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Area forecast discussion 
National Weather Service Riverton Wyoming
310 am MDT sun may 3 2015

Short through Tuesday night

The showers asct with a weak southern stream vorticity lobe that was over
the Great Basin earlier this evening will only produce some brief
sprinkles or light showers in southwest Wyoming this morning. In the
meantime...a cool front ejecting from a northern stream shortwave
with the asct vorticity maximum tracking east along the Montana Canadian border
will be dropping down across northern Wyoming and appears that is
has reached as far south as wrl. This front will get lodged up
against the Continental Divide today thus keeping the best
instability and inverted v soundings along and west of The Divide
as somewhat more stable air drifts in behind the front east of The
Divide today. Have mention of scattered thunderstorms with small hail
and gusty winds west of The Divide and a few showers with isolated
thunder with gusty winds east. Monday will feature more widespread
thunderstorm activity with lifted indices down to minus 3 in some
areas. WRF quantitative precipitation forecast indicates relatively strong and more focused
convective induced precipitation along and west of The Divide Monday
compared to the more widespread lighter showers asct with probable
virga showers that it appears to be indicating for this afternoon.
Tuesday should be even more active as far as convection GOES given
the anticipated upstream strong closed low making landfall onto
the Pacific northwest/British Columbia coast along with an arm of lower
pressure extending east southeast into Wyoming. In fact the GFS
indicates an 700 mb circulation over Pathfinder Reservoir Tuesday with
the Euro showing a weaker low but no discernible 700 mb circulation
over Johnson County Tuesday. Negative lifted indices and weak convective available potential energy
coupled with the 700 mb circulation should result in quite an active
convective day Tuesday/Tuesday evening. Temperatures will not fluctuate much
through Tuesday.

Long term...Wednesday through Sunday

Synopsis...a trough of low pressure over the western U.S. Will bring
cooler conditions with chances of showers and thunderstorms
Wednesday through Saturday. The system and will lift into the High
Plains on Sunday with decreasing chances of showers.

Discussion...medium range models have shown good overall agreement
and run-to-run consistency over the last 36 hours in the development
of trough across the western U.S. Wednesday through Friday. The
two weather makers during this period are a sub-tropical upper low
near 30n/140w and a northern stream upper low near 50n/150w. The
sub-tropical low will get kicked into the SW U.S. On Monday and
lift north-northeast across the Central High plains into strong downstream
ridge on Wednesday. This system will bring increasing showers and
thunderstorms up from the south on Wednesday...however the
heaviest and most widespread precipitation is expected east of the
area...along and east of the storm track.

Northern stream upper low will dig south along the West Coast
Wednesday and Thursday...roughly along 120w. There may be somewhat
of a break in precipitation Wednesday night into Thursday between
the two systems...the first upper low out of the sub-tropics lifting
into the northern plains Thursday morning. Upper low along the West
Coast is expected to cutoff from the flow Thursday and then slowly
wobble eastward across the Great Basin Friday and Saturday.
Upstream Pacific trough is forecast to move into the eastern
Pacific...across 140w around 00z Saturday...and act as the kicker
for the Great Basin low that is expected to accelerate out of
northern Utah NE across Wyoming into the High Plains Saturday night
and Sunday. Numerous showers and a few thunderstorms are expected
across much of the area Thursday afternoon through Saturday out
ahead of the upper low...the best chance for heavier precipitation
on Friday night and Saturday from southeast into central Wyoming.
Snow levels are expected to be between 7kft and 8kft across most of
the area Thursday through Saturday.


Aviation.../12z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

A weak cold front will stall out along The Divide later this
morning. This will result in a northerly to easterly surface flow
today into tonight. Although boundary layer moisture will increase
in wake of the front early this morning...low clouds are not
expected. The air mass will be relatively stable for much of the
day...but will begin to become slightly unstable after 21z Sunday
with peak heating and some convection is possible especially closer
to The Divide. Will mention thunderstorms in the vicinity at klnd and kcpr...but only have
vcsh at terminals farther north. This convection should weaken by
around 03z Monday...but some isolated activity could linger past 06z
Monday. VFR conditions are expected...except for brief MVFR/IFR
conditions near convection.

West of The Divide...kbpi/kjac/kpna/krks routes

A slightly unstable atmosphere with adequate moisture will allow for
isolated to scattered showers and thunderstorms across much of the
area...mainly in the afternoon and evening. Inverted v
soundings...steep lapse rates and cool temperatures aloft suggest
strong outflow winds to 45 kts...and small hail will be the main
hazards. Enough coverage to mention thunderstorms in the vicinity at all terminals during the
afternoon and evening. The convection will weaken by around 03z
Monday...but some activity could linger past 06z Monday. VFR
conditions are expected except for brief MVFR/IFR conditions
possible near any convection.


Fire weather...

Westerly zonal flow will be over the area through today with weak
low pressure troughs in the northern branch of the flow and
subtropical moisture moving up from the Great Basin in the
southern branch of the flow. Diurnally driven scattered showers
with isolated thunderstorms reforming late this morning and
increasing through the afternoon will result. Cloud cover and scattered
precipitation will have a cooling effect today. With moderate
flow aloft and convective cells moving 25 to 30 miles per hour...wind gusts
to 45 miles per hour will accompany the thunderstorms along with small hail.
The coverage of showers and thunderstorms is expected to increase
this afternoon. With strong convective mixing and moderate flow
aloft good or better smoke dispersal is expected this afternoon.
These mainly late day thunderstorms are expected to persist
through at least Tuesday.


Riw watches/warnings/advisories...


Short term...lipson
long term...aem
fire weather...lipson

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