Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 262004
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
204 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 Thursday, 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.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued at 202 PM MDT Fri May 26 2017

VFR conditions are expected today. Late this afternoon and
evening, scattered showers and isolated thunderstorms will move
into northeastern WY and spread across mainly the southern half of
western SD. MVFR/local IFR conditions will be possible in
showers/storms especially across the Black Hills. Conditions will
gradually improve Saturday morning.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...13


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