Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 161744

Area Forecast Discussion
National Weather Service Riverton WY
1144 AM MDT SUN OCT 16 2016

.SHORT TERM...Today through Tuesday night

Imagery shows a large deep broad upper level trough across the EPAC
and western CONUS. An accompanying jet of 120+ kts will also arrive
by this morning along with decent upper level divergence and
associated lift. Current surface obs show a strong north south
oriented Pacific cold front over the western CONUS with a weak
stationary front lying against the spine of the Divide across
Wyoming. Light precipitation (snow above 10000 feet) falling across
portions of western Wyoming.

Today through Monday: Modest moisture and relatively warm
temperatures this morning and through much of the day today will
keep accumulating snow levels on the high side mainly above 9500
feet until this evening when colder air aloft arrives and snow
levels drop to valley floor level overnight. Heaviest
precipitation/snowfall coincide with the arrival of the nose of the
jet and slightly better moisture. The western mountains, including
the Salt, Wyoming and Tetons ranges, in addition to the Wind River
mountains and Upper Green River foothills will be the areas most
favored for significant snowfall from Sunday night through the day
on Monday. A Winter Weather Advisory has been issued for these
locations and over that time period reflecting 6 to 12 inches of
snow accumulation mainly above 8000 feet in the mountains and 2 to 6
inches in the foothills of the Upper Green River. Additionally this
morning...a number of atmospheric conditions, such as amplified
mountain waves, increasing flow aloft, warm surface lee side
temperatures and a tightening surface pressure gradient come together
to give high wind at times over the foothills of the region and
across the wind corridor. A High Wind Warning has been
continued/issued through this morning. High wind will again be
possible on Monday across the foothills and the wind corridor...but
will not issue a watch at this point as may be confusing with
current Warning.

Monday night 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 trough axis approaches/arrives across western Wyoming Tuesday
afternoon and evening with increased moisture and dynamics producing
more snowfall across the western mountains. At this time is appears that
snowfall totals with this portion of the system will remain below
advisory levels...but worth watching and monitoring.

.LONG TERM...Wednesday through Sunday

Synopsis...Cool and unsettled conditions will remain over the area
Wednesday as an upper trough slowly exits east.  Isolated to
scattered rain and snow showers over the north and central will
diminish Wednesday evening.  A ridge of high pressure will bring dry
weather with a warming trend Thursday and Friday.  A mainly dry
cold front will bring a slight cooling trend late next weekend.

Discussion...Synoptic pattern at the beginning of this period
consists of deepening upper low over the Gulf of Alaska, downstream
ridging over the Intermountain West and upper trough over the high
plains.  GFS remains slower than ECMWF with progression of upper
trough into the plains Wednesday with ECMWF trending slower - toward
GFS over the last few runs. Best instability across the eastern
third of Wyoming on Wednesday with rain and snow showers expected
to be most numerous across Johnson and Natrona counties. Cold air
aloft (-6c at H7, -24c at H5) will likely result in some snow
pellet showers across the lower elevations.

Broad ridge over the Rockies and southwest U.S. will bring
temperatures into above normal range across the area Friday and
Saturday. Both GFS and ECMWF show a fast moving shortwave along the
U.S./Canadian border Saturday/Saturday night which looks to be
mainly dry for our area, possibly kicking winds across the wind
corridors Saturday, and pushing a dry Canadian cold front south
across the area Saturday night and Sunday.




VFR conditions will occur most the rest of the terminal sites this
afternoon, with some MVFR conditions at KRKS. Scattered showers will
occur through the afternoon.After 00Z, rain and mountain snow will
move back into the west with MVFR conditions prevailing by 06Z
Monday with possibly IFR around KPNA. Strong  wind aloft over the
mountains will cause considerable mountain turbulence through the
period. Conditions should improve later Monday morning after the
frontal passage.


Breezy to windy conditions along with mountain turbulence today. Most
of the shower activity will remain along and west of the Divide
today. Mountain tops will be obscured at times due  to this activity.
Otherwise VFR conditions will prevail today and most of tonight.
The wind will decrease after 00Z in most locations. Some showers and
local MVFR conditions will be possible later tonight at KRIW and
KLND as a front approaches.



Fire danger low to moderate all areas today...lasting through the
beginning of the work 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
begin to influence the forecast area today with increasing
precipitation west of the Divide and strong winds east of the Divide
today and Monday in the foothills and across the wind corridor
between South Pass and Casper. Winds could gust as high as 50 to 60
mph at times.


.RIW Watches/Warnings/Advisories...

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Monday for WYZ012-014-024-025.



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