344
FXUS65 KRIW 010738
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
138 AM MDT Sun Nov 1 2026

.KEY MESSAGES...

- A weather system crosses the area today into tomorrow and
  brings periods of rain to many along with some mountain snow.

- Gusty to strong wind likely (greater than a 3 out of 4 chance)
  the eastern half of the area on Monday, with the strongest
  wind in Johnson and Big Horn Counties.

- More tranquil weather returns for the rest of the workweek
  except for a chance of showers Wednesday.

&&

.DISCUSSION...
Issued at 136 AM MDT Sun Nov 1 2026

The weather has cooperated for the most part for evening activities
for Halloween. We are now entering what I call the 151 days (or 152
if it is a leap year) of winter in western and central Wyoming. This
is when we usually start expecting more wintery precipitation than
rain. I say usually because the past couple of winters have been
unusually warm and dry. We will have the precipitation, and there
will be some snow, but not really in the lower elevations. Going to
try something different tonight with the discussion, I will discuss
the overall weather pattern over the next couple of days first with
the players. Then, I will focus on the three main impacts,
precipitation amounts, potential snowfall and amounts and finally
potential for strong to high wind in portions of the area.

The low that will be bringing the precipitation today is now moving
onshore in northern California and will open a trough crossing the
Rockies late tonight and into tomorrow. The latest model trends have
been downward somewhat though with QPF, with the bulk remaining to
the south over Utah, Colorado and Arizona. We will see some here
though, with the heaviest precipitation in the west through midnight
tonight, tapering off after midnight after the trough axis passes
through the area. For areas East of the Divide, although there will
be some showers around this morning, the most numerous should be
tonight through Monday morning. Lee cyclogenesis will occur over
eastern Colorado late today and then the low will strengthen as it
moves northeastward into the Northern Plains states. This will
likely bring some wrap around moisture that will keep precipitation
going somewhat longer. However, it will also introduce a dry slot
that could keep amounts down somewhere. And this complicates the
rain and snow forecast somewhat. And finally, with the deepening
flow, the pressure gradient will tighten across the area. This will
bring the likelihood of strong wind to much of the area and
potentially high wind to portions of Johnson County. These main
impacts should end Monday evening as the low moves away to the east.
Now for the individual forecast elements.

Precipitation amounts...As was stated earlier, these have trended
down somewhat with core of the moisture remaining to the south.
However, all areas should see some precipitation. The wettest areas
at this time look to be 1) Far southern Wyoming, which will be
closer to the core of the deeper moisture; 2) The western facing
slopes in the western mountains with a period of favorable
orographic flow and 3) Favored northerly upslope flow areas East of
the Divide like the Wind River Basin south of Riverton and the
Lander Foothills, around Casper, the southern end of the Bighorn
Basin and the eastern Bighorns. As a counter, the least amounts
would be in 1) far northern Wyoming, which will be away from the
core moisture and 2) Areas of downslope, like the eastern Bighorn
Basin, the northern Wind River Basin and the Green River Basin.
However, there are still model differences on the placement of the
heaviest rain and the potential dry slot. Other differences are with
the amount of wrap around, as some have it further west than others.
As for amounts, most areas have at least a 1 in 2 chance of a
quarter inch of QPF or more, with the exception of the
aforementioned downslope areas. As for greatest chance of three
quarters of an inch or more, this would be mainly in the
aforementioned upslope areas. Some areas will likely see more than
an inch of QPF through the system though. Most should end by sunset
Monday, with all precipitation finished by sunrise on Tuesday.
CONFIDENCE IN AMOUNTS...Moderate to low.

Snow...For the most part, this looks like only the mountains for
this. Snow levels remain fairly high through this afternoon, ranging
from 8000 feet in the far west to 9500 to 10000 feet in eastern
portions of the area. The cooler air then works in as flow turns
north, falling as low as minus 3 to minus 4 by Monday afternoon.
This would drop snow levels down to 7000 to as low as 6500 feet.
However, the deepest moisture would be out of the area by then.
There could be some snowflakes mixed in or possibly a brief period
of wet snow in a place like Dubois, Meeteetse or areas along US 20
26 like Hiland, but accumulation would be slim to none. Most
mountains will see at least some accumulation though. The amount we
are most concerned with is 6 inches, which is needed for most
advisories. In the western mountains, this is largely restricted to
the highest elevations, so we will not issue highlights here. The
southern Absarokas have a 1 in 2 chance, but these are in areas with
no population and no impact. The Wind River Range does but we
already have an advisory for the western Wind River Range. I thought
about extending it to the eastern Winds but will hold off for now.
The other area of concern is the Bighorn Range, mainly the northern
portion west of Buffalo and northward. There is an area of a greater
than 1 in 2 chance of over 12 inches but it is in a relatively small
area with few people of impacts. In addition, the worst impacts
would not be until after midnight tonight. So, we will hold off of
issuing highlights for now. One complicating factor is the potential
for strong wind to increase impacts, we will touch on that in the
next section. And finally, we have Casper Mountain. There could be a
period of upslope flow here, but a lot depends on how much wrap
around moisture is present. Almost all snow here would be on Monday,
so we will hold off on any potential highlights for now. There is a
1 in 2 chance of 6 inches or more above 8500 feet on the highest
portions of the mountain. Almost all snow should end by sunset
Monday.
CONFIDENCE IN AMOUNTS AND PLACEMENT...Moderate.

Wind...Wind should remain fairly light through the day today except
for the favored southwest flow areas, even here is nothing unusual
for Wyoming though. The main time of concern will be from later
tonight after the flow turns northward and the pressure gradient
tightens, especially across northern Wyoming. There will be around
12 to 14 millibar pressure gradient from the eastern border of
Wyoming to Greybull. The 700 millibar winds are also rather
impressive, increasing to 50 knots across Johnson County from 12Z to
21Z on Monday. The NBM ensemble guidance gives a greater than 4 out
of 5 chance of wind gusts past 55 mph across northern Johnson
County. With this, we will hoist a high wind watch for this area
starting at 3 am and ending at 5 pm Monday. I also gave thought to
one for Bighorn County, but with the chance less than 1 out of 2 and
the main chance in the higher elevations I held off. The day shift
will have to watch this though and a warning may be needed if the
wind speeds trend up. Most areas east of a Cody to Riverton Line as
well as portions of southwestern Wyoming have at least a 1 in 2
chance of gusts past 40 mph so most places will see a blustery and
cool day, it will definitely feel like November in Wyoming. Wind
should decrease after sunset Monday as the low moves away and the
pressure gradient decreases.
CONFIDENCE IN WIND SPEED AND TIMING...Moderate to high.

Tuesday sees a return to more tranquil weather with somewhat warmer
temperatures. The next potential maker will be a fast moving low
moving in for Tuesday night into Wednesday. There is a rather
substantial model spread on this though, with some bringing some
showers to the state Wednesday and others keeping the system south
of the area as high pressure builds in from the north. Riding should
then bring mainly dry weather for the end of next week with near to
somewhat above normal temperatures.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 948 PM MDT Sat Oct 31 2026

A wet weather system will impact the area over the next 24 hours.
Rain will increase in coverage across western portions of the area
tonight with some snow possibly mixing in at KJAC, KBPI, and KPNA
through Sunday morning. Then, rain will spread north and east Sunday
morning continuing through the afternoon. There does appear to be a
lull in rain Sunday afternoon into Sunday evening as a dry slot
pushes up into the area, but confidence is low in where this dry
slot will set up. Conditions will most certainly drop to MVFR/IFR at
all terminals during the TAF period, with prevailing MVFR/IFR
conditions at most terminals beginning Sunday morning. There is a
decent chance (up to 25%) that CIGS may drop to LIFR at terminals at
some point during the TAF period as well. For now kept all terminals
IFR as the lowest flight category.

A cold front will drop down into Wyoming from the north Sunday
evening with breezy to windy conditions behind the front. For KBPI
and KPNA wind will increase to 10-15 knots, with gusts up to 25
knots by mid-morning Sunday and subside by mid-afternoon Sunday.
Wind will be a bit breezy at KRKS beginning Sunday morning
continuing through the period. Otherwise, wind should remain around
10 knots or less through the period. Frequent mountain obscuration
will begin tonight and continue through the period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late tonight through Monday morning for
WYZ010.

Winter Weather Advisory from 8 AM this morning to 5 AM MDT
Monday for WYZ014.

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe