Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 292116
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
316 PM MDT Wed Mar 29 2017

.SHORT TERM...(This evening through Thursday)
Issued at 1223 PM MDT Wed Mar 29 2017

Very challenging forecast upcoming for our developing upper low,
especially for later Thursday night and Friday. Models continue to
show a temperature profile for the central zones, particularly
Fremont County that is right on the edge of rain and snow on Friday.
Significant precipitation is a given but the snow level will be the
toughest part. The 5-wave pattern is still showing an overlying
ridge over the area that may still have some effect on this upper
low (keep temps borderline or push it a tad further south as has
been the trend in some of the guidance) but it`s still very close to
heavy wet snow for elevations above 5000 to 5500 feet in the Wind
River Basin and Lander Foothills from early Friday morning and
through the day Friday. Easier forecast for the Wind Rivers and
Green Rattlesnake ranges where we will be issuing winter storm
watches for late Thursday night through late Friday night for heavy
snow and strong ne wind. Potential for blizzard conds over the
higher parts of South Pass and Beaver Rim with 40 to 55 mph winds
developing along with heavy snow (debated a blizzard watch but went
winter storm for now). Strong north winds will also develop across
the Wind River Basin. The other area to watch will be the I80
corridor as strong ne winds develop. If the rain changes to snow,
blizzard conditions are possible on Friday. Right now, it may stay
rain or a rain snow mixture but that`s another tough area. Back up
to tonight, the leading edge of this system will spread mountain
snow into the west with a cold rain or rain and snow mixture
developing in the valleys later. Most areas look to be sub-advisory
and with warm temps, snow levels will be up into the foothills for a
while. Some of the Tetons could see low end advisory amounts but a
sps for 3-6 with local 8 seems the way to go for now. Thursday into
Thursday night our digging system strengthens and closes off over
ern NV and wrn UT Thursday evening before heading to the 4-corners
or slightly south region on Friday. Significant upslope and
isentropic upglide develops in the central zones Thursday night,
especially around Fremont County where moderate to heavy pcpn is
expected to develop and continue through much of Friday, especially
in the morning. Heavy wet snow will fall in the foothills and
mountains with a cold rain or rain and snow mixture in the 5000-
6000 foot range. We will need to watch the Lander area especially
close as if it changes to heavy wet snow early Friday, heavy
accumulations are possible. Will issue a hard hitting SPS for now.
Amounts could vary widely within a couple hundred vertical feet
of each other. Beaver Rim and South Pass will be severely impacted
with heavy snow and wind with roads likely becoming
impassable/closed by Friday morning. The eastern sections and most
of the northern sections will see rain with some mixture above
5500-6000 feet. As the coldest air arrives Friday evening, many
areas could see a brief period of light snow in the central zones
(for areas that stayed rain during the day Friday) before ending
late. Saturday will generally be dry and chilly to start the day
but with significant warming late in the afternoon, especially
across the north.

.LONG TERM...(Thursday night through Wednesday)
Issued at 1223 PM MDT Wed Mar 29 2017

SATURDAY NIGHT WILL FEATURE A CONTINUATION OF A PRECIP FREE WINDOW
AS A POSITIVELY TILTED TRANSITORY RIDGE TRACKS EAST ACROSS THE CWA.
THE NEXT IN A SERIES OF PACIFIC TROUGHS WILL MAKE LANDFALL LATER
SUNDAY AS IT DEEPENS. INSTABILITY IN NORTHWEST WYOMING AS WELL AS A
WESTERLY DIVERGENT FLOW OUT AHEAD OF THIS APPROACHING TROUGH WILL
INITIATE MOUNTAIN SNOW AND VALLEY RAIN SHOWERS IN FAR WESTERN WY
SUNDAY, SPREADING EAST INTO NORTHERN WYOMING SUNDAY NIGHT. THE GFS
TRACKS AN ASCT MID AND UPPER LEVEL LOW EAST ACROSS NORTH CENTRAL
WYOMING MONDAY AS THE TROUGH AXIS NEARS THE ROCKIES, ALBEIT, LIKE
MANY OF THE TROUGHS THAT HAVE PARADED THROUGH AS OF LATE, THE MAIN
ENERGY TRACKING WELL TO OUR SOUTH ACROSS THE 4 CORNERS AREA BY
MONDAY NIGHT. THE CLOSED LOW OVER WY WILL PROVIDE PLENTY OF COLD AIR
ALOFT WITH H7 TEMPS AROUND -10C TO RESULT IN WIDESPREAD INSTABILITY
AND SNOWFALL WITH SFC TEMPS WELL ABOVE FREEZING OVER THE AREA ALONG
WITH ISOLD THUNDER. THE ECMWF TRACKS A LESS DEFINED, BAGGIER LOW
EAST, FARTHER SOUTH ACROSS SLC WHICH WOULD TEND TO GIVE US MORE
STRATIFORM PRECIP. THE CANADIAN KEEPS THE TROUGH ENERGY EVENLY SPLIT
WITH THE NORTHERN STREAM ENERGY AFFECTING THE CWA FAIRLY
SIGNIFICANTLY. THE CANADIAN IS ALSO THE OUTLIER IN THAT IT IS 24
HOURS SLOWER THAN THE GFS/ECMWF. I SIDED WITH A COMBO OF THE GFS AND
THE ECMWF. I HAVE INDICATED NUMEROUS RAIN AND SNOW SHOWERS ACROSS
THE CWA FOR MONDAY AND MONDAY NIGHT WITH A MIXTURE OF RAIN AND SNOW
BETWEEN THE TEMPS OF 37 AND 42 DEGREES. THERE SHOULD BE QUITE A BIT
OF VARIABILITY OF SNOW ACCUMULATION FROM ONE PLACE TO ANOTHER WITH
THE NATURE OF THIS SYSTEM. TYPICAL EARLY SPRING WEATHER. THE WEATHER
SYSTEM WILL EXIT THE AREA TO THE EAST TUESDAY. THE FAR WEST VALLEYS
SHOULD REMAIN IN THE 30S DURING THE DAY MONDAY WITH HIGHS IN THE
UPPER 40S AND LOW 50S EAST. TUESDAY AND WEDNESDAY WILL BE MILDER.
WEDNESDAY WILL BE PRECIP FREE AS YET ANOTHER POSITIVELY TILTED RIDGE
BUILDS IN FROM THE WEST.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1223 PM MDT Wed Mar 29 2017

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

Mid and high- level clouds will increase this evening with some
lowering around KJAC by 06Z/Thu. Have included VCSH in the KJAC
terminal by 06Z/Thu as atmosphere begins to moisten ahead of the
next wave approaching from the west. Mountain tops will be
obscured between 06Z-00Z/Fri as moisture significantly increases
by 12Z/Thu. MVFR and local IFR conditions at KJAC just prior to
12Z/Thu and continuing through the day Thursday. Some showers will
spread to KRKS a little after 18z Thursday.

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

Conditions will be VFR through the forecast period. General trend
will be for increasing mid and high-level cloudiness during the
late afternoon and nighttime hours with thickening and lowering
clouds Thursday. Some showers will move into the KLND vcnty
around 22z.

&&

.FIRE WEATHER...
Issued AT 1223 PM MDT Wed Mar 29 2017

Snow will move into the western mountains tonight with rain or snow
developing across the western valleys and basins with low fire
danger the next several days. East of the divide, some gusty wind
will occur in the Cody Foothills tonight. Otherwise, expect a mild
day Thursday with showers developing over and near the mountains and
foothills during the afternoon. Thursday night and Friday, a large
storm system will move into the area with widespread rain and snow.
Fire danger will be quite low Friday and Saturday with and behind
this storm.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Thursday night through late Friday
night for WYZ014-015-019.

&&

$$

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


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