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Area forecast discussion
National Weather Service Riverton Wyoming
222 am MDT sun Jun 25 2017

Short term...(today through Tuesday night)

Imagery shows upper high p centered over the desert SW and nrn mx
with a ridge axis extending to the NW through the Mojave and Great
Basin deserts to the Pacific northwest. Best moisture is being diverted across
the swrn Continental U.S.. broad trof lies across the ern Continental U.S.. Wyoming is east of
the ridge under NW flow. The surface has high p across the nrn High
Plains including most of Wyoming with N to NE surface flow towards a heat
derived low p over the desert SW. No precip across Wyoming except for a
few virga showers falling and evaporating across the ern portion of
the state.

Most rivers across the fa, including the big and little wind rivers,
are responding positively (decreasing in flow and gauge height) to
the cooler Post frontal temperatures the past couple of days. The
Green River near La Barge in nern Lincoln County is, however, still
high and steady but should begin to decrease today.

Today and tonight, the forecast area continues east of the ridge under northwest flow
aloft and mostly seasonally cool dry high p and nrly flow at the
surface. Monday, the upper ridge begins to translate ewd over the
Central Plains with lower p pushing into wrn WY, redirecting surface
flow into the forecast area from the E and se. Moisture return into the fa,
however, will have to wait until later Monday night and into Tuesday
as the ridge axis passes through the County Warning Area and into the cntrl High
Plains while Lee side low p develops across the surface over cntrl and
wrn Wyoming. Low to mid lvl moisture will invade Wyoming from the se with mid
lvl moisture moving in from the NW.

By Tuesday morning a weak embedded upper level SW will ride through
the flow W of the ridge axis and across nwrn Wyoming along with a surface
front. Scattered showers and thunderstorms will quickly move
into/through the Yellowstone area (more isolated to the south) at 30
to 40 kts relatively light rainfall...small hail and very gusty
winds that could border on severe at times from mid-day on. With
seasonally warm/hot temperatures continuing Tuesday, dpt depressions
will be in the 50 to 60 deg range and dcape will run 1000-1500 in
the afternoon supporting strong wind gusts from the
convection...particularly across the Big Horn basin and adjacent
foothills/mountains. Tuesday evening, Post frontal decreasing temps
and over ncntrl and nwrn Wyoming will increase relative humidity and lower LCLs with
deeper available moisture in place for a short period and heavier
rain possible in the evening with somewhat less wind across the NW
and Big Horn basin. Heaviest rain of the day/night, with a few
possible severe storms, will occur across Johnson County for a few
hrs Tuesday evening as the front, outflow boundaries and deeper ll
moisture all provide a better environment for both marginally large
hail and strong gusty winds. Possible moderate to heavy rain to
continue with the passage of the upper SW trof through at least

Long term...(wednesday through saturday) upper trough over the northern rockies will bring
cooler weather, with chances of showers and thunderstorms across
the north and central Wednesday and Thursday. Breezy and mainly
dry conditions will prevail across the south. Warmer and mainly
dry conditions are expected on Friday. Westerly flow next weekend
will bring seasonal temperatures with slight chances of showers
and thunderstorms across the northern third of Wyoming.

Discussion...upper low currently near Juneau, Alaska will drift east
into west central Canada by Wednesday morning with broad cyclonic
flow to the south across the northern rockies. GFS and European model (ecmwf) in
fair agreement rotating a shortwave into SW Montana Wednesday
evening, across northern Wyoming and out into the northern High
Plains Thursday morning. Best thunderstorm coverage is expected
from Yellowstone National Park through the north central, with a few
strong thunderstorms possible mainly across Johnson County Wednesday
afternoon and evening. Cooler conditions will prevail on Thursday
behind this system and its attendant cold front, with better
moisture and instability shifting out into the plains. European model (ecmwf) and
GFS both show a transitory ridge over the area Friday, so looking
for mainly dry and warmer conditions. West-east oriented Pacific jet
stream is expected to translated the northern rockies over the
weekend, with the GFS further south than European model (ecmwf). Ensemble means keep
this storm track near the U.S./Canadian border. Thus, leaning
toward dry conditions with near normal temperatures Saturday and
Sunday. The outlook into the Fourth of July looks mainly dry across
the area with near or above normal temperatures.


Aviation...(for the 12z tafs through 12z Monday morning)

VFR conditions will occur through 12z Monday. Wind speeds will be
less than 12 knots through the period. Very isolated showers may
occur over the mountains from 23z through 03z.


Fire weather...

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of The Divide and over portions of
southern Wyoming. These areas will generally coincide with relatively low
afternoon relative humidity values and gusty winds...especially Monday and Tuesday.
Winds will flow out of the north 10 to 15 mph today...west to
southwest 10 to 20 on Monday...and west 20 to 30 on Tuesday...all
with higher gusts...across this area of concern. No appreciable
precipitation expected today. Monday night through Tuesday night
will experience fast moving isolated to scattered showers and storms
crossing the northern half of Wyoming from west to east. These
showers and storms will produce little rainfall through the daytime
hours with strong gusty outflow winds of 40 to 60 mph possible at
times. Heavier rainfall and some hail possible in the evening,
especially across portions of north central and northeastern
Wyoming. Lightning should be on an occasional basis. Otherwise,
smoke dispersion will be good to excellent each afternoon and


Riw watches/warnings/advisories...


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