Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 270900

Area Forecast Discussion
National Weather Service Riverton WY
300 AM MDT FRI MAY 27 2016

.Short Term...Friday through Sunday Night

Most showers are dying off as we speak and the balance of the late
night Thursday night looks mainly dry. Temperatures should continue
the slow warming trend of the last couple of days as thicknesses
rise. A shortwave ridge will move over the area today. This should
limit thunderstorm a bit. All in all, it looks fairly similar to
yesterday. That means some isolated to scattered mainly afternoon
showers and thunderstorms with the best chance over the mountains
with the high elevations heat source. Also, steering winds are
rather weak in most areas so most storms will be anchored to the
mountains. The exception would be in the northwest where storms
could move a bit. We will again have the threat of gusty winds and
small hail with the thunderstorms, but that describes any
thunderstorm in Wyoming. Most of the convection should again die off
by midnight.

Another shortwave will then cross the state on Saturday. However,
coverage of convection could be a bit less. There will be somewhat
drier air around the area. In addition, the shortwave will move
through in the morning when instability is limited. The exceptions
there would be across the east when the wave would arrive later,
especially in Johnson County where there will be a bit more upper
level divergence to aid in thunderstorm development. There could
still be an isolated storm in the mountains of the west, but most
places will be dry most of the time. Temperatures will also continue
a slow climb with many basin areas cracking the 70 degree mark for
the first time in a while.

Temperatures will warm a bit more for Sunday and thunderstorm
coverage will shrink once again as heights and thicknesses continue
to rise across the Rockies. There could still be an isolated storm
across the west but these will be few and far between. With the
models showing more sunshine, we raised high temperatures a few
degrees and future shifts may want to raise them some more if the
trend continues.

.LONG TERM...Memorial Day through Friday

Synopsis...A low pressure trough will move across the northern
Rockies on Memorial Day bringing increasing showers and a few
thunderstorms into the west Monday morning spreading into central
Wyoming during the afternoon. A few strong to severe thunderstorms
are possible across the northern one-third of Wyoming. Several
inches of snow will be possible in the northern mountains behind
this system Monday night and Tuesday. A ridge of high pressure
will gradually build into the region Wednesday through Friday,
bringing more summer-like weather by the end of the week with
slight chances of thunderstorms mostly confined over and near the
northern mountains.

Discussion...Upper low on the eastern flank of an Omega-block
(the upper high centered over Alaska) will be dislodged and push
east across the northern Rockies on Memorial Day. ECMWF remains on
the fast and further north track of guidance with GFS showing a
slower and further southward track. WPC prefers a blend of the two
solutions and see no reason to differ.

The main concern for Memorial Day has been the potential for severe
weather depending on timing of trough passage.  Though there remains
timing differences with upper trough, models in general agreement
placing surface trough near Wyoming`s eastern border by Monday
afternoon with the higher risk for severe thunderstorms east of the
area.  Downslope westerly flow across Fremont and Natrona counties
will also hinder stronger storms, but cannot rule out some stronger
wind gusts.  The best shear and instability will be across the
north, from Yellowstone National Park and along and north of a Cody
to Buffalo line, where a better environment for storms with hail
will be found.

Wraparound colder N-NW flow behind the system will likely bring some
snow to the northern mountains and possibly Yellowstone National
Park Monday night into Tuesday morning.  The northern Bighorn Range
will be most favorable for accumulating snowfall.  Otherwise, cooler
NW flow and slight instability behind the system will bring isolated
to scattered showers and a few thunderstorms along and east of the
Divide Tuesday afternoon and evening.

A ridge of high pressure is expected to strengthen over the Great
Basin and Intermountain West Wednesday through Friday and into next
weekend.  H7 temperatures are expected to be 12-13c over the west
and central by Friday.  This ridge should bring down much of the
remaining snowpack above 9000 feet through next weekend with
corresponding rises in rivers and creeks.


.Aviation.../12Z issuance/

West of the Continental Divide...KJAC/KRKS/KPNA/KBPI Routes

Showers lingering over the mountains should be the main activity
through around 18z today. Clearing skies over the valleys and basins
may result in areas of patchy fog, with KJAC looking to be most
susceptible to development. Any developed fog should dissipate by
18z when the next push of moisture arrives. This moisture will also
come with some instability to result in isolated showers and
thunderstorms across the area. Ongoing storms may result in mountain
top obscuration. Brief periods of heavy rain could result in periods
of MVFR conditions at area terminals.

East of the Continental Divide...KCPR/KCOD/KRIW/KWRL/KLND Routes

Generally dry conditions anticipated through around 18z this
afternoon. Clearing skies noted across northern Wyoming, with some
lowered ceilings possible across the northern terminals, especially
KCOD. Moisture and instability returning during the afternoon will
help to keep off isolated shower activity, with thunderstorms
possible after 20z for mot locations. Most thunderstorm activity
should diminish by 04z this evening, though rain showers may persist
through 06z. Drier air then moves over the area to keep VFR
conditions present through the remainder of the TAF period.


A slow drying and warming trend will take place through the weekend.
Temperatures will slowly warm with relative humidity lowering each
day. There will still be some mainly afternoon and evening showers
and thunderstorms around, especially in and adjacent to the
mountains. Any thunderstorm could have erratic winds. Otherwise,
winds should remain light to moderate outside of showers or
thunderstorms. Coverage of thunderstorms will shrink each day with
many of the lower elevations mainly dry on Saturday and Sunday.

.RIW Watches/Warnings/Advisories...


SHORT TERM...Hattings
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