Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 290859
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
259 AM MDT SUN MAY 29 2016

.SHORT TERM...Today through Tuesday night

Imagery shows general trof across the CONUS west of the MS river
valley. Weak embedded disturbance currently moving across northern
WY...with a stronger embedded trof upstream and moving into the PAC
NW. Surface has general weak low pressure across WY with an also
weak nearly stationary front across sern/srn WY. A couple of light
showers moving across northern WY this morning associated with the
weak upper disturbance.

Today, upper level flow remains flat and slow for the most part
while modest moisture moves into the southern/southwestern portion
of WY. Convective precipitation will be isolated to scattered and
mainly afternoon based across south/southwest/west central WY...with
a lesser area of showers/thunder across the far northwest corner of
Wyoming associated with the leading edge of another weak upr
shortwave. The western/southwestern mountains will stand the best
chance of seeing measurable precipitation - with a few spots in the
Salt and Wyoming ranges up to perhaps the southern Tetons getting up
to a tenth of an inch of rainfall. Very small hail and gusty winds
will be the usual accompaniment to this relatively sparse rainfall.
Stronger storms of the day will be near the western border areas
with ID/UT. Tonight into Monday, the weak shortwave trof, mentioned
above, will move/sag across WY from west to east with a few light
showers continuing overnight primarily near terrain features.

Monday, the upper level trof continues to move across the region,
being reinforced by a much deeper trof/low moving into MT. Lee side
troffing across ern WY and the Front Range of CO will be the focus
in the late afternoon and evening for two areas of low pressure
circulations to form over - one over the high/cntrl plains of
WY/SD/NE and the other over sern CO/SERN KS. Two main areas of
convective precip can then be expected across the region east of the
divide: one, across southwestern WY in/near/north of the area of
maximum convergence between the two low pressure systems; and two,
across northern/northeastern WY associated with a couple of fronts
generated under the influence of the upper trof over MT. The front
will push through the region during the late afternoon/evening
period. This will also be the region that will see the stronger
storms of the day with gusty winds and small hail a possibility.
However, there will be a small threat for a few severe storms
with/near this front in the late afternoon/early evening across
Johnson county and to the east across northeastern WY. Deep shear >=
40 kts and CAPE near 1000 J/Kg +/- may produce rotating storms with
large hail and strong wind the main threat. There will also be a low
chance for a brief tornado with a storm just ahead to on the front
late in the late afternoon as LCLs drop quickly and these storms
have their best chance at becoming surface based for a short
period...or produce a spin-up on the storm`s outflow.

Tuesday morning through Tuesday night, will be post frontal and cool
with some lingering showers across the central mountains early. Then
by afternoon, a weak front associated with a fast moving upper level
disturbance will move through the northeastern FA with showers -both
rain and snow - across the Bighorn Mountains and foothills.
Clearing and relatively cold overnight all locations.

.LONG TERM...Wednesday through Sunday

Synopsis...A ridge of high pressure will strengthen over the area
bringing warmer, more summer-like weather Thursday through the
weekend.  Afternoon/evening thunderstorms are expected to be
isolated, mostly across the north Wednesday through Friday, and
across the west over the weekend.

Discussion...The inevitable summer ridge will setup across the Great
Basin Thursday and Friday and persist through the weekend.  On
Wednesday, temperatures will return to near normal values with a dry
and mostly stable northwest flow. Slight instability with light
upslope flow may result in a few thunderstorms along the eastern
slopes of the Absaroka and Bighorn ranges.

Both GFS and ECMWF show a 590dm high pressure system centered across
western Utah Thursday afternoon with gradual strengthening into
Saturday.  Shortwaves will be shunted well to the north along or
north of the U.S./Canadian border, but smaller scale mountain/valley
breeze boundaries combined with slight instability will result in a
few thunderstorms blossoming and moving off into the north central
basins. Most high temperatures will be in the 80s to lower 90s
across the lower elevations Friday through Sunday with 60s and 70s
in the mountains.  These warm temperatures will accelerate melting
of remaining snowpack above 9000 feet with corresponding rises in
creeks and streams.  Drainages along the east slopes of the Wind
River Mountains will likely see the most significant rises where
some Sno-Tels still show 15-20 inches of Snow Water Equivalent above
9500 feet.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR conditions will prevail through the period. Showers and
thunderstorms will develop over Western WY this afternoon into the
evening hours. Local MVFR conditions will occur near storms as well
as the possibility of wind gusts to 30 knots. Showers will linger
through 06z.

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

VFR conditions to prevail through the period. Isolated showers
will occur after 00z through 09z.

&&

.FIRE WEATHER...

Fire danger low as green-up and seasonally cool temperatures with
elevated humidity levels continue across most of Wyoming. Isolated
to widely scattered showers and storms possible by mid-afternoon
through the evening period across mainly southern Wyoming with
little overall rainfall expected with any of this activity and less
than a tenth of an inch of precipitation at the best locations in
the southwestern/west central mountains. Winds will flow out of the
west to southwest west of the Divide at less than 15 mph, except
near stronger showers and storms which could put down gusts up to 40
mph. Wind will be mostly light and variable east of the divide.
Monday will be a little cooler across the forecast area...especially
across northern Wyoming. Chances for showers and thunderstorms will
increase east of the divide as an upper level disturbance and a
couple of cold fronts move across the region. A few storms may
become strong to severe over Johnson county by late afternoon and
early evening.

&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Braun
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...Braun


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