY tonight and then through the CWA
Saturday. CWA will be under weakly coupled jets tonight with 80kt
jet streak departing via the eastern Dakotas into MN and approaching
80kt jet streak chugging across northern UT into southeast WY.
Divergence aloft will continue to sync with increasing low level
frontogenesis/850-700mb theta-e advection producing a shield of
precipitation tonight over the southern two thirds of the CWA.
MUCAPE less then 300J/kg will support isolated TS through this
evening across the southwest half of the CWA and then overnight
across the far south. 1/4-3/4" QPF expected across southern half of
CWA by Saturday morning. Main precipitation shield will move east of
the CWA Saturday morning, but steep lapse rates and weak SBCAPE will
support some -SHRA/TS during peak heating Saturday afternoon/evening.
Temperatures will be near guidance.

Sunday and Monday, northwest flow aloft and weak cyclonic flow will
support diurnal CU development, but air will be fairly dry so any
PoPs should be limited to the terrain of northeast WY/Black Hills
Sunday afternoon and over the Black Hills Monday afternoon.
Temperatures will be near guidance.

Tuesday through Friday, upper ridge moves into the northern plains
with somewhat drier and warmer air, but upper low developing over CA
will moves into the southwest US breaking down ridge and likely
forcing split flow. Thus, confidence is low for the actual warm-up
and dry PoPs, especially by Friday.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 526 PM MDT Fri May 26 2017

Scattered showers and isolated thunderstorms will occur tonight
over northeastern Wyoming, the Black Hills area, and over
southwestern and south central South Dakota. LCL MVFR conditions
can be expected in and near any showers or thunderstorms. Over
northeastern Wyoming, the Black Hills and southwestern South
Dakota, widespread MVFR conditions will occur from 03Z this
evening until around 13Z on Saturday morning. Widespread MVFR
conditions are expected over south central South Dakota from 11Z
Saturday morning until around 18Z Saturday.

&&

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

&&

$$

DISCUSSION...Helgeson
AVIATION...10



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