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fxus65 kriw 192100 

Area forecast discussion
National Weather Service Riverton Wyoming
300 PM MDT Thu Sep 19 2019

Short term...(tonight through saturday)
issued at 255 PM MDT Thu Sep 19 2019

A trough over the Great Basin will slowly pull across The Rockies
Friday and Saturday. Models are in good agreement with a better
defined mid-level circulation developing across eastern Idaho
tonight. Have lowered pops and precipitation amounts tonight as
models are showing relatively weak forcing ahead of the system.
However this is expected to change rapidly towards sunrise Friday.
This circulation is then expected to push across northwest Wyoming
Friday into southeast Montana Friday night. The heaviest and most
widespread precipitation is expected to occur ahead and near the
circulation (friday into Friday evening) across western and
northwest portions of the forecast area. Some significant
mountain snowfall is expected. Farther southeast, models are
showing a dry slot with possibly very little precipitation in the
Rock Springs to Casper corridor. Cannot rule out a few robust
thunderstorms over the north/far east Friday afternoon/early
evening. Instability will be weak, but shear decent enough with
forcing for possibly a strong thunderstorm or two.

700mb temperatures and soundings would indicated snow levels by
12z Friday around 9500 feet over the far west to 11,000+ feet
over the east. A cold front will push across the forecast area
Friday, with snow levels by 00z Saturday around 8500 ft west, and
9500 to 10000 ft east. There is enough cold air to have snow
levels drop to around 7000 feet late Friday night.

On Saturday, a moist northwest flow will occur with snow likely
over the northwest mountains. This looks to be high pops, but low
precipitation amounts.

Issued a Winter Weather Advisory for the absaroka mts. And
Yellowstone despite not expecting the widespread 6 to 12 inches
of snow as this is an early season snowfall with some Mountain
Pass impacts.

Long term...(saturday night through thursday)
issued at 255 PM MDT Thu Sep 19 2019

Showers will linger across the northwest Saturday evening and
overnight as the trough will be slow in departing. Enough moisture
in cyclonic northerly flow to necessitate at least low precipitation
chances in this region. A shortwave ridge will quickly replace this
trough for Sunday with a sunny and seasonal day in store. Ridge axis
will traverse the forecast area Sunday afternoon with gusty
southwest wind expected from eastern Sweetwater County northeast
through Natrona County. Progressive zonal flow will bring the next
trough across the intermountain west and toward Wyoming on Monday.
Models have differing solutions for Monday regarding this trough.
GFS brings an open wave directly across the forecast area Monday
afternoon and evening, while both the European model (ecmwf) and Canadian drive this
trough toward the Desert Southwest. Tend to favor the idea of the
energy dropping more southwest with just general troughiness across
the forecast area Monday. Thus, have scattered precipitation chances
across the northwest mountains and isolated chances in the valleys.
Conditions elsewhere Monday will be generally dry, especially east
of the Continental Divide, with seasonal temperatures expected. The
flow aloft will be west-northwest Tuesday and Wednesday in the wake
of the open wave trough. Any embedded ripple within this flow could
lead to light showers across the far northern mountains. Otherwise,
it will be dry and seasonal Tuesday and Wednesday. Wind speeds
across southwest Wyoming will begin to ramp up Wednesday as the next
trough takes shape across the Pacific northwest. These winds will
further increase on Thursday and likely more so Friday as the flow
backs more to the southwest, the pressure gradient tightens, and the
mid-level flow strengthens. The southwest flow and warmer mid-level
temperatures could lead to Thursday being warmer than normal and
possibly set the stage for elevated fire weather conditions.


Aviation...(for the 18z tafs through 18z friday)
issued at 1128 am MDT Thu Sep 19 2019

Trough over the intermountain west will progress slowly toward the
forecast area Thursday afternoon and evening. Gusty south to
southwest wind will increase for most terminals by 21z/Thursday with
a decrease seen around sunset. All terminals to be VFR through at
least 06z/Friday. The incoming trough will begin to spread showers
across kjac, kbpi, and kpna between 11z-15z/Friday. Kjac will see
MVFR conditions Friday morning with the potential for IFR. Kbpi and
kpna will be low-end VFR with possibly brief periods of MVFR with
heavier Showers. Mountain tops will be obscured by 12z/Friday with
mountain obscurations becoming more frequent along and west of the
Continental Divide by 18z/Friday. Central terminals and krks will
see VFR conditions with increasing southwest wind toward 18z/Friday.
A frontal boundary across the northern Bighorn Basin will provide
weak upslope flow for kcod with low-end VFR and light rain showers
possible after sunrise Friday.


Fire weather...
issued at 255 PM MDT Thu Sep 19 2019

Elevated to critical fire weather conditions (strong gusty winds,
low rh) will continue through early evening. Otherwise the main
focus will be a Fall storm system that will bring widespread
valley Rain/Mountain snowfall for western and northern Wyoming
Friday into Saturday. The heaviest precipitation will occur Friday
into Friday evening. Areas between Rock Springs and Casper might
end up not seeing much rainfall. Accumulating snowfall is expected
generally above 8000 feet with this storm system but the bulk of
the accumulating snowfall will be above 9000-9500 feet.

Warmer and drier conditions return Sunday with elevated fire
weather conditions possible between Rock Springs and Casper.


Riw watches/warnings/advisories...
Winter Weather Advisory from 6 am Friday to noon MDT Saturday
for wyz001-002.

Red flag warning until 7 PM MDT this evening for wyz280.



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