Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 162115
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
215 PM MST Thu Nov 16 2017

.SHORT TERM...(This evening through Sunday)
Issued at 212 PM MST Thu Nov 16 2017

Updated to add the lower elevations of Johnson County into the
high wind warning. The mountain wave affecting the Lander
Foothills has lined up with the Big Horn mtns with mountain wave
underway. Best timing looks to be through about 03z this evening.
Already gusted to 58 knots at the Buffalo airport.

Major winter storm is already underway in the far west mountains
with snowfall amounts up to 11 inches in the Tetons already. Cold
front stretches from central MT to ern ID. As this cold front,
moves in this evening the lower elevation rain will change over to
snow either later this evening or early Friday morning. Snowfall
rates will increase again along this frontal forcing and
increasing upper dynamics. As the colder trough moves in, lapse
rates steepen considerably with even some small capes of
100-200J/kg showing up Friday. Intense snow showers with even a
little lightning is quite possible Friday which would just aid in
the very heavy snow rates along with good forcing, orographics and
optimal dendritic growth zones temps. 1 to 2 inch per hour
snowfall rates are quite possible at times in the far west,
especially in the mountains. Another area to watch is the Star
Valley where very favorable wnw flow around Alpine and the
surrounding area could result in some very heavy snow on Friday
(and even some late tonight). Decided to upgrade this area to a
warning with high potential of heavy snow shower periods into at
least late Friday afternoon. In fact, most of the area between the
Tetons, Salt/Wyoming Ranges over to the Winds will likely see some
accumulating snow into 03z Saturday. Jackson Valley should also
see a period of decent snow/snow showers later tonight and Friday.
The Pinedale/Bondurant zone will likely need a highlight for
Friday. East of the divide, we have a lot of wind potential
through Friday (both pre and post-fropa). Lander as feared
yesterday is undergoing their second surge with 55 to 65 mph
winds blowing in the area. I see this potential through the
evening. Wind corridor is windy but nothing special. High wind
warning is fine but fairly localized so far. On Friday, we`ll have
to watch the I80 corridor with rain changing to snow Friday
morning and a period of heavier snow showers and 45 to 55 mph
winds by Friday afternoon. 1 to 3 inches seems reasonable across
the area with the heavier amounts east and southeast of RKS.
Another area of accumulating snow is possible along the Cody
Foothills and far northern Bighorn basin on Friday. A lot of north
to northwest wind for the nrn 2/3rds with the wly flow along I80.
Had to manually adjust the grids since most of the guidance is
too light. With the steepening lapse rates east of the divide
Friday, snow showers will likely be out and about just about
anywhere with the highest concentration in and near the higher
terrain. Friday crew will have to watch this. Gradual improvement
Friday night with a dry weekend. Chilly start Saturday, then slow
improvement Sunday with basin/valley inversions to deal with.

.LONG TERM...(Sunday night through Thursday)
Issued at 212 PM MST Thu Nov 16 2017

Major winter storm is already underway in the far west mountains
with snowfall amounts up to 11 inches in the Tetons already. Cold
front stretches from central MT to ern ID. As this cold front,
moves in this evening the lower elevation rain will change over to
snow either later this evening or early Friday morning. Snowfall
rates will increase again along this frontal forcing and
increasing upper dynamics. As the colder trough moves in, lapse
rates steepen considerably with even some small capes of
100-200J/kg showing up Friday. Intense snow showers with even a
little lightning is quite possible Friday which would just aid in
the very heavy snow rates along with good forcing, orographics and
optimal dendritic growth zones temps. 1 to 2 inch per hour
snowfall rates are quite possible at times in the far west,
especially in the mountains. Another area to watch is the Star
Valley where very favorable wnw flow around Alpine and the
surrounding area could result in some very heavy snow on Friday
(and even some late tonight). Decided to upgrade this area to a
warning with high potential of heavy snow shower periods into at
least late Friday afternoon. In fact, most of the area between the
Tetons, Salt/Wyoming Ranges over to the Winds will likely see some
accumulating snow into 03z Saturday. Jackson Valley should also
see a period of decent snow/snow showers later tonight and Friday.
The Pinedale/Bondurant zone will likely need a highlight for
Friday. East of the divide, we have a lot of wind potential
through Friday (both pre and post-fropa). Lander as feared
yesterday is undergoing their second surge with 55 to 65 mph
winds blowing in the area. I see this potential through the
evening. Wind corridor is windy but nothing special. High wind
warning is fine but fairly localized so far. On Friday, we`ll have
to watch the I80 corridor with rain changing to snow Friday
morning and a period of heavier snow showers and 45 to 55 mph
winds by Friday afternoon. 1 to 3 inches seems reasonable across
the area with the heavier amounts east and southeast of RKS.
Another area of accumulating snow is possible along the Cody
Foothills and far northern Bighorn basin on Friday. A lot of north
to northwest wind for the nrn 2/3rds with the wly flow along I80.
Had to manually adjust the grids since most of the guidance is
too light. With the steepening lapse rates east of the divide
Friday, snow showers will likely be out and about just about
anywhere with the highest concentration in and near the higher
terrain. Friday crew will have to watch this. Gradual improvement
Friday night with a dry weekend. Chilly start Saturday, then slow
improvement Sunday with basin/valley inversions to deal with.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 212 PM MST Thu Nov 16 2017

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 the rest of 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. 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. An isold lightning strike in the far west Friday afternoon
will not be out of the realm of possibilities.

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

Expect VFR conditions along with brisk veering SW to NW 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. 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 the rest of this afternoon.

&&

.FIRE WEATHER...
Issued AT 212 PM MST Thu Nov 16 2017

A major winter storm will impact the west through Friday with rain
changing to snow in the valleys and continued snow in the
mountains along with quite a bit of wind. Wind will also be a
concern east of the divide through Friday with sharply colder
temperatures behind the cold front on Friday. Scattered to
numerous snow showers will occur east of the divide on Friday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM MST this evening for WYZ019-020.

High Wind Warning until 11 PM MST this evening for WYZ018.

Winter Weather Advisory from midnight tonight to 5 PM MST Friday
for WYZ013.

Winter Storm Warning until 5 PM MST Friday for WYZ012-014-024.

High Wind Warning until 8 PM MST this evening for WYZ010-011.

Winter Weather Advisory until 5 PM MST Friday for WYZ001-002-015.

Winter Storm Warning from midnight tonight to 5 PM MST Friday
for WYZ023.

&&

$$

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


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