Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 160555
Area Forecast Discussion
National Weather Service Riverton WY
1055 PM MST Wed Feb 15 2017
.SHORT TERM...Tonight and Thursday
Dry conditions and clear skies continue today, as the upper level
ridge over the Northern Rockies and west-central Canada begins to
fold over. Gusty winds continue this afternoon across wind-prone
locations, such as the Wind Corridor and northern portions of the
Cody Foothills. The ridge axis will slowly migrate eastward
tonight into Thursday, as a Pacific storm system slams across the
Pacific Northwest. The ridge will cause this system to become
disorganized and weaken somewhat as it moves over the region.
Clouds and moisture will enter far western WY early Thursday
morning and slowly spread over central portions by Thursday
afternoon. With the clouds, there will be a slight chance for snow
in the higher elevations. Any snow that falls will be light.
Chances for snow will increase late Thursday afternoon and spread
across areas west of the Divide.
.LONG TERM...Thursday night through Wednesday night
Things look to turn more active in this period as a series of
Pacific systems move into the area. The first one will move through
on Thursday night. This one does have some left front quadrant jet
energy with it. However, it is a quick hitter with most of the snow
falling over a 6 hour period or so, at this point probably 9 pm to 3
am. At this time, that looks like too little time for any kind of
highlights to be needed. Meanwhile, quiet weather will continue for
the most part East of the Divide. There will be some breezy to windy
conditions continuing across the southwestern Wind Corridor but it
does not look to rise to the level of high wind at this point.
This system will move out by Friday morning with a lull during the
day on Friday. Both the GFS and European are dry on Friday afternoon
so we removed any precipitation for this period. The next system
then moves in from the southwest for Friday night, especially later
at night and into Saturday. This one at least initially does not
have a ton of moisture to work with so any precipitation through
Saturday looks to be fairly light. Again, areas East of the Divide
look to remain largely dry with temperatures near to somewhat above
normal. We reduced POPS on Saturday night near Casper and if trends
continue future shifts may want to remove them.
Better moisture looks to move into the west for later Saturday night
and especially on Sunday with somewhat better upper level support as
well. Some warmer air may try to move in during the day through and
bring a mix of rain and snow to the valleys, especially further
south. It should stay all snow in the mountains though. Again, not
much should jump the mountains. We maintained some slight POPS for
Sunday and Sunday night but most areas should be dry most of the
The models begin to diverge starting the first part of next week.
The latest GFS brings the next system into the west on Monday
morning. Meanwhile, the European is about 12 hours slower and holds
off until Monday night. For now, we split the difference. At this
point, we split the middle. The models also differ on timing of the
systems for Tuesday and Wednesday so for the most part we left
things alone until the details become clearer. We agree with the
previous shift that there is a potential high wind event for Monday
night and into Tuesday for the usual suspects, AKA the lee of the
Absarokas and the southwestern wind corridor.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Very patchy fog possible this morning. Otherwise the main concern
will be marginal low-level wind shear (LLWS) at KJAC terminal with
southerly flow over the normal northerly drainage wind through about
15Z. A weak system approaching the area will result in increasing
mid-high level cloudiness Thursday. Between 22Z Thursday and 04Z
Friday clouds will lower with snow breaking out over the west. The
best chance for MVFR and brief IFR conditions in snow and low
ceilings will be KJAC. Model soundings showed the possibility of
some LLWS redeveloping at KJAC in the late afternoon/early evening.
Mountains will become obscured in most areas during Thursday evening
with the least impact over South Pass.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
Patchy fog possible mainly in the river bottoms later
tonight/Thursday morning. Otherwise, breezy to windy west/southwest
winds or LLWS will be observed from South Pass to Casper, lee side
of mountain ranges and adjacent foothills. Thursday afternoon into
the evening, considerable mid-high level cloudiness will overspread
the area associated with a weak system. Considerable mid-high
cloudiness will remain through much of Thursday night with mountain
tops being obscured at times. Precipitation associated with this
system is expected to remain along and west of the Divide.
Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.
Fire Danger low for all of WY through the weekend, as a good
existing snow pack will combine with seasonal to below seasonal
temperatures. Winds begin to pick-up east of the Divide out of the
southwest, while remaining relatively light out west. Smoke
dispersion poor to briefly fair in the afternoons through
Thursday...fair to good east of the Divide Friday. Areas of light
snow to return to the region Thursday night....continuing
periodically west of the Divide through the weekend.