Hoback Junction, Wyoming Weather Forecast Discussion

280
FXUS65 KRIW 021741
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1141 AM MDT Thu Oct 2 2025
.KEY MESSAGES...
- Stronger thunderstorms possible in western Wyoming Friday;
  with elevated fire weather East of the Divide.
- Much cooler and wetter this weekend with periods of rain in
  the lower elevations and snow in the mountains.
- There is at least a 1 in 2 chance of sub freezing temperatures
  across the central basins Sunday night.
&&
.DISCUSSION...
Issued at 312 AM MDT Thu Oct 2 2025
We have now entered October. The transition into fall is gradual at
first, then a bit more abrupt. In September, I usually use the
phrase "one step forward, two steps back." As we head into October,
it is more "one step forward, three steps back." And this will
especially be the case this weekend, when we have a decent
likelihood of seeing the first decent snow in the mountains.
Concerns are few today though, as most of the area will be under
control of ridging over the western Plains. Radar does show some
moisture over Idaho, but most of this should remain to the west of
the state. There could be an isolated shower over western Wyoming,
but the chance is at most 1 out of 5 with most locations seeing
nothing. With the southwest flow, temperatures will remain above
normal, as much as 15 degrees in some locations today.
The transition will begin on Friday as a deep trough moves onto the
Pacific coast and moves towards the Northern Rockies. Most
locations will have one more warm day on Friday, with highs well
into the 80s in some locations. There will be two threats, one
on each side of the Divide. In areas east of the Divide, the
threat will be strong to potentially a few severe thunderstorms.
There are some factors for and against though. There will be
some decent upper level divergence. Also, as the trough and
upper level low approach, models are showing a decent amount of
shear across the west, with up to 35 knots of speed shear
between 700 and 500 millibars and decent directional shear as
well. There will a 90 knot jet streak moving over the area as
well. However, instability parameters are not as impressive.
Lifted indices only fall as low as minus 2 and surface CAPE
generally maxes out at around 600 J/Kg. My opinion at this time
is that we will see a few stronger storms, possibly an isolated
severe storm, but they should not be widespread. As we shift to
areas east of the Divide, the main threat looks to be elevated
to possibly critical fire weather. The pressure gradient will be
tightening ahead of the approaching trough and cold front, with
700 millibar wind increasing to as high as 35 knots. Ensemble
guidance gives at least a 1 in 2 chance of wind gusts over 40
mph across the favored Wind Corridor, from eastern Sweetwater
County to southern Johnson County. The chance of wind gusts past
55 mph is at most 1 out of 4 and largely confined to the
highest elevations of the Green Mountains, so wind highlights
should not be needed. As for humidity, it is close. The ensemble
mean bottoms out as low as 17 percent across the aforementioned
area. With most guidance not showing an extended period of sub
15 percent humidity, we have elected to forgo any Fire Weather
Watches for now. If things trend down, the day shift can issue
watches this afternoon.
The abrupt change will begin later Friday night as the cold front
moves into western Wyoming. The 700 millibar temperatures will fall
from 5 Celsius or higher on 00Z Friday to around 1 Celsius by
Saturday morning and as low as minus 2 by Saturday evening. That
could bring snow levels as low as 7500 feet in western Wyoming
by this time. At the same time, rain will spread into central
Wyoming. Most guidance is indicating the heaviest precipitation
across northern Wyoming, which will be closest proximity to the
upper level low. Much of the northern half of the state has at
least a 4 out of 5 chance of over a quarter of an inch of QPF
from Saturday through Sunday night, with much of the same area
having at least a 1 in 2 chance of over a half an inch of QPF
over the same period. The precipitation looks more steady than
convective though. And, given the mainly dry weather over the
past week, the risk of flooding looks low at this time. The
heaviest precipitation looks to fall on Saturday as the low
moves over the area. The first low will be followed by a second
system, this one with origins in Canada. As a result, this one
will be colder, dropping 700 millibar temperatures down to minus
4 Celsius by Sunday morning, which could lower snow levels as
low as 6500 feet. Ensemble guidance is giving a greater than 3
in 5 chance of at least 6 inches of snow over the northern and
central mountains above 8000 feet through Monday morning, with
some location above 10000 feet having as much as a 1 in 2 chance
of a foot of snow or more. With much of the snow occurring at
night, this could be the first snow that impacts travel over the
mountain passes. It is too early for any highlights, but at
this point, I give it at least a 1 in 2 chance of some winter
weather highlights starting sometime Saturday and continuing
through Sunday. Snow levels may even fall to around 5500 feet at
times Sunday morning across northern Wyoming as well. And this
could bring some snow flurries or maybe even a coating of snow
in places like Dubois, Meeteetse and even Cody. High
temperatures will be a slap in the face as well, with some
locations over 30 degrees colder on Sunday when compared to
Thursday and Friday.
And this leads to the final question. How quick this second system
moves out. There are differences, with the ECMWF model around
18 to 24 hours slower than the GFS. And this brings the question
of a possible freeze on Sunday night. Many areas have at least
a 1 in 2 chance of temperatures below 32 degrees on Sunday
night, although this could be mitigated if clouds hang around.
Again, too early for any highlights, but this has the chance of
being a widespread frost or freeze for much of the area.
Following this, things look more tranquil starting on Tuesday as
ridging builds back over the area, bring moderating
temperatures and mainly dry conditions.
&&
.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1140 AM MDT Thu Oct 2 2025
Dry and VFR flight conditions are anticipated to prevail this TAF
period. Winds have already increased to 15-20kts from the southwest
at KCPR. At most other terminals, southwest winds increase above
10kts between 19Z-21Z. The exception is KPNA where winds are
forecast to stay just below 10kts. KRIW, KLND, KRKS, and KBPI have
the greatest chances (80-90%) of seeing frequent gusts around 20kts
this afternoon. Other than winds staying at or above 10kts at KRKS
and KLND overnight, winds generally diminish below 10kts between 00Z-
01Z at all other terminals.
Anticipate mid to high level clouds to increase across northwestern
Wyoming this evening and overnight. Winds are expected to increase
and become gusty again towards the end of this TAF period,
especially at KCPR. Other terminals become gusty after this TAF
period.
Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence
forecasts.
&&
.FIRE WEATHER...
Issued at 312 AM MDT Thu Oct 2 2025
Fire weather concerns remain low today. On Friday afternoon,
elevated to possibly critical fire weather looks likely as wind
increases ahead of an approaching weather system, with wind
gusts past 40 mph possible from eastern Sweetwater County
through southern Johnson County. Humidity looks borderline at
this time, with most locations seeing humidity in the
aforementioned area under 20 percent but likely remaining above
15 percent. Concerns should end on Saturday as cooler and wetter
weather moves into much of western and central Wyoming. There is
a high likelihood for wetting rain for most areas Saturday.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Hattings
AVIATION...Gerhardt
FIRE WEATHER...Hattings

NWS RIW Office Area Forecast Discussion