Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 280848
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
248 AM MDT SAT MAY 28 2016

.SHORT TERM...Today through Monday night

Imagery shows large trof across the western CONUS with a ridge
across the east and an embedded sw trof over the cntrl plains.
Another small embedded sw currently located ovr s cntrl MT. Surface
has general weak low pressure across WY with a weakening slow moving
front off to the west and a slightly more mobile portion of the same
front moving through cntrl MT. Tonight/this morning, isolated
showers/thunder from north to south across cntrl Wyoming.

Today, surface low develops weakly across southeastern WY with
boundary stretching back across the southern portion of the state
while the front currently across MT swings through northern/eastern
WY mainly as a clipper. Flow aloft becomes nearly zonal to slightly
southwesterly. Most precipitation will occur around the front or the
mountains of northern Wyoming and the boundary/weak front across the
south. Showers/storms will be of low CAPE, modest shear, low QPF,
very small hail (if any) and gusty wind variety with little chance
for any severe parameters to be met. Tonight, skies clear quickly
after sunset.

Sunday, flow remains modest and rather flat aloft while some weak
moisture tries to infiltrate the southern portion of WY. Most
convective precipitation will be isolated to widely scattered
afternoon based, centered around the area of moisture across south
and southwest WY. The western/southwestern mountains will stand the
best chance of seeing measurable precipitation - with a few spots in
the Salt and Wyoming ranges 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 form of accompaniment to the relatively
sparse rainfall. Stronger storms of the day will be near the western
border areas with ID/UT. Sunday night into Monday, another weak sw
trof will move across WY from west to east with a few spurious light
showers continuing overnight into/through the Monday morning hours.

Monday, lee side troffing across eastern Wyoming will be quickly
followed by the formation of a modest surface cyclone across the
high/cntrl plains with rapid frontogenesis occurring across
southern/southeastern MT. A couple of clipper fronts will then move
to the south across nrn/cntrl/ern Wyoming in the afternoon and
evening. Another area of low pressure will occur further south ovr
nern NM/sern CO. This will give a maximum in convergence between the
two systems across sern WY/nern CO and to the east of this area.
This will also be the region that will see most of the severe
storms of the day. The second clipper will push through the nrn
and ern portions of the forecast area through the late
afternoon/evening period. Showers/storms will accompany this front
with gusty winds and small hail a possibility for the most part.
However, a severe storm or two will be possible near this front in
the afternoon/early evening across Johnson county and points
eastward, with large hail and strong wind the main threat within
these rotating storms. There will also be a low chance for a brief
tornado with this(these) storm(s) near to just behind the front
late in the afternoon/early evening as LCLs drop quickly and these
storms have their best chance at becoming surface based.

.LONG TERM...Tuesday through Saturday

Synopsis...an upper level low pressure system will drift east across
the northern high plains Tuesday with some rain and mountain snow
showers wrapping back across northern Wyoming in cool northerly
flow.  A ridge of high pressure will build into the area Wednesday
and Thursday bringing a warming trend with summer-like temperatures
5-10F above normal expected Friday into next weekend.

Discussion...GFS and ECMWF in better agreement with track of upper
low into the northern plains on Tuesday with both placing the H5 low
center near the MT/ND/SD triple point 12z Tuesday, drifting east
across southern North Dakota during the day.  An overall trend of
placing wraparound precipitation further to the north and east will
place mainly the Bighorn Range in line for the best chances of
receiving accumulating snowfall on order of 3 to 5 inches from
Powder River Pass and northward Tuesday morning with snow levels
near 7500 feet. The best chances for valley rain showers will be
along and north of a Cody to Buffalo line.  A brisk northerly wind
will develop Tuesday afternoon east of the divide with highs mostly
in the 50s to lower 60s.

A ridge of high pressure will then bring a more summer-like pattern
to the area Thursday through Saturday with a few afternoon
thunderstorms developing along mountain/valley breeze boundaries,
mainly along eastern slopes.  Warming mountain temperatures will
accelerate the melting of remaining mountain snowpack above 9000
feet with corresponding rises in creeks and rivers expected.

&&

.AVIATION.../12Z ISSUANCE/

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

Isolated showers and storms over northwest WY late morning through
early evening. Local mvfr cigs/vsbys are possible with storms.
In southern WY this afternoon isolated showers and storms will
occur into the early evening. KRKS airport will be close to this
activity and VCSH will be included in the TAF. Otherwise VFR
conditions to prevail over the region through tonight.

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

Isolated showers will end by 12z in the eastern part of the area,
mainly from KBYG to KCPR. New showers and storms will develop by 18z
over the Bighorn Mountains southward to Casper Mountain. This activity
will occur through 00z and then diminish by 02z. MVFR cigs/vsbys
will be likely with storms from 18z to 01z. Otherwise, VFR
conditions will prevail over the rest of the area today and tonight.

&&

.FIRE WEATHER...

Fire danger low today and Sunday as green-up and seasonally elevated
humidity levels continue across Wyoming. Isolated to widely
scattered showers and storms possible by mid-day through the early
evening period across mainly northern, eastern and 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 Bighorn Mountains. Winds will flow out of the west to
southwest west of the Divide at 10 to 20 mph, but will be light and
variable to the east. Sunday will see the warmest temperatures of
the weekend with a few locations in the Wind River and Bighorn
Basins seeing RH values drop into the the upper teens (percent).
Winds across the entire forecast area will be on the light side.

&&

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

$$

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


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