Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 161824 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1124 AM MST Thu Nov 16 2017

.SHORT TERM...Thursday through Saturday night

Again, with the contrast in weather, we will split the discussion
into West and East of the Divide.

West of the Divide...Snow has broken out with the initial surge of
moisture across the western mountains. Interesting problem occurring
this evening. Web cams at the town of Jackson show mainly rain
while it has been snowing at the Jackson Hole airport, about 350
feet higher. The 00Z GFS shows 700 millibar temperatures at right
around minus 4C at 6Z, which is just about right. It also shows
warmer air working in overnight as does the NAM so we should see a
gradual mix with and change to rain through the night to around
7000 to 7500 feet by morning. Valleys still look to be mainly rain
through the day and even the evening. Periods of snow will
continue across the mountains however, although both the NAM and
GFS show a bit of a lull from later this morning into the
afternoon. We went ahead and upgraded the watches to warnings for
the Salt and Wyoming range as well as the West Slopes of the Wind
River Range, although the heaviest snows would likely stay above
pass level. Still some question about Star Valley, and since snow
would not start until later tonight, we will punt that decision
back to the day crew.

The heaviest snow still looks to be tonight, especially after
midnight as the cold front passes through and drops snow levels down
to the valley floors. The best dynamics will move through at this
time as well so snowfalls rates of over an inch an hour could occur
and really cause some travel problems. Still some question about
Friday though. There will still be unstable northwest flow so some
snow will likely continue. The question will be where and since the
snow will more of the banded variety it is very difficult to
pinpoint exactly where this will occur. Of greater concern is what
may happen along the Interstate 80 Corridor on Friday. Both the NAM
and GFS continue to show a decent area of precipitation developing
in the left front quadrant of an 140 knot jet streak that will be
moving across Colorado. This will be occurring during a period of
falling temperatures as well. The question is will the heaviest
precipitation coincide when the cold air moves in. Something to
watch as I-80 could get slick for a while. Conditions should then
gradually improve late Friday and Friday night as the system
moves away to the east. At this point Saturday looks mainly dry
but cool, but nothing unusual for this time of year.

East of the Divide...Here the main concern shifts to wind. Satellite
imagery early tonight showed some drying over the Wind River Range
so we upgraded the high wind watch to a warning. It has not
materialized yet, but this can develop quickly so we will let it
ride. Across the Absarokas and Cody Foothills, the models continue
to show up to 65 knots at 700 millibars so we will leave the high
wind warning in that area as is as well. Winds have begun to
increase with the strongest winds likely occurring around sunrise.
We also upgraded the high wind watch across the Green Mountains and
Natrona County to a warning. the Camp Creek RAWS and Wyoming
Boulevard have already seen wind gusts to around 50 mph and these
should increase through the day as 50 to 55 knot 700 millibar winds
mix to the surface as the front approaches and mixing increases.
Many areas will see a gusty breeze though, especially this
afternoon as the pressure gradient tightens ahead of the
approaching cold front. Most of the high wind threat should end
after sunset tonight. As for temperatures, it will be a rather
mild day for most areas with good mixing and downsloping flow.
Highs in the 60s will be possible in some of the windier areas. As
for snow, there will be some in the mountains where a Winter
Weather Advisory has been issued, although most of the snow will
be away from the passes. Snow will spread into the Big Horns
tonight as well, although at this point amounts will likely be sub
advisory.

Friday looks to be a blustery and cooler day with some showers
around. The problem here is that for most areas the precipitation
will be of the banded variety and rather difficult to nail down
exactly where it will pop up. We do agree with the previous shift
that the best chance would be across the far north and in some of
the normal convergence areas. High temperatures will be tricky in
some areas however depending on the timing of the frontal passage.
All in all, it will be a slap in the face compared to the mild
weather we have experienced over the past few days. All
precipitation should end on Friday night. At this point, Saturday
looks dry and temperatures close to seasonal averages along with
less wind.

.LONG TERM...Sunday through Thursday
Issued at 230 AM MST Thu Nov 16 2017

Dry weather is expected Sunday and Sunday night with breezy areas
in the central wind corridor through Sunday night. Moisture
returns to Western WY Monday and Monday night with some snow in
the mountains and light snow in the far west valleys. A NW flow
aloft continues Tuesday and Wednesday with some moisture in the
flow. This would lead to periods of light snow for the mountains.
The valleys would see light snow or rain at times Tue and Wed.
After some drying in the west next Wed night and Thursday morning,
the long range models hint at precipitation developing next Thurs
afternoon. Plenty of time to watch this potential weather
scenario play out. For areas east of the divide, the weather looks
dry Sunday through next Thursday. The northern Bighorn mountains
may see some light snow at times Mon and Tue in that moist NW flow
pattern. High temperatures look to be near to slightly above
normal through the period with 40s to lower 50s and 30s to lower
40s in the western valleys. Low temps about normal with teens and
20s for the most part.

&&

.AVIATION...18Z Issuance

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

A potent southwesterly stream of moisture will keep rain and snow
falling in the valleys through today along with snow, heavy at
times, in the mountains along with obscured terrain. Rain and snow
will turn to all snow in the western valleys after 08z tonight. Low
cigs and vsbys due to precip will result in MVFR/IFR conditions
continuing across Western Wyoming through 12Z Friday. After 12Z
Friday, expect more localized areas of MVFR/IFR conditions behind
the strong Pacific cold front when heavy snow showers stream
across the area along and behind the front within the veering
northwest flow behind the front. Alpine will be especially
affected by these snow showers as will the higher terrain. Expect
the lowest cigs/vsbys at KJAC followed by KPNA, and then KBPI
through the period. KRKS will experience west southwest winds at
20 to 30 knots through the period.along with occasional MVFR
conditions in rain showers through the period.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Expect VFR conditions along with brisk veering winds through 18Z
Friday. A strong Pacific cold front will sweep the snowfall
obscuring the Continental Divide over to the east side of the
Divide, resulting in isolated shower activity and isold MVFR
conditions later tonight and Friday morning. KLND can expect
occasional southwest winds of 35 to 45 knots through 06z tonight.
KCPR will see 23033g43 knots through 02Z. LLWS is expected around
both KCOD and KLND through today.

&&

.FIRE WEATHER...

A Pacific system will bring mountain snow and valley rain to areas
West of the Divide today, with snow levels lowering to the valley
floors tonight. East of the Divide, expect a dry, mild and breezy to
windy day with high winds possible in the lee of the Absarokas and
from the Red Desert through Natrona County. However, relative
humidity should remain above critical levels. Mixing and smoke
dispersal will be range from poor in the west to very good from
eastern Sweetwater County through Natrona County.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 AM early this morning to 5 PM MST
Friday for WYZ002-015.

Winter Storm Watch from late tonight through Friday afternoon
for WYZ023.

High Wind Warning from 9 AM this morning to 6 PM MST this
evening for WYZ019-020.

High Wind Warning until 7 AM MST this morning for WYZ018.

Winter Storm Warning from 5 AM early this morning to 5 PM MST
Friday for WYZ014-024.

Winter Storm Warning until 5 PM MST Friday for WYZ012.

Winter Weather Advisory until 5 PM MST Friday for WYZ001.

High Wind Warning until 11 AM MST this morning for WYZ003.

&&

$$

SHORT TERM...Hattings
LONG TERM...Ross
AVIATION...Lipson
FIRE WEATHER...Hattings



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