Area Forecast Discussion
Issued by NWS Riverton, WY

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558
FXUS65 KRIW 281743
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1142 AM MDT Fri Apr 28 2017

.SHORT TERM...Friday through Sunday night

The snowstorm is currently underway and will continue through the
day. So far, the hardest hit areas are the Green and Rattlesnake
range as we as portions of Johnson County. So far, and I then size
so far the storm has behaved itself for the most part. The main
brunt will begin now as flow begins to turn northeasterly and
enhances the upslope component of the storm. The question continues
to be where will the heaviest snow set up. And, there is still a bit
of uncertainty as to exact location. We do have some concerns that
higher amounts of snow may fall in places like Lander so they could
be upgraded to a warning. This will be a game time decision.
Otherwise, everything looks OK at this point so we will let most of
the highlights ride. And again, we will run into the problem
during the day that once we get to about 9 or 10 am with the high
sun angle, accumulations, especially on paved surfaces may let up.
As a result, the worst conditions will through the morning with
improvement in the roads this morning with some improvement in the
afternoon. Conditions should begin improving in the north after
noon and the improving everywhere by Friday evening as low
pressure moves away from the area. Most of the steady snow should
be over by midnight with only some leftover showers late at night.
Meanwhile, West of the Divide some snow showers will continue.
However, additional accumulations should be rather light so at
this point we should not need any highlights.

Saturday looks to be a much drier day. However, with a bit of
instability lingering, there will still be some mountains showers
around. Any accumulations will be inconsequential however.
Temperatures will also remain rather cool with clouds hanging around
in the morning and much of the solar energy going toward melting the
snow rather than heating things up. There should be some breaks in
the clouds after noon however to make things feel a bit warmer. With
a mainly clear sky expected Saturday night and early Sunday, we did
add patchy fog to areas that are expected to see a significant
amount of snow today.

The next system will approach the west on Sunday. The models have
trended a bit slower with it`s arrival, so we trimmed POPS in the
morning and also removed them from the Bighorn range during the day.
The models always seem to think the Bighorn`s are wetter than the
Amazon rain forecast. Anyway, any showers during the day would be
restricted to the west with dry and milder weather East of the
Divide. Some showers will spread east on Sunday night. Given the
more important events of the day, we made few changes this far
out.

.LONG TERM...(Monday through Friday)
Issued at 127 AM MDT Fri Apr 28 2017

A couple of quick moving shortwaves in the strong northwest flow
will impact the forecast area Monday and Tuesday. The stronger of
the two will slide southeast across the area Monday night and
Tuesday with scattered to locally numerous mountain snow/valley
rain showers. Snow levels Monday night/Tuesday could lower to
valley floors above 5000 feet, but went with rain or rain/snow
mix. Overall precipitation amounts look to be on the light side
with some convection giving pockets with heavier amounts. Could
even see a few thunderstorms Tuesday.

Upper level ridging builds into the area Wednesday and Thursday
with mainly dry conditions and a warming trend. Global models show
the ridging flattening, or at least the ridge axis shifting east
into the Plains ahead of digging trough over the West Coast
Friday. Could see convection late Friday ahead of this trough.
Models diverge significantly on how this trough will evolve next
weekend with the GFS and Canadian models splitting the trough,
while the ECMWF shows a deep trough pushing across the Rockies.
Plenty of time to watch this system.

Below average temperatures are expected Monday and Tuesday, with
temperatures warming to above average by Thursday and Friday.

&&

.AVIATION...18Z Issuance

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

Large scale storm continues across WY this afternoon but it is
slowly moving to the southeast, resulting in scattered snow showers
west of the divide through about 03Z Sat.  Main impact for these
terminals will be wind shift to the northeast and gusts over 20 kts
also through 03Z Sat.  KRKS will see a higher chance of snow and a
much longer period of northeast wind with gusts around 30 kts.
Higher terrain continues to be obscured through at least 12Z Sat.
Conditions improve mid day Saturday with weaker wind and mostly
mountain cloudiness.

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

Widespread snow continues with the large storm system this
afternoon, with the biggest impact across central WY.  KCOD and KWRL
should see improving conditions this afternoon with a slight. t
chance Central WY will continue to see snow through about 06Z Sat.
of showers early this afternoon. LIFR conditions likely through 21Z
from KCPR to KRIW to KLND in heavy snow; improving by 00Z Sat. with
MVFR after 03Z Sat. Mountains certainly obscured with most clearing
occurring after 18Z Sat.

&&

.FIRE WEATHER...

A cool and wet pattern looks to persist today. Low pressure
will move across the area over the next couple of days and bring a
mix of rain and snow today with mainly snow into this evening.
The precipitation will be banded and difficult to pinpoint the
heaviest amounts. Mixing and smoke dispersal will be generally
fair. Drier weather will return tomorrow although it will remain
rather cool followed by warmer temperatures for Sunday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT tonight for WYZ030.

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

Winter Weather Advisory until midnight MDT tonight for
WYZ005>008-017-018-022.

Winter Weather Advisory until noon MDT today for WYZ003-004.

Winter Weather Advisory until 6 AM MDT early this morning for
WYZ001-002.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...McDonald
FIRE WEATHER...Hattings



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