Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210553

Area Forecast Discussion
National Weather Service Riverton WY
1153 PM MDT Sat May 20 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 248 PM MDT Sat May 20 2017

The next shortwave to influence our weather is diving southeast
in the northwest flow along the Idaho/Montana border. Showers and
isolated thunderstorms will result as the shortwave tracks
southeast across the area with more coverage along and west of the
divide and across the northern CWA with less elsewhere. With the
track of the main vort lobe taking it roughly along the
Idaho/Wyoming border through Sunday morning, this would favor a
brief upslope event for the East Slope of the Wind River Mountains
and Lander Foothills later tonight and Sunday morning. The MOS
guidance for Lander/Riverton suggests this with the lowering
numbers for cigs. The precip in this area late tonight and Sunday
morning should be more of the stratiform variety. This period of
precip will run right into the next round of convection later
Sunday as instability increases everywhere and capes increase
across the far west. The thunderstorms that occur this weekend
will feature small hail with the low wet bulb zeros but with low
cloud bases and relatively light steering winds, gusty winds
should not be over the top. Then drier air will punch in from the
north during Sunday evening with clearing from north to south.
Then early Monday morning, a northern stream shortwave on the back
side of the slowly exiting main trough will invade from the north
and bring some stratiform precip and brisk northerly winds to
areas east of the divide. The increased northerly winds will
mainly affect the Northern Bighorn Basin and Northern Johnson
County with the Buffalo area seeing wind speeds just shy of high
wind criteria according to the MOS guidance and surfacing NNW
700mb 40 knot winds Monday. Snow levels should drop to around 7000
feet over the Bighorn Mountains early Monday morning with precip
continuing through the day Monday which should add up to 3 inches
of snowfall over the Bighorn Mountains by late Monday. Precip will
be most prevalent over Johnson and Natrona Counties as far as
lower elevation precip goes with the Bighorn Mtns receiving the
most from this event. Snow levels will be higher at points west
and south of here farther from the cold trough with snow levels
perhaps as high as 10K over the Wind River mtns where less precip
will fall and same with the Absarokas where and inch or two could
accumulate over the highest elevation in the northern portion of
the Absaroka Range. Precip will linger in Natrona County Monday
evening before exiting to the southeast after midnight. On Tuesday
a ridge will struggle to arrive from the west as the mean ridge
remains along the Pacific coast but we should finally begin to
warm up and dry out as the jet stream shifts to our northeast
along with lighter winds.

.LONG TERM...(Sunday night through Saturday)
Issued at 248 PM MDT Sat May 20 2017

Medium-range forecast models are providing very similar solutions
with regard to the overall synoptic pattern evolution through the
forecast period. Series of shortwaves in northerly flow will carve
a trough across the mid-Mississippi Valley Tuesday night while the
next upper low sets-up over southern BC/Alberta. This will leave
Wyoming between these two systems with a shortwave, transient ridge
in place Tuesday night and Wednesday. With little cloud cover and
700mb temperatures of +10C to +12C, have nudged basin/valley
temperatures upward a tad as we should see 80-84F at many locations.
Wednesday will provide the most active snowmelt day. However, this
will be short-lived as the southwest Canada trough ejects another
piece of energy into the Pacific Northwest Wednesday night and
Thursday. Could be some isolated showers across the northwest
Wednesday night as Pacific moisture begins to funnel into the state
ahead of this next area of low pressure. The best chance for
showers and isolated thunderstorms Thursday and Friday will be
over the western mountains and the far north as moist, southwest
flow spreads across the state. Upper low sits and spins over WA/OR
Thursday and much of Thursday night before beginning a shift to
the east. As this system presses eastward, we will see increased
precipitation chances Friday night and especially Saturday.
Accordingly, temperatures will gradually cool from highs of 10-15F
above normal Wednesday, to more seasonal averages Friday and
possibly slightly below normal Saturday. GFS is a bit faster than
the ECMWF in tracking the low across our north on Saturday, a move
that would nudge cooler air south into Wyoming on the backside of
the low and increase the chance of showers across the north. For
now, have followed the blends as there is a great deal of time
before next Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1152 PM MDT Sat May 20 2017


Isolated to widely scattered showers will linger mainly along and
south of a KBPI-KPNA line through Sunday afternoon as an upper
level low pressure system continues to dive south across the
northern Great Basin. Drier and more stable air will prevail
across northwest Wyoming, vicinity KJAC. Showers across southwest
Wyoming will dissipate early Monday evening, leaving Few-sct
FL110-130 across the area overnight.


Isolated showers across the area will continue to dissipate
through 09z. Low level upslope northeast flow may result in the
development of MVFR ceilings along the east slopes of the Wind
River Range, extending into vicinity KLND by 12z, possibly as far
east as vicinity KRIW. An upper level disturbance will move south
across the area Sunday morning and early afternoon with isolated
to widely scattered showers developing ahead of this system. Most
of this activity will blossom in the late morning and afternoon
vicinity and south of a KRIW-KCPR line. Some drying and clearing
will then occur north to south late Sunday afternoon through
Sunday evening. A cold front will plunge south into northeast
Wyoming 09z-12z Monday, near a vicinity KCOD-KWRL-40SM north KCPR
line by 12z Monday. Lower MVFR/IFR clouds are expected mainly
along and east of the Bighorn Range through 12z, remaining north
of KCPR until later Monday morning.


Issued AT 248 PM MDT Sat May 20 2017

Unsettled weather will continue through the weekend. An upper
level disturbance will move across the area this weekend and bring
the chance of mainly afternoon and evening thunderstorms as well
as late night and morning showers both for the rest of today and
Sunday. Wind should generally remain light to moderate except
around any thunderstorm. Relative humidity should remain well
above critical levels with temperatures continuing to average
below normal. Another weather disturbance will bring more
precipitation, increasing north winds, and cool temperatures to
mainly Northern and Eastern Wyoming east of the divide. Expect
snow above 7000 feet in the Bighorn Mountains where around 3
inches of snow could accumulate Monday. Expect drying and warming
to take place by Tuesday as high pressure tries to build in from
the west. Mixing and smoke dispersal will be generally fair to




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