Area Forecast Discussion
Issued by NWS Rapid City, SD

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305
FXUS63 KUNR 270856
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
256 AM MDT Tue Jun 27 2017

.DISCUSSION...(Today Through Monday)
Issued at 248 AM MDT Tue Jun 27 2017

The axis of the upper ridge is over the Rockies, and a
trough over the Pacific Northwest is beginning to break down the
ridge. Some high clouds are streaming into the northern plains
through northwest flow. With the surface high over IA and the
surface trough approaching the CWA, winds are southerly and gusting
to 30 mph in some locations. Temperatures at 3 AM are in the
upper 50s and 60s, with even some 70s across the downsloped
northern foothills.

Thermal ridge will shift over western SD, and temperatures will rise
to the mid 80s to lower 90s. Tight pressure gradient ahead of the
approaching surface trough will create breezy southerly winds of 20-
30 mph, with gusts near 40 mph at times. Min RH will be around 20%
across southwestern SD and parts of northeastern WY, and winds
there will be breezy, but only about 15-20 mph. Farther east, dew
points will rise into the 50s, and around 2000 J/kg ML CAPE will
develop across the western SD plains by mid afternoon.
Thunderstorms are expected to develop across northeastern WY and
southwestern SD mid afternoon, with some strong storms possible.
However, the better chances for severe weather will be farther
east across the western and central SD plains with better
instability, more towards evening when the cap breaks down. CAMs
are showing a line of storms developing across western SD and
moving into central SD this evening ahead of the cold front. Main
threats will be large hail and damaging winds. Low level bulk
shear and helicity show a small potential for a tornado across
south central SD, with better chances farther east. Storms will
shift into eastern SD late this evening, with a few trailing
storms possible across western SD overnight.

Temps will be about 10 degrees cooler on Wednesday behind the cold
front. Another weak wave associated with the upper trough will
produce some showers/storms across northeastern WY and the Black
Hills. ML CAPE up to 1000 J/kg and 0-6 km shear around 50 kts could
support and isolated strong to severe storm. Then chances for
showers/storms will expand across the CWA overnight and into
Thursday, as the main trough crosses the northern plains. Cool air
will continue to wrap into the region, with highs Thursday and
Friday in the 70s. Mainly dry weather is expected for Friday and
Saturday, as an upper ridge builds over the western CONUS, and a
warming trend will begin.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Tuesday Night)
Issued At 1038 PM MDT Mon Jun 26 2017

VFR conditions are expected through tonight. A southerly low
level jet will bring some low level wind shear to Northeast WY
and western SD through the night. A cold front will settle into
the region Tues, bringing veering winds and eventually sct SHRA/TS
late afternoon/evening.


&&

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

$$

DISCUSSION...Pojorlie
AVIATION...JC



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