Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 020604

Area Forecast Discussion
National Weather Service Riverton WY
1102 PM MST Thu Dec 1 2016

.SHORT TERM...Tonight Through Sunday

Multi-pieced upper low/trough is moving across the nw. One vort max
is moving through about the Dubois area attm with elongated band of
weak upglide lined up north-south ahead of it with lowering mid
clouds and higher terrain flurries. More convective elements are
seen developing behind it across the far west. Wrn part of this
trough is moving into far wrn ID and most of that energy will stay
just southwest of us but may have some influence in the far swrn
corner later tonight into Friday morning. The nrn piece over wrn MT
will likely weaken as it drops south and does not actually clear
the area until Friday evening. Might still see flurries/very light
snow along its path later tonight and Friday morning. May be a
little over the higher peaks through the day Friday. Behind our
Dubois shortwave, we start to see some weak upslope with potential
for lowering clouds and a brief couple hour period of light snow.
Best bet still looks to be around Lander but even the higher
terrain between Beaver Rim/Jeffrey City and Casper Mtn will see
some weak upslope develop. Nothing significant but still a
challenging forecast with all the moving parts of this fragmented
trough. Not much break then until upglide returns to the nw later
Friday night and spreads swd during the day Saturday with
widespread light snow or flurries. Several inches of snow will
likely fall again in the mtns, especially around the Tetons and sw
part of YNP. Gap flow signature increases significantly along the
Cody Foothills Friday night into Saturday morning with threat of
strong to possibly high winds. H7 thermal gradient increases to as
much as 12-14C in the wrf. Mtn top flow is still rather weak
(20-25kts) but there is also some lee troughing. WRFNMM is hitting
some high winds in the two favored gap flow areas but I`m not
confident enough now to hoist any watches with the overall
unfavorable bigger picture. Could still happen and I expect some
stronger winds but I`ll let the next crew look at it again and
handle with a SPS for now. The flow becomes even more favorable
Saturday night into Sunday with stronger mtn flow, better lee side
troughing, and approaching right front quad of upper jet. Windy
period for sure with threat of high winds. In the west, flow
flattens and strengthens Saturday night into Sunday with upglide
signature strengthening across the west later Saturday night into
Sunday with potential for significant snow and wind across the
west, especially by Sunday morning (maybe earlier in the nw).
Approaching jet will also be sagging swd providing better support
by Sunday afternoon. Winds will increase across many areas east of
the divide with strengthening upper low over ern MT ahead of
approaching cold front just beyond the short term period.

.LONG TERM...Sunday Night Through Thursday

Much colder temperatures will be the main story of the long term
portion of the forecast. Appears that the cold air will arrive
Monday with daytime highs probably occurring in the morning.
Ongoing snowfall in the west will continue through the day on
Monday with light snow spreading east. Increased the chance of
snow across the northern Big Horn Basin and Johnson County Monday
afternoon, but would not be surprised if these need to be
increased as we get a better handle on this weather system. Global
models continue to show a broad swath of snow across south central
and southeast Wyoming Monday night. Have adjusted snow chances
upward from model blends and have meshed well with neighboring
WFOs. Both the GEM and the 00Z/EC runs show a much deeper,
cyclonic flow across the northern Rockies and Intermountain West
through Wednesday. Cold air will continue to invade from the north
with by far the coldest readings of the season expected Tuesday
and Wednesday. High temps are likely to be 20-30F below normal
with highs only in the teens. Model forecasts have too warm of air
in the basins and plains Tuesday through Wednesday and have
adjusted downward. Overall, snow chances will be low both days.
However, if this changes and there is a blanket of snow on the
ground, even these adjusted temperatures will be too warm. A
better chance of snow returns to areas along and west of the
Continental Divide on Thursday as trough pushes east and upper
level flow gradually backs to the west. This will usher in another
round of Pacific moisture, especially across Teton County and
Yellowstone National Park. Warmer air will begin to replace the
cold air associated with the trough, so some temperature
moderation is anticipated Thursday.




Through mid-morning areas of light snow/flurries with MVFR to Low
VFR ceilings are expected from LND to CPR with an emphasis around
Casper. Clouds are expected to scatter out between 17Z and 21Z
Friday with SKC in most areas by 00Z Saturday. Winds will be light
through 00Z Saturday. An approaching system Friday night will result
in increasing mountain top winds and lee side cirrus. After 06Z
Saturday...some low level wind shear looks likely at KCOD and could
occur at other places even at KCPR but will only include it at KCOD
for now.


Stratocumulus with ceilings between 025-050 KFT AGL will be common
overnight into Friday morning with some flurries. There is a
possibility some areas over the far west could see decreasing
cloudiness around sunrise which could lead to some fog formation,
but the more likely scenario will be stratocu deck remaining in far
western valleys. Clouds should scatter out to even becoming SKC
between 18Z Friday and 00Z Saturday. Friday night, increasing
mountain top winds with mid-high level cloudiness overspreading the
area. There could be even some light snow/flurries after 06Z
Saturday north of Jackson terminal mainly impacting Yellowstone.



Upper level disturbance will pass slowly southeast across the area
tonight and Friday with considerable clouds and scattered areas of
very light snow or flurries, mainly tonight into Friday morning.
Another system will spread another round of light snow to the far
west later Friday night and Saturday. Strong west wind will develop
along the Cody Foothills Friday night and continue at times over the
weekend. Significant snow and wind may hit the far west later
Saturday night through Sunday with increasing wind for many areas
east of the divide Sunday.&&




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