Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 300818
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
218 AM MDT Tue May 30 2017

.SHORT TERM...(Today through Thursday night) Issued at 217 AM MDT
Tue May 30 2017

Imagery shows a non-progressive amplified ridge over the wrn CONUS
with a trof on the west side of the ridge stretching down the west
coast...a splitting flow situation. The SFC has general (inverted)
high pressure across WY ATTM. Lighted isolated precip has ended
across WY.

Today - the FA will remain under NW flow aloft with just enough
instability in the afternoons and evenings to get a few isolated
showers/thunder mainly over the higher terrain, or closely adjacent
foothills, of the FA. Relatively high based showers/storms providing
a little rain and gusty winds. Otherwise, general conditions call
for seasonal to below seasonal temps and relatively light winds
(outside of a stronger winds associated with a shower or storm as
DCAPE values will range up to around 500 J/KG giving gust
possibilities of 30 to 40 kts).

Wednesday, the wrn CONUS ridge axis will begin to move over wrn WY,
with plenty of sun and much warmer temperatures beginning to induce
lower SFC pressure east of the Divide. Additionally, a minor ridge
riding wave shows up in mdls that will move through the the upper
lvl flow during the day and perhaps help in breaking the
cap...especially over the higher terrain of wcntrl/swrn
WY...providing a few chances for some mainly isolated elevated
convection. Again, relatively little precip is expected overall with
any shower or storm but gusty winds with DCAPE ranging from 500 J/KG
to 700 J/KG. CAPE values east of the Divide will be higher
overall...but much stronger capped and without a strong enough
trigger to work with to initiate storms...so not expecting much of
anything ATTM. This will also mark the beginning of a couple of
seasonally very warm days where strengthening potential for some
flooding will develop along creeks, streams and rivers due to
increasingly rapid mountain snowmelt.

Thursday, the upper pattern finally begins to budge again and the
west coast trof moves towards WY...spurring on frontogenesis across
cntrl ID and wrn MT as it does. The ridge will obligingly translate
east of the FA, allowing upper lvl sw flow and somewhat colder air
aloft to begin to filter over the FA...increasing instability and
weakening the cap. By mid to late afternoon...isolated to widely
scattered showers/storms will break out across the FA with a
developing SFC trof east of the Divide helping to get/keep them
going. Thursday night, the upper level trof begins to move into wrn
WY along with an associated weakening cold front. This will also
allow for an extended period of widely scattered precip chances to
remain across the cntrl and nrn FA overnight. Same story
here...mainly gusty wind producers (classic inverted V
soundings...up to 800 J/KG DCAPE)...light/moderate rain...with
perhaps a little small hail possible late afternoon/evening across
the Absarokas, Cody Foothills and Big Horn Basin where stronger
effective shear will be present.

.LONG TERM...(Friday through Monday) Issued at 217 AM MDT Tue May
30 2017

Synoptic pattern will feature strong upper ridge over central Canada
Friday through the weekend.  Incoming Pacific shortwaves will move
across the northern Rockies into the backside of this ridge,
splitting underneath it and riding up the backside, eventually
pinching off an upper high early next week.  As with most blocky
patterns with fairly weak systems moving through it, preference is
for more gradual transitions to the overall pattern which is more in
line with the ECMWF than the GFS.

On Friday, ECMWF and GFS agree on moving first shortwave across
north central/northeast Wyoming into the northern High Plains in the
morning.  GFS brings a stronger trailing shortwave east across
Montana late Friday-Friday night while ECMWF is weaker and further
north with this system.  Forecast leaves slight chance PoPs across
the north.  Temperatures will be 5-10 degrees cooler than on
Thursday behind cold frontal passage Thursday night.  Upper ridge
and dry, warmer weather will prevail on Saturday.  Next Pacific
system is forecast to lift across the NW U.S. into the northern
Rockies Sunday-Sunday night.  Low level east/southeast flow will
bring higher CAPES east of the Divide with thunderstorms expected to
move in/initiate over the high terrain in the west in the afternoon
and move off into the central basins late afternoon-evening.  A few
strong thunderstorms will be possible in this setup, with the Casper
area and southeast Wyoming most favorable for the stronger storms.

Forecast confidence is lower Monday and Tuesday as Upper High
pinches off over central Canada, with a few upper lows caught up in
weak upper level winds, or what basically looks like two way traffic
on a one-way street across the northern Rockies.  Forecast keeps
slight chances of showers and thunderstorms across the north on
Monday followed by a cooling trend on Tuesday.  The good news is
that we are not seeing a hot summer ridge or strong southwest flow
that could bring on rapid snowmelt and significant flooding as we
move into early June.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 217 AM MDT Tue May 30 2017

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

VFR conditions will prevail through 12Z Wed. Scattered showers and
isolated thunderstorms will develop after 20Z and continue through
03Z Wed. Gusty wind to 35 knots and small hail are likely with some
of the storms. The best concentration of activity will be in NW
WY...from a KJAC to KCOD line. Showers will become isolated after
03z and end around 06Z Wednesday.

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

VFR conditions will prevail through 12Z Wed. Isolated showers and
thunderstorms will develop after 20Z over and near the mountains.
This activity will end by 05Z Wednesday. Gusty outflow wind to 35
knots will occur near the storms. KCOD airport will see the best
chance for showers and storms. KLND airport will also have VCTS due
to its close proximity to the mountains.

&&

.FIRE WEATHER...
Issued AT 217 AM MDT Tue May 30 2017

Fire danger low across the forecast area through today as all fuels
are currently in green-up and high temperatures and minimum RH
values remain at or below seasonal and above 15 percent
respectively. Isolated showers or storms again today most likely to
occur over the higher terrain and closely adjacent foothills. Gusty
winds with this activity possible into the evening. Wednesday, fire
danger will elevate some across southern WY as RH values drop into
the lower to mid teens (percent) and west winds gust 20 to 25 mph at
times. Smoke dispersion will be good to excellent in the afternoons.
Flooding potential will also begin to increase Wednesday, Thursday
and Friday as seasonally very warm temperatures greatly increase
snowmelt.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...Ross
FIRE WEATHER...Braun



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