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Area forecast discussion 
National Weather Service Riverton Wyoming
321 am MDT Sat may 30 2015

Short through Monday night

We are currently experiencing one of the driest nights in a while.
With saturated soils in many areas along with mostly clear skies
and light winds...will leave fog in some of the basins and valleys
for this morning.

Another prominent closed low pressure center is currently spinning
along the British Columbia coast with a trailing weak shortwave now
along the Pacific coast. This shortwave will rapidly track east
across Montana today thus flattening the weak ridge over US with most of
the energy from this weak shortwave to our north...the best convective available potential energy
will be over the west central and north today. will
not be that unstable today so have painted the highest scattered
probability of precipitation today over the western and northern mountains with just showers and
isolated thunderstorms in these areas expected for this afternoon
and evening. With light steering winds...most of the convection will
be anchored to the mountains temperatures will be around 10 degrees
warmer today with highs in the 70s at the lower elevations east of
The Divide and 60s west.

On Sunday the ridge will reamplify. This will enhance the southwest
flow on the backside of the high ushering in the warmest air of the
calender year thus far with many areas east of The Divide at the
lower elevations reaching the 80s for the first time and 70s west.
This southwest flow will also usher in some southern stream moisture
that will combine with increasing instability. An old surface
boundary that will still be draped across northeast Wyoming from the
frontal passage that occurred Thursday night will separate higher dew points
in the 50s and high temperatures in the 70s in Johnson County with highs in
the 80s and dew points in the 40s elsewhere...except highs in the
70s west of The Divide as mentioned above. This boundary will act as
a focus for thunderstorms over Johnson County. In addition...with a
smaller temperature/dew point spread expect more in the way of locally heavy
rain and small hail in Johnson County Sunday afternoon and evening.
Elsewhere...gusty winds and lightning should be the dominant
features with the thunderstorms moving in with the southwest
unstable flow.

On Monday the ridge continues to amplify resulting in scorching
temperatures compared to what we have become accustomed to lately with some
areas in the Big Horn basin reaching 90 degrees. A well defined
shortwave will track northeast across the area in the southwest flow
on the backside of the ridge resulting higher based but stronger
thunderstorms so will throw in small hail and gusty winds everywhere
for now. Storm Prediction Center just misses a slight risk for Johnson County by about a
half mile Monday.

One side note...increased southwest winds through the period...
mainly in the absarokas and East Slope of the wind
account for the increased southwest flow on the backside of the

Long term...Tuesday through Saturday


Aviation.../12z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

Local areas of fog will be possible through 14z with local MVFR/IFR
conditions. This includes the kcpr Airport. Otherwise VFR conditions
will prevail over the region. Isolated showers and storms will occur
over and near the mountains from 20z today until 02z Sunday. A few
showers will move east off the mountains and across the lower
elevations from 21z today until 03z Sunday.

West of The Divide...kbpi/kjac/kpna/krks routes

Local areas of fog or low clouds over northwest Wyoming overnight through 15z.
This may impact the kjac Airport...thus a mention in the terminal
forecast. Otherwise VFR conditions will prevail over the region.
Isolated showers and storms will occur after 19z over the western
mountains. Some of these showers or storms will move over the lower
elevations after 20z today until 03z Sunday.


Fire weather...

Drier and warmer weather for much of the area today except a few
thunderstorms could occur in the north. Isolated to scattered
showers will return Sunday afternoon across northern and central
portions. This will mark a return to another active weather
pattern through the middle of next week. Smoke dispersal each day
will be good to excellent...except for short periods in the very
early morning hours. A Haines of six will affect portions of
southern areas early next week.


Riw watches/warnings/advisories...


Short term...lipson
long term...wm
fire weather...lipson

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