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Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
455 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 Iowa 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 relative humidity will be around 20%
across southwestern South Dakota 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 South Dakota plains by mid afternoon.
Thunderstorms are expected to develop across northeastern Wyoming and
southwestern South Dakota mid afternoon, with some strong storms possible.
However, the better chances for severe weather will be farther
east across the western and central South Dakota plains with better
instability, more towards evening when the cap breaks down. Cams
are showing a line of storms developing across western South Dakota and
moving into central South Dakota 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 South Dakota late this evening, with a few trailing
storms possible across western South Dakota 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 Wyoming 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 County Warning Area 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 12z tafs through 12z Wednesday morning)
issued at 450 am MDT Tue Jun 27 2017

VFR conditions are expected through early this afternoon.
Southerly low level jet will bring low level wind shear to the
area through 15z. A cold front will move through the region this
afternoon and evening bringing scattered thunderstorms and rain with local MVFR
conditions along with the possibility of hail and gusty winds.


Unr watches/warnings/advisories...
South Dakota...none.

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