957
FXUS65 KRIW 302135
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
335 PM MDT Mon Jun 30 2025

.KEY MESSAGES...

- Mostly dry and warm through Tuesday. An isolated virga
  shower/dry thunderstorm is possible Tuesday afternoon,
  capable of producing 40-60 mph outflows.

- Shower and thunderstorm coverage increases Wednesday onward.
  Locally heavy rainfall is possible in a stronger thunderstorm
  on Wednesday. Localized flash flooding concerns increase on
  Thursday.

- Hottest days this week are Tuesday and Wednesday, with
  widespread highs in the upper 80s and 90s.

&&

.UPDATE...
Issued at 1230 PM MDT Mon Jun 30 2025

Current (18Z) satellite water vapor imagery shows the upper-level
ridge slowly building into the region. This relatively weak flow
aloft combined with high pressure at the surface is aiding in
the quiet weather and seasonable temperatures today.

The forecast remains on track with the aforementioned upper-level
ridge shifting east through Wednesday just as a deepening upper-
level low, currently evident on water vapor imagery off the
California Coast, begins to move onshore. This pattern enables the
Intermountain West to tap into some mid-level Gulf moisture bringing
isolated shower and thunderstorm potential Tuesday. Coverage is
expected to be limited with weak support aloft and dry low-levels
(thanks to hot temperatures and low dewpoints). Dewpoint depressions
are still forecast to range from 40-60F, especially across southern
Wyoming. With these large dewpoint depressions, any shower or
thunderstorm will be capable of 40-60 mph dry downdrafts. With
greater moisture on Wednesday, a slight uptick in shower and
thunderstorm coverage with localized heavy rainfall is still
anticipated.

The temperature forecast both Tuesday and Wednesday remains on track
with mean 700-mb temperatures ranging from 13-18C. These warm 700-mb
temperatures, deep-mixing, and sunshine will support highs in the
80s and 90s both days.

&&

.DISCUSSION...
Issued at 123 AM MDT Mon Jun 30 2025

The cold front that dropped southward Sunday morning will continue
its progression across the CWA this morning, bringing easterly winds
across southern portions. The front will eventually stall and wash
out by midday as winds west of the Divide turn more southwesterly.
Dry conditions will result across most of the area today, as an
upper level ridge builds over the Four Corners to the northern end
of the Great Basin. Temperatures will remain seasonal with light
winds and mostly clear skies. Some high-based CU will build over the
western mountains late in the afternoon. Today will be the quietest
day of the week.

The aforementioned ridge, in combination with a closed low just off
the CA coast, will allow for an increase in moisture from the south
beginning on Tuesday and continue into Wednesday. This could be the
start of the monsoon season. Hot temperatures return Tuesday, with
readings in the upper 80s to near 90 degrees west of the Divide and
readings in the 90s east of the Divide. Virga showers/thunderstorms
are likely (as CAPE values will be up to 500 J/kg) with a 20-30%
chance to occur over western and central portions of the CWA. This
activity looks to develop after 20Z and gradually end across much of
the area through sunset. The main threat will be wind gusts of 50 to
60 mph, as dewpoint depressions will be quite large (45 to 60
degrees difference). Isolated showers may linger through the
overnight hours, but details are fuzzy at this time.

Convective activity looks to remain somewhat limited and more
isolated Wednesday, as CAPE values will be around 500-800 J/kg and
lifted indices of minus 2 to minus 4. This in despite of increasing
moisture and precipitable water values increasing to 0.6" to 0.9".
This will introduce the potential for heavy rain as an additional
threat for thunderstorms.

The closed low will begin to weaken and fill as it moves onshore
Wednesday night and the ridge shifting eastward over the Plains,
resulting in a more southwesterly flow aloft. This will also result
in PWAT values approaching (and possibly exceeding) 1 inch. This
will be near record levels as the max value for early July is around
0.96". The remnant low will move over the Four Corners during this
time, heading toward southwestern portions of the Cowboy State. The
added lift from this low (and additional subtle shortwaves ahead of
the low) will add to the lift for widespread showers and
thunderstorms. The increasing PWAT will also add to this buoyancy.
Most of the convective activity looks to be over mainly western
portions Thursday afternoon, spreading eastward across the rest of
the forecast area through the evening. Needless to say, localized
flash flooding could occur with the stronger storms in the afternoon
and early evening.

The flow pattern becomes a bit flat for the Fourth of July (Friday),
as the remnants of the low continue to slowly propagate eastward.
This would result in the thunderstorms being more focused over areas
east of the Divide. Another round of thunderstorms is possible
across western portions Saturday afternoon, as a shortwave moves
over southern ID.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 335 PM MDT Mon Jun 30 2025

VFR conditions and light winds (10kts or less) will occur through
the TAF period. Skies will remain mostly clear, with a band of FEW
to SCT high clouds passing over and shifting west to east this
evening and overnight. Clear skies return by sunrise. Tuesday
afternoon, showers and thunderstorms become more likely (20 to 30%
chances) for most terminals west of the Divide as well as KCOD. For
other terminals, chances in any shower or thunderstorm impacts are
less than 20%. PROB30 groups have been included for those terminals
with the most likely chances for shower and thunderstorm activity
Tuesday afternoon. Gusty outflow winds would be the main threat with
any showers or thunderstorms.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Gerhardt
DISCUSSION...LaVoie
AVIATION...Hensley