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Area forecast discussion 
National Weather Service Riverton Wyoming
248 am MDT Sat Oct 1 2016

Short through Monday night

The last of this evenings convection has cleared the area and
dissipated over southern Montana. Currently, the nearest area of
convection is a thunderstorm cluster moving north across central
Idaho within a stream of moisture rounding the base of the next
upstream low off the Pacific northwest coast. These cells appear to
be related to the first vort lobe that will track northeast across
Idaho and western Montana ahead of the main low with another vort
lobe taking the same track this afternoon, all within the broad
circulation around this closed low. This second vort lobe that will
brush our far western zones as well, as marginal instability and
southwest difluent flow will result in a good chance of afternoon
and evening showers and thunderstorms in western Wyoming later
today. As this low moves inland across Oregon and northern
California Sunday, the southwest steering flow will increase along
with increasing instability resulting in an increase in thunderstorm
activity across the west half of the cwa by Sunday afternoon. Then
on Sunday night, the mid and upper level low will track to
Idaho/Nevada/Utah with Wyoming underneath an increasingly strong
southern difluent flow ushering in more moisture. This will likely
result in another round of a rapid succession of thunderstorms
moving south to north across the area Sunday night in the same
fashion as two Thursday nights ago, but with with not quite as much
rainfall. The NAM/GFS/Euro all seemed to have come to a general
consensus as to tracking this low northeast across northwest Wyoming
by Monday. This will mean that sfc temperatures will not be as cool
as originally anticipated on Monday during the time all the dynamics
of this storm system come together. Widespread rainfall can be
expected Monday, especially over the north and west, closer to the
track of the low. With the left front quad of the jet ovhd as well
as q vector forcing, especially closer to the track of the low,
plenty of measurable rainfall on the order of one half inch to one
and a quarter inches could fall across the northwest third of the
County Warning Area Sunday afternoon and night, Monday, and Monday night. Snow
levels will drop to around 8500 feet Monday to 7500 feet late Monday
night as colder air aloft streams in from the west southwest. High
temperatures Monday should be 10 to 15 degrees cooler on Monday in
most areas. As the almost vertically stacked low pulls out into
eastern Montana, wrap around moisture will continue the rain and
snow shower activity closer to 12z Tuesday. The 700mb low will be a
bit too far north for classic rain and snow banding off the favored
mountain areas across the north half of Wyoming, in spit of the weak
instability and moisture around. By 12z Tuesday, have painted the
Big Horn Mountains, extreme northern absaroka and highest Wind River
mtns with around 4 inches of total snow during this precip event,
but that is still a long ways off so plenty of time to adjust.
South southwest winds should increase to 35 to 45 mph from krks to
kcpr and 25 to 35 mph across the Wind River and Big Horn basins as
well as Johnson County and the Big Horn Mountains, south and east
of the storm center where 700mb winds are progged to blow at
50 knots.

Long term...Tuesday through Saturday

00z guidance suite in good agreement with a stacked low over
eastern Montana at 12z Tuesday. This low is expected to slowly
push northeast into south-central Canada late Tuesday/Tuesday
night. Breezy to windy conditions are expected Tuesday along with
isolated to scattered mountain Snow/Valley rain and snow showers
with an emphasis across the northwest and north. Snow levels
looks to be around 6500 feet. The forecast area remains on the
cyclonic side of the upper jet through Thursday. This combined
with enough moisture and instability for a slight chance of some
diurnal valley Rain/Mountain snow showers becoming enhanced at
times by some perturbations in the flow. Thursday night and Friday
look to be mainly dry as some ridging takes hold of the area. This
looks to be short-lived as a low amplitude trough is projected to
track east across the northern rockies Friday night/Saturday time
frame. For now the bulk of the moisture and precipitation is
expected to be north of the area.

Temperatures are expected to be below average, but nothing extreme Tuesday
through Thursday with more seasonal temperatures Friday and Saturday.


Aviation.../12z issuance/

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

Patchy fog or low clouds will occur in the far west valleys until 16z.
Kjac terminal site may be impacted by these conditions with MVFR to IFR
cigs/vsbys at times. Otherwise...scattered showers and thunderstorms
will develop over the west after 19z and continue through 03z. Isolated
showers will occur after 03z. Local MVFR conditions may occur with storms.

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

Isolated showers and thunderstorms will occur after 20z through 04z.
Isolated showers will occur after 04z. Otherwise VFR conditions will
occur through the period.


Fire weather...

A large Pacific low pressure trough off of the Pacific northwest
coast will move inland and eventually across western and central
Wyoming through Wednesday bringing increased clouds and showers
with isolated afternoon and evening thunderstorms. Temperatures
will cool through Wednesday with the onset of more widespread
precipitation Monday and Monday night...snow levels will lower
to around 7500 feet at night rising to around 8500 feet on
Tuesday. Precipitation amounts could become quite significant...
especially in the north and West Mountains.


Riw watches/warnings/advisories...none.



Short term...lipson
long term...wm
fire weather...lipson

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