Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180519
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1119 PM MDT Mon Apr 17 2017

.SHORT TERM...(This evening through Thursday)
Issued at 224 PM MDT Mon Apr 17 2017

As a long wave low pressure trough slowly passes through the
Northern Rockies, a series of short wave troughs will move through
the main trough in westerly flow aloft. The smaller troughs will
determine the timing and coverage of convection over the western
and northern mountains and valleys with showers and isolated
afternoon and early evening thunderstorms tonight, becoming more
numerous on Tuesday and Tuesday night, and spreading across the
southwest. Snowfall is expected above 7800 feet with rain to rain
snow mix below in the Valleys. West to southwest winds will be
rather strong over the mountains and along and near the east
slopes of the mountains and through the wind corridor from Rock
Springs to Casper with mountain waves in the lee of the Absaroka
and Wind River mountains.  Temperatures will cool on Wednesday
and Wednesday night with more widespread precipitation east of
the Continental Divide with snow levels falling to around 6000
feet late Wednesday night and Thursday morning. As the main
low pressure trough moves east of the area, cold unstable
northwest flow will produce showers mainly north and east,
especially over the mountains with gradual clearing over the west
and southwest.

.LONG TERM...(Tuesday night through Monday)
Issued at 224 PM MDT Mon Apr 17 2017

The circulation now over 43N/147W will swing a digging negatively
tilted trough across Wyoming Thursday night and Friday. Just as
other weather systems have been doing this season, the main energy
will track across the 4 Corners area with a baggy upper closed low
over the Central Rockies. And, much like previous systems as of
late, a mid level low will shoot north and east across our southern
and eastern CWA Friday, which usually means significant precip along
or near its track, as in this case. The Canadian is 6 to 12 hours
slower than the GFS with this soln. increased pops slightly, mainly
north and west of the track of the mid level low for a big swath of
likely pops. For now, have indicated about 2 to 3 tenths of an inch
of precip within much of this area for Thursday night with more
significant precip slated for Friday along with 4 inches of snowfall
in the Southern Absaroka Mtns just for Thursday night during which
time we can expect significant QG forcing. Rain should turn to snow
in the Western Valleys Thursday night and continue as rain and snow
Friday. Snow levels east of the divide should start out at about
7500 feet Thursday evening and lower to about 6000 feet by Friday
morning. Some instability Thursday evening and in the western and
central zones Friday will add to the intensity, somewhat, of the
heavier embedded showers, thanks to the cold pool aloft. The low
will pull out into the Central Plains Friday night with the precip
decreasing in intensity and coverage. After a precip free day
Saturday thanks to a transitory ridge, Sunday will feature some
precip out west as difluent west southwest flow ensues ahead of the
next shortwave that will track east across the Northern Rockies.
Skies will clear by Monday afternoon. High temps should range from
the 40s in the lower elevations of the west except 50s Sunday and
Monday and the 50s and then the 60s during the same times frames
east of the divide.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday)
Issued at 1115 PM MDT Mon Apr 17 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Unsettled weather over the area Tuesday and Tuesday night as rain
and mountain snow will occur. Expect areas of MVFR conditions and
mountain obscuration. Most of the terminal sites will remain VFR
through the period, but there will be times of MVFR conditions.
Please see individual terminal sites for more details.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail at the terminal sites through 00Z Wed.
Scattered rain showers will develop in the central and south by 18Z
with isolated thunderstorms possible through 03Z Wed. The north will
see isolated showers Tuesday afternoon. Widespread rain will develop
over central and southern WY after 01Z Wed and continue through 06Z
Wed. Expect MVFR conditions to prevail in these areas Tuesday night.
Please see individual terminal sites for more details.

&&

.FIRE WEATHER...
Issued AT 224 PM MDT Mon Apr 17 2017

As a long wave low pressure trough slowly passes through the
Northern Rockies, a series of short wave troughs will move through
the main trough in westerly flow aloft. The smaller troughs will
determine the timing and coverage of convection over the western
and northern mountains and valleys with showers and isolated
afternoon and early evening thunderstorms tonight, becoming more
numerous on Tuesday and Tuesday night, and spreading across the
southwest. Snowfall is expected above 7800 feet with rain to rain
snow mix below in the Valleys. West to southwest winds will be
rather strong over the mountains and along and near the east
slopes of the mountains and through the wind corridor from Rock
Springs to Casper with mountain waves in the lee of the Absaroka
and Wind River mountains.  Temperatures will cool on Wednesday
and Wednesday night with more widespread precipitation east of
the Continental Divide with snow levels falling to around 6000
feet late Wednesday night and Thursday morning. As the main
low pressure trough moves east of the area, cold unstable
northwest flow will produce showers mainly north and east,
especially over the mountains with gradual clearing over the west
and southwest. Most areas will have good or better smoke
dispersal Tuesday afternoon with stronger transport winds and
deeper mixing. The areas with more widespread showers on
Wednesday will have decreased smoke dispersal in the poor to fair
range with colder moist air increasing stability and developing
inversions over the valleys.

&&

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

&&

$$

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



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