Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210809

Area Forecast Discussion
National Weather Service Riverton WY
209 AM MDT Thu Jun 21 2018

.SHORT TERM...Today through Saturday night

A shortwave will enter E ID/SE MT this afternoon and develop
isolated showers and thunderstorms by late afternoon over
northwestern portions. Some of this activity will spread into the
Bighorn Basin by 00Z Friday and continue eastward into
Sheridan/northern Johnson Co. by 03Z, as further destabilization
occurs in those areas. These storms could be strong to severe, with
bulk shear close to 50 kt, CAPE values of 1000-1500 J/kg, steep mid
level lapse rates and PW values of 0.50" to 0.75". Temperatures will
return to more seasonal readings, as the ridge builds over the
region and west to southwest winds become more predominant.

NW flow will develop by Friday, as the ridge flattens and shifts
eastward over the Plains. The shortwave from Thursday will slow and
become situated over southern/central MT. This circulation could
develop into a weak low and rotate additional shortwaves over the
forecast area. This will be in addition to other minor shortwaves
embedded in the NW flow. This will result in scattered showers and
thunderstorms over northern and central portions, with storms moving
from northwest to southeast. Showers could linger over northern
portions Friday night. Any convective activity that develops across
the south will be isolated. Temperatures will once again be slightly
cooler than normal.

An area of low pressure will develop over the PACNW Saturday morning
and begin to dig southeastward through the day. Conditions are
expected to remain dry across the forecast area ahead of this
system. Showers and thunderstorms will develop across the western
half of the forecast area by 00Z Sunday and progress eastward
through the evening hours. Locally heavy to flooding rains will be a
possibility once again, as precipitable water values remain 100 to
150% of normal and the showers and storms become widespread across
much of the forecast area Saturday night.

.LONG TERM...Sunday through Thursday

Upper level storm system tracks across WY Sunday and into the high
plains by Monday morning. The exact track is critical for
precipitation location. If the upper circulation tracks across
southwest WY, then many areas east of the continental divide would
see a decent chance of rain. If the track is through central WY,
then the best rain chance is in the eastern zones from Buffalo to
Casper. The 00Z ECMWF model tracks the circulation through central
WY, while the 00Z GFS model tracks it through SW WY which favors
more areas east of the divide. Will not stray much from going
forecast which hits many areas east of the divide with rain Sunday
into Sunday night. Drier day Monday as the system moves away.
Tuesday will be dry with breezy areas. A weather system moves
through northern Montana Tue night into Wed and drags a cold front
into northern WY. A few showers or an isolated storm may clip the
far northern WY border Tue night. The cold front becomes more or
less stationary over the north Wed afternoon. Will keep Wed dry,
but an isolated mountain shower or storm is possible. Next
Thursday will see isolated showers and storms over the area with
breezy areas in the afternoon. High temperatures Sunday will be on
the cool side, then warming up quickly for Mon and Tue. A little
cooler over the north Wed with little change in central and south.
Next Thursday will be warm.


.AVIATION...For the 12Z TAFS through 12Z Friday

Through 15Z, very patchy fog will be possible mainly in the western
valleys. A quick round of isolated showers and thunderstorms are
expected mainly north of a KJAC-KRIW-KCPR line between 20Z Thursday
and 02Z Friday. The most confidence is across the far northwest and
north-central areas and will have VCTS for those terminals. Although
there is a chance of convection farther south, will keep TS
mention out of those TAF terminal since confidence is lower. The
convection is expected to rapidly shift away during the early
evening. Then after 09Z, some light shower activity could push
into far northwest Wyoming.



Temperatures will be much warmer today (10 to 15 degrees warmer in
most cases) and much drier. Relative humidity values will range
from the middle teens to the 20s across most lower elevation
zones. Windy conditions will occur this afternoon over western
and southern portions. Isolated to scattered showers and
thunderstorms will develop across all but southern and
southwestern zones later this afternoon and becoming isolated by
midnight. Some storms across northern and eastern zones could be
strong to severe with hail and strong wind gusts. Mixing and smoke
dispersal will range from very good to excellent.





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