864
FXUS65 KRIW 200507
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1107 PM MDT Tue May 19 2026

.KEY MESSAGES...

- A clear sky allows low temperatures Tuesday night to fall into
  the 20s for many lower elevation locations. While it will be
  warmer Wednesday, high temperatures remain below normal.

- Precipitation arrives from the northwest late Wednesday night
  and becomes widespread Thursday. Cooler temperatures by
  Thursday evening could lead to a period of light snow in
  upslope-favored areas around Lander and Beaver Rim.

- Forecast trends continue to favor widespread high temperatures
  in the 70s to mid 80s for the three-day Memorial Day holiday
  weekend. Other than a few late day showers, conditions look
  dry each day.

&&

.UPDATE...
Issued at 1234 PM MDT Tue May 19 2026

Snow from yesterday is clear on satellite today, with a large
portion of central and southern areas white. Melting will occur
through the next couple days as daytime highs slowly increase.
However, temperatures do remain on the cool side the next few days.
As a result, Freeze Warnings are once again in effect for portions
of the area that have seen an early start to the growing season.
Lows tonight should be in the upper-20s to lower-30s east of
the Divide, where the warnings currently are.

&&

.DISCUSSION...
Issued at 145 AM MDT Tue May 19 2026

Areas of clouds still linger over central and southern portions of
the CWA this morning, as expected. Low clouds have developed in the
Upper Green River Basin, in the wake of the residual snow that fell
through Monday evening. Low clouds/fog is also expected to develop
near Pathfinder Reservoir in southern Natrona County as well as far
eastern Sweetwater County near Wamsutter and Creston Junction. These
clouds are expected to last through mid-morning. Travelers on I-80
and US-189/191 should exercise caution due to the reduced
visibility. Otherwise, clouds will continue to scatter out
through the rest of the morning with partly cloudy skies by 18Z.
There is a 20-30% chance for fog/low clouds to develop in the
Wind River Basin and areas near Casper. Cumulus clouds will
begin to develop over the western mountains after 18Z, with
isolated virga showers possible over northern portions of
Yellowstone NP, the Cody Foothills, Hoback Junction and
Lander/Fort Washakie. Temperatures will be about 10 to 20
degrees below normal today, with sub-freezing temperatures
expected again tonight for most areas except northern portions
of the Bighorn Basin.

The warming trend continues Wednesday, but temperatures stay
below normal for central and southern portions of the CWA.
Conditions will be dry for most areas, with showers possible
over far northern portions irt the next storm system approaching
the Cowboy State from the north. Precipitation chances will be
confined to northwestern portions Wednesday night, with snow
levels between 7000 and 8000 ft, as the storm drops southward.
The associated cold front will drop southward over the CWA
through the overnight hours, extending from Buffalo to Kemmerer
by sunrise Thursday morning. The front will then slowly push
through Natrona and Sweetwater counties through the rest of the
day. Most of the precipitation with this storm will occur along
and behind of the front, being widespread across the CWA through
the day Thursday. Rain could change over to snow along and
north of a Meeteetse-to-Lovell line after 15Z, as the cold core
from the storm pushes into the Bighorn Basin. The challenge will
be if snowfall rates will be heavy enough to overcome the high
May sun angle. Otherwise, any snow is likely to stay in grassy
areas through the day. There could be a similar change over
later in the afternoon at Lander. Both of these locations
(mainly Cody and Lander) will have a good upslope component with
north- northeast winds. There could be a 3 to 5 hour window for
snow accumulation between 23Z and 04Z (5pm to 10pm), as the sun
begins to wane in the evening. A leeside low will develop over
the Black Hills around this time, generating a line of showers
and thunderstorms over western portions of SD and NE. Drier air
will quickly push in over western portions of the CWA as the
storm moves over the aforementioned areas through the rest of
the night into Friday morning. Precipitation will quickly end
from west to east after 03Z as a result of the storm becoming
better organized over SD/NE. However, precipitation looks to
linger over Johnson and Natrona counties through the rest of the
night. Rain or a rain/snow mix is likely at this time, with
snow levels as low as 6000 ft. Initial snowfall amounts around
6 inches are forecasted over the Absaroka and Wind River
Mountains and 6 to 10 inches over the northern half of the
Bighorns.

Temperatures rebound more quickly on Friday, staying at or slightly
below normal as the storm quickly exits over the Plains. A pleasant
holiday weekend is shaping up, with highs in the 70s and 80s by
Memorial Day.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1042 PM MDT Tue May 19 2026

West of the Divide...KJAC/KRKS/KPNA/KBPI Terminals

Northwest flow aloft backs to the west as a shortwave drops south
toward the forecast area late Wednesday afternoon. Weak showers
develop over the mountains 19Z-21Z/Wednesday and move east into the
basins and valleys around KJAC and KBPI. Any impacts would be brief
and confidence is not high enough to include PROB30, with KJAC being
the most likely terminal. Shortwave energy and deeper moisture
reaches northwest Wyoming around KJAC late in the period, so a
PROB30 group has been added beginning at 04Z/Thursday. In general,
cloud cover increases and begins to slowly lower after 00Z/Thursday.
Wednesday afternoon westerly surface wind 10-18kts decreases to 7-
10kts around 02Z/Thursday. Mountain tops occasionally obscured after
00Z/Thursday over northwest Wyoming.

East of the Divide...KCPR/KCOD/KRIW/KLND/KWRL Terminals

All terminals to be VFR through the forecast period. Weak showers
develop over the Absaroka and Wind River Ranges 19Z-21Z/Wednesday
and slowly track east into the basins during the afternoon. Impacts
are likely to be confined to gusty outflow wind 25-35kts, with
convection around KCOD the most likely to produce these gusts.
Otherwise, KCPR to see 10-18kt westerly wind during the day with
occasional late day westerly wind 9-13kts at KRIW and KLND. The
arrival of more robust shortwave energy in northwest Wyoming leads
to a better chance of light rain showers at KCOD late in the period.
This shortwave will begin to push a cold front accompanied by
northerly surface wind through the Bighorn Basin after about
03Z/Thursday. Mountain tops obscured after 00Z/Thursday in the
Absaroka Range.

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

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 8 AM MDT Wednesday for WYZ005-006-011-
016>020.

&&

$$

UPDATE...Wittmann
DISCUSSION...LaVoie
AVIATION...CNJ/LaVoie