Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 180430
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1030 PM MDT Sun Sep 17 2017

.SHORT TERM...This evening through Wednesday
Issued at 215 PM MDT Sun Sep 17 2017

During the short term the upper level pressure pattern will shift
from a Gulf of Alaska low center with a trough extending south
southeast along the west coast with rather strong west southwest
flow over western and central Wyoming to a closed low over Alberta
and Saskatchewan with a trough extending south across the
Northern Rockies and Great Basin as a high amplitude high pressure
ridge builds into the Gulf of Alaska. This will bring a series of
fast moving short wave troughs into western Wyoming through
Wednesday. Tonight will be dry with some fog in the western
valleys and northern and central basins with increasing southwest
winds across the southwest spreading through the wind corridor
from around Rock Springs to Casper. The windy areas will warm
tonight while the valleys and basins will have cold air trapped
under an inversion. Overall temperatures will warm but freezing
temperatures will occur in the western valleys and mountains.
Clouds will increase on Monday with some rain and higher
elevations snow showers developing over the western and northern
mountains. The showers will become rather widespread across the
northern and western mountains with some spreading into the lower
elevations Monday night and Tuesday. Embedded thunderstorms are
expected with the showers on Tuesday afternoon with small hail,
local heavy rain/snow and gusty winds. The heaviest precipitation
will be over the northern and western mountains with snow levels
lowering as colder air aloft moves in with the deepening low
pressure trough aloft. Snow levels will lower to around 8000 feet
and a low as 7500 feet in the northwest. From Monday night through
Tuesday night the mountains, mainly above 9000 feet, may see from
5 to 10 inches of wet snow with a rain snow mix with lower snow
accumulations from 9000 to 8000 feet. Temperatures will cool from
Monday night through Tuesday night. A break will occur in the
moisture feed from late Tuesday night into Wednesday morning with
a brief break in the precipitation before more showers move into
the northwest Wednesday afternoon. The southwest will become quite
windy Tuesday with the wind spreading across southern fremont and
Natrona counties continuing through Wednesday.

.LONG TERM...Wednesday night through Sunday night
Issued at 215 PM MDT Sun Sep 17 2017

Synopsis...The second, reinforcing upper low will aid digging the
longwave trough over the Great Basin Wednesday night. This will back
the flow to the SW once again and the core of the 100-120kt jet
located from central CA to E MT, clipping NW WY. As a result, a
strong baroclinic zone will return at the beginning of the forecast
period. 700mb temps will range from +9/+10C from W Sweetwater to
Natrona (with 40-50kt winds), to 0/+1C over Yellowstone. Although
the surface gradient continues to look weak, the Wind Corridor could
again be impacted. The cold front will slowly make its way eastward
Wednesday night, before finally moving through most of the forecast
area by 00Z Friday. This will all be associated with the first of
several shortwaves, that will rotate around the base of the trough
and through the forecast area. The jet will be over the eastern half
of WY by 12Z Friday, generally staying in place as the low/trough
begins to fill Saturday and begins to exit the area Sunday.

Discussion...Precipitation should be confined to northern portions
of the Big Horn Basin and the west Wednesday night, along the S side
of the jet. This will spread eastward Thursday, along and behind the
cold front, becoming widespread Thursday night and impacting all but
Natrona and eastern portions of Sweetwater. Precipitation will
impact the entire forecast area by Friday, due to the upper level
dynamics of the jet and embedded shortwaves. By Saturday, the focus
of this activity will mainly be east of the Divide. However,
scattered showers will continue west of  the Divide. Guidance become
split by Sunday, with the GFS the faster solution and ending
precipitation over the area. The ECMWF continues the precipitation
through the day Sunday, before finally ending Sunday night.

Temperature profiles continue to support valley rain/mountain snow
through this event. Snow levels will range between 6500 and 7500 ft,
possibly dropping to 5500-6000 feet Saturday morning.

&&

.AVIATION...For the 06Z TAFS through 06Z Tuesday
Issued at 1030 PM MDT Sun Sep 17 2017

VFR conditions will occur at most terminal sites through 06Z
Tuesday. Breezy to windy conditions will occur Monday at most
terminal sites from late morning through sunset. A cold front will
move into NW WY around 06Z Tue and push eastward to a KBYG-KRIW-KEVW
line at 12Z Tuesday. Rain showers with high mountain snow will begin
in NW WY between 00Z and 03Z Tue. This precipitation will spread
across much of Western WY by 12Z Tue. MVFR to IFR conditions will
develop over the northwest by 03Z Tue and increase in areal coverage
from 06Z to 12Z Tue in the west. Mountains will become more obscured
with time Monday night in the west. Please see terminal forecasts
for more details.

&&

.FIRE WEATHER...
Issued AT 215 PM MDT Sun Sep 17 2017

A series of fast moving short wave troughs will move into western
Wyoming through Wednesday. Tonight will be dry with some fog in
the western valleys and northern and central basins with
increasing southwest winds across the southwest spreading through
the wind corridor from around Rock Springs to Casper. The windy
areas will warm tonight while the valleys and basins will have
cold air trapped under an inversion. Overall temperatures will
warm but freezing temperatures will occur in the western valleys
and mountains. Clouds will increase on Monday with some rain and
higher elevations snow showers developing over the western and
northern mountains. The showers will become rather widespread
across the northern and western mountains with some spreading into
the lower elevations Monday night and Tuesday. Embedded
thunderstorms are expected with the showers on Tuesday afternoon
with small hail, local heavy rain/snow and gusty winds. The
heaviest precipitation will be over the northern and western
mountains with snow levels lowering as colder air aloft moves in
with the deepening low pressure trough aloft. Snow levels will
lower to around 8000 feet and a low as 7500 feet in the northwest.
From Monday night through Tuesday night the mountains, mainly
above 9000 feet, may see from 5 to 10 inches of wet snow with a
rain snow mix with lower snow accumulations from 9000 to 8000
feet. Temperatures will cool from Monday night through Tuesday
night. A break will occur in the moisture feed from late Tuesday
night into Wednesday morning with a brief break in the
precipitation before more showers move into the northwest
Wednesday afternoon. The southwest will become quite windy Tuesday
with the wind spreading across southern fremont and Natrona
counties continuing through Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Baker
LONG TERM...LaVoie
AVIATION...AR
FIRE WEATHER...Baker



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