Area Forecast Discussion
Issued by NWS Riverton, WY

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311
FXUS65 KRIW 210445
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1045 PM MDT Fri Sep 20 2019

.SHORT TERM...(Today through Saturday)
Issued at 357 AM MDT Fri Sep 20 2019

Much cooler and wetter weather is expected for the area,
especially over the western Mountains.

A prominent circulation is seen spinning over the Great Basin.
This circulation is progged to track northeast across Northwest
Wyoming through this evening. Ahead of this circulation and
associated strong shortwave, a strong difluent south southwest
flow along with sufficient instability has already spawned
significant nocturnal convection earlier this evening along with
considerable lightning. However, according to spotter reports,
not nearly as much rain has fallen from these cells compared to
what was indicated on radar. A northerly push at the sfc also
appeared to have enhanced the growth of these cells as a result
of sfc convergence.

As this weather system progresses northeast, instability will
again develop ahead of it in the warm sector today, with decent
capes across the Big Horn mountains as well as over the adjacent
lower elevations of Johnson County. In fact, SPC has our northern
zones east of the Divide in a marginal risk for this afternoon as
a result of the instability and synoptic features, which would
include the increasing south southwesterly diffluence and the
wind directional shear created by the 700mb low over Northwest
Wyoming. A dry slot could occur from RKS to CPR ahead of this
system, south and east of where most of the convection will take
place today.

Then our attention shifts to the snow potential for the Western
Mountains. Both the GFS and the Euro agree on cooling the 700mb
temps significantly through Saturday around the backside of the
trough/low center with the GFS lowering h7 temps down to -3.5C
and the Euro, -5C. Decided to expand the winter weather advisory
to include the rest of the Western Mountains, including the Wind
River Mountains to account for Togwotee Pass. It appears the
coldest and wettest weather will occur over Northwest Wyoming
where the low will track. Will keep the valid time of the advisory
going from 6 am this morning through 12 noon Saturday.
Significant snowfall will occur above 8500 feet in Northwest
Wyoming and around 9500 feet across the Wind River Mtns. After
most of the precip quits on Saturday in most areas, the Western
Mtns, especially the Northwest Mtns, will continue to receive
wrap around moisture and snowfall from the exiting low well into
Saturday.

.LONG TERM...(Saturday night through Thursday)
Issued at 357 AM MDT Fri Sep 20 2019

With the trough quickly departing east on Saturday night, the
ridge will build in quickly behind it, bringing much more tranquil
conditions to the region Sunday. Models have become more
consistent with the next trough moving into the Pacific Northwest.
Models now agree on a solution to drop the trough southward
through Nevada into the Four Corners region, or even further west
down the lower Colorado basin. This will bring only weak
troughing to Wyoming proper, so rain chances will be minimized for
Monday, focused on the far NW portion of the state, and even this
may be overdone. Broad zonal flow across the northern US will
bring a slight chance for a shower Wednesday if any weak impulses
sweep through. Longer range models hinge on the next large H5
trough rolling southeastward into the northern Rockies.
Considerable model differences noted on how sharp of a southward
drop this trough will take, but WY will at least see mid level
winds switch more form the southwest, which generally leads to an
increased amount of mid level moisture available, so a chance of
showers is still a possibility Thursday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)

Will continue with VFR to MVFR conditions due to ceilings across
west and northwest Wyoming through around 18Z Saturday. Some local
IFR conditions will also be possible at times. Expect frequent
mountain obscurations until around 18Z Saturday west of the
Continental Divide.

At least VCSH conditions will be possible at most of the TAF
sites through the majority of the forecast period. Most locations
east of the Continental Divide will experience VFR conditions
through the balance of the forecast period.

&&

.FIRE WEATHER...
Issued AT 357 AM MDT Fri Sep 20 2019

The main focus will be a fall storm system that will bring
widespread valley rain/mountain snowfall for western and northern
Wyoming today into Saturday. The heaviest precipitation will
occur today into this 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,
especially if that area remains dry during the next couple of
days.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MDT Saturday for WYZ001-002-
012-014-015-024.

&&

$$

SHORT TERM...Lipson
LONG TERM...Straub
AVIATION...Troutman
FIRE WEATHER...Lipson



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