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Area forecast discussion
National Weather Service Riverton Wyoming
347 PM MDT Sat may 27 2017

Short term...(this evening through tuesday)
issued at 246 PM MDT Sat may 27 2017

Not much in the way of convection this afternoon so far with the
only cells of significance drifting off the southern Big Horn mtns
into northern Natrona County with weaker cells blowing south off
the south end of the Wind River mountains into Sweetwater County but
not making it as far south as Interstate 80. Otherwise, today has
turned out to be a pleasant start to the Holiday weekend with
temperatures at the lower elevations east of The Divide in the 60s
and 50s west. No large scale cold precipitation events are
expected at least for the next several days. Sunday afternoon and
evening should prove to be more active as far as convection GOES
as a weak wave in the northwest flow approaches followed by the
jet stream taking a dip to the southwest Monday with a weak
frontal boundary moving in from the northeast late Sunday night
and Monday morning. Small hail and gusty winds should be the main
features with any convective cells Sunday afternoon/eve with the
wet bulb zero still relatively low. Any thunderstorms over the Big
Horn mtns on Sunday could produce snowfall above 9000 feet with
relatively cool air lingering aloft over northeast Wyoming. Behind
the sfc boundary Monday and Tuesday, the air will remain
relatively stable north of The Owl creek mtns east of The Divide
and in most areas east of The Divide Tuesday where not much in the
way of convection is expected with late day scattered
thunderstorms relegated to areas west of The Divide, especially
over the higher elevations. With high pressure slowly building in
from the west, temps aloft will slowly increase from the southwest
ranging from 1.5c in Johnson County Tuesday afternoon to 11c in
southwest Wyoming. Temperatures will slowly warm Sunday through
Tuesday, with the exception of temperatures leveling off Monday
east of The Divide behind the weak surface boundary. High temps by
Tuesday should range from the upper 70s to low 80s at the lower
elevations east of The Divide to upper 60s and low 70s west.

Long term...(tuesday night through saturday)
issued at 246 PM MDT Sat may 27 2017

Tuesday night through Saturday

Shortwave ridging starts the period before a splitting trough starts
to slowly approach from the west Wednesday into Thursday. Current
timing has this splitting trough moving across late Thursday into
Thursday night...possibly lingering into Friday. Big picture then
shows the mean ridge reestablishing itself late next week and
beyond. New GFS shows this trend with the next upper low over the
pacnw next weekend just clipping the nwrn corner just beyond this
period. Weak shortwave Tuesday night may help focus a few storms
over the far west into Tuesday evening/Tue ngt. On Wednesday,
ripples in the SW flow and daytime heating will likely help fire
some storms in the west. In the east, some late day se flow around a
midwestern surface high may allow some higher dew points to hover
near Johnson County for some storm potential there. This signature
looks better Thursday though as surface low forms around srn Wyoming/nrn
Colorado which May Draw higher Theta air wwd into at least Johnson County
where some stronger storms are possible. In the west, approaching
splitting negatively tilted trough will aid in more widespread
showers/storms out west which will move east overnight and linger
into Friday. Heights start to recover Saturday as we are in
between our split trough off to the east and the Pacific northwest large
upper low. Probably enough energy in the SW flow to fire at least
some late day higher elevation storms.


Aviation...(for the 00z tafs through 00z Sunday afternoon)
issued at 246 PM MDT Sat may 27 2017

West of the Continental Divide...kbpi/kjac/kpna/krks terminals

VFR conditions will prevail thru Sunday. Slight instability will
result in isolated showers and thunderstorms in the vicinity of the
Continental Divide with this convection tracking off the southern
end of the Wind River Range but should dissipate before reaching
vicinity krks. This activity will dissipate with sunset, leaving
prevailing sky clear-few fl130-150. More late day showers and
thunderstorms are expected Sunday afternoon, especially over and
near the mountains.

East of the Continental Divide...kcod/kcpr/klnd/kriw/kwrl terminals

Slight instability through this evening will result in isolated
showers and thunderstorms along Mountain Valley/breeze boundaries
along the south and eastern slopes of mountain ranges and moving
south-southeast. This activity will be mostly anchored to the high terrain and
not impact any terminals. Kriw-kcpr routes will be most vulnerable
to a few thunderstorms in the vicinity of the southern Bighorn
range. Convection will dissipate with sunset, leaving sky clear-few fl130-
150. A better coverage of showers and thunderstorms are expected
Sunday afternoon, especially over and near the mountains.


Fire weather...
issued at 246 PM MDT Sat may 27 2017

Fire danger low across the forecast area through the Holiday
weekend as all fuels are currently in green-up and much of the
forecast area has received some form of precipitation over the
last few days. Precip chances decrease over the next 3 days, being
isolated to widely scattered at best and most likely to occur
over the mountains and closely adjacent lower elevations. On
Monday and Tuesday most of the precipitation will mainly be
relegated to areas along and west of The Divide. Additionally,
high temperatures and minimum relative humidity values will remain at or below
seasonal and mainly above 20 percent respectively. However, by
Tuesday afternoon the minimum relative humidity could dip to the upper teens in
southwest Wyoming. Winds too will remain relatively light through
the period. Smoke dispersion will be good to very good Sunday,
then lowering to fair to good on Monday and Tuesday.


Riw watches/warnings/advisories...


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