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fxus65 kriw 201711 

Area forecast discussion
National Weather Service Riverton Wyoming
1111 am MDT Wed Jun 20 2018

Short through Friday night

The upper level low is over SW Montana this morning and will begin to
shift southeastward over the forecast area today. An upper level
ridge continues to build over the Great Basin, which will direct the
upper low over northwest Wyoming. The ridge will continue to build northward
today, which will help to finally get rid of the low. Residual
energy will move southeast over north-Central/Northeast portions.
This looks to be the focus for the stronger convection today.
Showers and thunderstorms will be more isolated to scattered today,
confined over northern and central portions. This will be largely in
part to the aforementioned ridge. Northerly winds will be in place
across much of the forecast area today, in the wake of a cold front
Tuesday night. Gusty winds of 15 to 25 mph will be possible the
morning over favored areas in Big Horn and northern Johnson
counties. As a result of these winds, temperatures will once again
be about 10 degrees below normal.

The ridge will continue to build over the region Wednesday night
into Thursday, ending most (if not all) precipitation over the
forecast area by 06z Thursday. A shortwave will enter east Idaho/southeast Montana
Thursday afternoon and develop isolated showers and thunderstorms by
late afternoon over northern portions. Some of this activity could
continue through the overnight hours across the north, as the
shortwave moves across S Montana. These storms could be strong to severe,
with bulk shear close to 50 kt, cape values of 800-1500 j/kg, steep
mid level lapse rates and precipitable water 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.

Northwest 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 Montana. This circulation could
develop into a weak low and rotate additional shortwaves over
northern portions of the forecast area. This will be in addition to
other minor shortwaves embedded in the northwest flow. This will result in
isolated to scattered showers and thunderstorms over northern and
central portions, with storms moving from northwest to southeast.
Showers could linger over northern portions Friday night.
Temperatures will once again be slightly cooler than normal.

Long term...Saturday through Wednesday night

Weather system out near 40-45n/170w is the eventual upper low that
is still forecast to dig se across US over the weekend. The Euro and
Gem has also come into agreement with the GFS that this low will
move into Idaho during the day Saturday and then dig se into srn
Wyoming on Sunday. A fairly strong system out near the wrn end of
the Aleutian chain will help to pump up the ridge behind this system
and allow it to dig se across the cowboy state. This trajectory will
increase the risk of some stronger thunderstorms in the far west
Saturday and then widespread showers/tstms shifting into central Wyoming
Saturday night through Sunday. Strong qg forcing and cyclonic nose
of the jet impacts the west during the day and moves quickly into
central Wyoming by 00z Sunday. Combine that with potential mid level
low(s) sliding from Sublette into ern/sern Fremont County and
eventually Natrona County and we could see significant rain
developing once again from wrn Wyoming into central Wyoming Saturday into
Saturday night and continuing along and east of The Divide Sunday,
especially across Fremont and Natrona counties and possibly up into
the Cody foothills and srn Big Horn basin. Front coming down into
Johnson may also be a focus of some stronger storms Saturday. Quite
a few showers/tstms may impact Sweetwater County too with potential
cold air funnels Sunday around the low. Definite increase once again
of flooding potential if 1-2 rain amounts with locally higher
amounts in the mountains and upslope foothills locations of central
Wyoming materialize. Considerable warmup Monday through Wednesday. Still
some mainly afternoon convection Monday, especially in and near the
mountains. More isolated coverage Tue-Wed with very warm to hot


Aviation...18z issuance

Things are mostly quiet right now. A few sites, mainly kcpr, klnd
and kjac continue to play follow the bouncing ceiling, alternating
between VFR and MVFR. Isolated afternoon/evening convection is
expected generally across north and northwest mountains and adjacent
foothills. This convection is expected to quickly end during the
evening hours, with skies clearing. Some fog could reform after 06z
Thursday. Further showers and thunderstorms are expected Thursday
mainly across areas north of The Owl creek and Bridger Range but
should hold off until after 18z Thursday.


Fire weather...

Temperatures will be similar to Tuesday across all but southern
and western portions, which will be 5 to 10 degrees warmer.
Isolated showers will be ongoing this morning across the northern
half of the state. Isolated to scattered showers and
thunderstorms will develop this afternoon across northern and
central portions, moving to the east-southeast. Mixing and smoke
dispersal will range between fair to good. Warmer temperatures and
lower relative humidity values on Thursday. Isolated thunderstorms
Thursday afternoon and evening could be strong to severe.


Riw watches/warnings/advisories...


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