Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 171717
Area Forecast Discussion
National Weather Service Riverton WY
1117 AM MDT MON OCT 17 2016
All winter weather highlights were dropped for the lower elevations
as snow has ended for the most part and road conditions are
improving. Have kept them for the mountains as web cams continue to
show reduced visibility and some snow showers.
.PREV DISCUSSION... /issued 233 AM MDT MON OCT 17 2016/
SHORT TERM...Today through Wednesday night
Imagery shows a large deep broad upper level trof across the EPAC
and western CONUS. An accompanying jet of 140+ kts is currently
flowing over WY with strong upper level divergence and associated
lift. Current surface obs show front south of WY with weak to modest
high pressure across WY and MT with a lee side sfc trof across
northern and central portions of WY. Upper level induced light to
moderate precipitation, both rain and snow, is falling across
portions of central and western Wyoming ATTM.
Today, colder air aloft has arrived and snow levels have dropped to
valley floor. Heaviest precipitation/snowfall associated with this
almost entirely upper level forced system is currently coinciding
with the arrival and passage of the strongest portion of the jet.
The western mountains, including the Salt, Wyoming and Tetons
ranges, in addition to the Wind River mountains and valleys are most
favored for significant snowfall from this morning through much of
the rest of the day. A Winter Weather Advisory continues for these
locations with the Upper Green River Valley (and Star Valley) added
into the mix. The Star Valley and Salt and Wyoming Ranges have
since been upgraded to a Winter Storm Warning as this seems to be
the preferred location for heaviest snowfall this morning with up to
5 inches already on the grass by midnight. Otherwise, 6 to 12 inches
of snow accumulation mainly above 6500 feet and 2 to 8 inches in the
foothills and valleys of southwestern and west central WY with
locally higher amounts can be expected. The warned areas could see
up to a foot across the lower elevations with up to 18 inches in the
mountains. In addition to the strong jet aloft...a number of other
atmospheric conditions, such as amplified mountain waves, increasing
instability aloft, relatively warm lee side temperatures and a
tightening surface pressure gradient will come together to give
another day of high wind across the wind corridor from roughly Rock
Springs to Casper. A High Wind Warning has been issued beginning
later this this morning and continuing through the rest of today.
Tonight through Tuesday night: The stronger portion of the upper jet
moves east of the FA although the body of the jet (100+kts) will
remain flowing over much of the southern portion of Wyoming.
Available moisture will decrease some as the flow goes nearly zonal
Monday night through most of the morning Tuesday before the main
trof axis...now located over/near California... approaches/arrives
across western Wyoming Tuesday afternoon and evening with increased
moisture and dynamics producing more snowfall across the western
mountains. ATTM is appears that snowfall totals with this portion of
the system will remain below advisory levels...but worth watching
Wednesday and Wednesday night, the upper trof finally clears WY and
precip ends from west to east...with ridging aloft and drying and
warming working across the FA. Weather looks benign for the most
LONG TERM...Thursday through Monday
Upper level ridging over the Rockies will remain in place through
the weekend with dry conditions and a warming trend. Models do
show a weak impulse pushing just north of the area
Thursday/Thursday night with the ECMWF hinting at some mountain snow
showers across the north. For now will keep the forecast dry. By
Friday and Saturday there could be elevated fire weather
conditions over Natrona County as a very dry air mass should be in
place, but winds and even RH look marginal at best right now.
Medium Range models show the potential for a trough and associated
cold front impacting the region next Monday/Tuesday time frame.
For now will introduce some slight PoPs out west, but keep the
east dry and mild.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Conditions will improve as the snow coverage has ended in the
valleys. Snow showers will linger in the mountains through 00Z, then
decrease to scattered snow showers. Strong wind is expected in
southern WY Monday with wind gusts to 55 mph before decreasing after
sunset. This includes the KRKS airport where strong wind is
expected. More snow showers may move in on Tuesday but should hold
off until after 18Z.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
Strong wind will develop after 15Z as a cold front moves through the
region. Wind gusts of 45 to 55 mph are expected in some locations.
The wind will decrease after sunset. VFR conditions will prevail at
the terminal sites through 18Z Tuesday.
Fire danger low to moderate all areas today...lasting through the
beginning of the work week...as minimum RH values remain above 25
percent over those areas in central Wyoming where fuels remain
critical. That said, a vigorous upper level weather system will
continue to influence the forecast area today with good
precipitation west of the Divide and even modest chances east of the
Divide. Strong winds, however, will once again be found across
portions of southern and central Wyoming today. Winds could gust as
high as 50 to 60 mph at times with sustained winds 20 to 40 mph.
High Wind Warning from 9 AM this morning to 6 PM MDT this
evening for WYZ016>020-028>030.
Winter Weather Advisory until 6 PM MDT this evening for WYZ012-
Winter Storm Warning until 6 PM MDT this evening for WYZ014-