Area Forecast Discussion
Issued by NWS Riverton, WY

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029
FXUS65 KRIW 282148
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
348 PM MDT Fri Apr 28 2017

.SHORT TERM...(This evening through Saturday)
Issued at 242 PM MDT Fri Apr 28 2017

The main circulation appears to have become established over
Western Colorado attm which is where the models have placed it for
21z today with the 500mb and 700mb circulation stacked upon one
another. The QG forcing will trend south of our CWA during the
night tonight as the low exits to the south and southeast. We
will lose the isentropic lift as well. Much of what additional
snowfall (and most of if not all of the precip will be snow with
h7 temps falling to -9c) occurs will be more purely upslope east
of the divide. Light snow or flurries will end this evening in the
west as general troughiness exits the area. The associated cold front
has reached most of Sweetwater County with KRKS having
experienced the wind shift at 20z this afternoon as colder air
continues to seep in. Will keep remaining highlights going east of
the divide through 06z tonight. Even roads that are just wet will
likely freeze and become slick along with another 1 to inches of
snowfall expected over much of the area. Mainly just flurries or
light snow showers will linger beyond midnight tonight, ending
from north to south east of the divide and over Sweetwater
County.

.LONG TERM...(Saturday night through Friday)
Issued at 242 PM MDT Fri Apr 28 2017

By Saturday morning, the core of the large storm system will have
moved south to southern CO and northern NM, moving the snow and
colder air to the southern Front Range. A few early morning
showers may still continue from the central WY mountains to the
southern WY border and into SW WY. The surface pressure gradient
will continue the strong northeast flow from around Muddy Gap into
Sweetwater County as well, with the wind speeds diminishing to
10-12 mph by 6pm. Elsewhere across the area on Saturday, skies
should gradually clear through the day over the lower elevations,
with mostly cloudy skies remaining over the higher terrain.
Temperatures will possibly hit the low 50s around Cowley and
Greybull, but remain in the 40s for another cool day.

As the large low moves into the southern and central plains with
high pressure building over California, northwest flow is expected
over WY Sunday through Wednesday.  The first trough from the Pac NW
looks to slides into the area from Sunday afternoon into Monday.  At
present, the models are indicating a weaker system mainly expected
to produce mountain precipitation and some showers in the foothills,
as well as northern Johnson County. Most of the energy is expected
to move more to the northeast, resulting in colder air and more
precipitation northeast of a line from Cody to Casper.  Some strong
northwest surface flow could develop across SW and central WY as
well as NE WY.  Another shortwave trough in the NW flow will move
through from early Tuesday through early Wednesday.  The core of
this system appears to be more along western WY into western CO,
once again bringing down a strong northerly flow east of the divide
and forcing an upslope situation. Mid-level temperatures don`t look
quite as cold as the Fri Apr 29 system, so we may see more of a
rain/snow mix as the lower elevations.  The timing of this storm
will start across western WY Tuesday afternoon and move into
northern and central WY Tuesday evening into Wednesday morning.

Later Wednesday should see quick clearing from the west as high
pressure builds more northward and eastward across the Great Basin.
The ridge axis will move slowly eastward Thursday and Friday, with
the ECMWF moving more quickly and allowing some instability into NW
WY by Friday afternoon.  Regardless, temperatures Thursday and
Friday will quickly revert to above normal for the lower elevations
with temps jumping to the 60s and low 70s.|

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)

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

As the large scale storm system moves to the southeast, a secondary
easterly push has moved into western WY.  A few snow showers are
possible this evening, dropping conditions from IFR to MVFR, but
conditions show stayed improved through the night.  The mountains
will continue to be obscured with mid-level moisture.  Northerly to
NE flow will also continue, with stronger NE flow at KRKS through
00Z Sun.  Skies should clear Saturday afternoon with mountain cumulus
behind the storm.  Additional storminess expected through Wednesday
morning in waves.

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

Widespread snow is starting to diminish late this afternoon. General
improved conditions in the Big Horn Basin will remain, while
conditions across central WY will slowly improve through 06Z Sat.
KLND likely to hold on to snow the longest though.  MVFR lower
ceilings likely through Saturday morning, with gradual clearing
through the day.  ENE flow to continue after 06Z Sat before
diminishing.  Mountains to remain obscured through about 18Z Sat.
Expect next system Sunday into early Monday and another on Tuesday.

&&

.FIRE WEATHER...
Issued AT 242 PM MDT Fri Apr 28 2017

A cool and wet pattern looks to persist through tonight. Low
pressure will move south to Colorado with a mix of rain and snow
continuing into this evening, diminishing late. Mixing and smoke
dispersal will be generally fair. Drier weather will return
Saturday, although it will remain rather cool followed by warmer
temperatures for Sunday. A faster moving precipitation event is
slated for early next week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT tonight for WYZ008-
017-018-022-030.

Winter Storm Warning until midnight MDT tonight for WYZ009>011-
015-019-020.

&&

$$

SHORT TERM...Lipson
LONG TERM...McDonald
AVIATION...McDonald
FIRE WEATHER...Lipson



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