Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 291004
Area Forecast Discussion
National Weather Service Riverton WY
304 AM MST Tue Nov 29 2016
.SHORT TERM...Tuesday through Thursday night
We still have some snow showers in areas east of the divide mainly
from around a Greybull to Lander line and east associated with some
wrap around moisture from a low over the Dakotas. These should
slowly decrease through the morning. The main concern today will be
wind. The high wind warning in northern Johnson County has already
verified with a wind gust to 59 mph. With 700 millibar winds still
strengthening into the morning, these wind gusts are expected to
continue before diminishing late today. Other areas like the Bighorn
and Wind River Basins will also experience gusty winds but should
remain below high wind criteria. These winds should decrease late
today and tonight as mid level winds decrease as the low over the
northern plains moves further away.
Meanwhile, out west things are mainly quiet with snow showers having
dissipated as flow turned to the west. There has been some fog in the
Jackson Valley this evening and we have added patchy fog to the
western Valleys this morning. As for precipitation out west, there
should only be some orographic snow showers and these should not
amount to much. And these should decrease in coverage tonight with
most areas being dry. Temperatures will likely be cooler than recent
days as well across all the area.
Another trough will move toward the west on Wednesday. This one at
this point looks fairly weak with not a lot of moisture associated
with it so any snowfall amounts are expected to minor at this point
through Wednesday night. Meanwhile, mainly dry and cool conditions
will be the rule East of the Divide with a ridge of high pressure
over the area. There will be the usual gusty breeze across the
southwestern wind corridor, but nothing unusual.
This trough will move across the divide on Thursday. Snow showers
are expected to continue across the west but again not a ton of
accumulation is expected. A few snow showers may cross the divide
east of the divide. The interesting part is Thursday night and there
is still some uncertainty. A weak upper level low is expected to
close off over eastern Utah and southwestern Wyoming. However, the
models do have some placement differences and this could make a
difference. Snow lovers from Lander to Natrona County will like the
GFS which puts the low in a sweet spot for some upslope flow and
some snow Thursday night. There is not a lot of moisture with this
system, so amounts should not be extreme. There could be low end
advisory amounts though. The NAM is drier and faster with only a few
flurries. For now, we kept the wetter solution.
.LONG TERM...Friday through Tuesday
Synopsis...An upper low pressure system over the northern Great
Basin Friday morning will drift southward, so some lingering light
snow across the southwest and west central will taper off in the
morning. Strengthening westerly flow will bring increasing snow
across the northwest mountains over the weekend, along with the
potential for high wind along the Cody Foothills Friday night into
An Arctic cold front is expected to plunge south across the area
early next week, with frigid temperatures settling in by Tuesday.
Discussion...Synoptic pattern at the outset Friday morning consists
of upper ridge over the eastern Pacific centered at 140W with
positive-tilted trough from NW Mexico into the northern High Plains.
Closed H5 low over the northern Great Basin will sink SSW across
Arizona into NW Mexico, before slowly progressing eastward over the
weekend. Strong 170+ knot anticyclonic jet streak is expected to
plow across the NW U.S. Friday into the northern Rockies Friday
night. This will be a favorable setup for high wind and mountain
wave activity along the Cody Foothills south to the Dubois area.
Moist westerly flow will also likely bring a sustained period of
light to moderate snow across the northwest mountains, Jackson Hole
and northern Star Valleys over the weekend.
Arctic airmass with source region mostly from eastern Siberia is
expected plunge southward across the Canadian Rockies on Sunday and
into Wyoming by Tuesday. GFS is faster with Arctic front moving
south across the area Sunday night/Monday, while ECMWF is around 12
hours slower. Both models have trended a bit higher with 1000-500mb
thicknesses (not quite as cold) Monday and Tuesday. As a first
modest guess, forecast brings the sharp cooling trend in on Tuesday
with highs mostly in the teens across the basins and valleys with
single digit to sub-zero lows Tuesday night.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Snow showers over and near the mountains will continue through
tonight. Expect some mountain obscuration along with MVFR to
IFR conditions in the vicinity of the mountains. The terminal sites
will maintain VFR conditions through the forecast period.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
VFR conditions will occur at the terminal sites through the period.
A gusty wind will be common in northern WY through this
afternoon. The wind will gradually decrease this evening. Some snow
showers will occur in the Bighorn mountains through 18Z, then become
isolated snow showers after 18Z. Expect mountain obscuration at
times with MVFR/IFR conditions through 21Z. Snow showers will be
ending after 01Z Wed.
A few snow showers will continue West of the Divide otherwise expect
a mainly dry day with some valley inversions and patchy morning fog
and light to moderate winds. There will be gusty north to northwest
winds in many areas East of the Divide with high winds likely over
northern Johnson County. Snow showers will be possible from around
the Bighorn Range and east but little accumulation is expected
except in the mountains. Mixing and smoke dispersal will be
generally fair to poor West of the Divide and good East of the
High Wind Warning until 5 PM MST this afternoon for WYZ010.