Green River, Wyoming 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

855
FXUS65 KRIW 160431
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1031 PM MDT Thu May 15 2025

.KEY MESSAGES...

- A few lingering showers and thunderstorms this evening with
  activity quieting down by the early morning hours Friday.

- Somewhat warmer Friday and Saturday with less coverage of
  showers and thunderstorms.

- A Pacific weather system will bring a 50 to 60% chance of
  widespread rain and mountain snow from Sunday through Monday.

&&

.UPDATE...
Issued at 1224 PM MDT Thu May 15 2025

As of 1200L, the radar remains fairly quiet with only some light showers
across western WY. Scattered showers have been steadily increasing
in coverage across most areas west of the Divide and across northern
WY over the last hour, and these are expected to continue to increase
in coverage across the rest of the region through the early afternoon.
Precipitation totals are much lower than yesterday, with likely only
a few hundredths rather than tenths of precipitation out of the heavier
showers. Thunderstorms will be much more isolated today as well,
at only 10 to 20% coverage/probability. Though gusty winds and small
hail are a possibility with any stronger storms, but those stronger
storms are much less likely than in previous days (5 to 10% chance).
Friday will see even less coverage of showers and storms with best
chances (20 to 30% chance of showers) across the northwest.

&&

.DISCUSSION...
Issued at 326 AM MDT Thu May 15 2025

We have had some changes over the past few days. Storm movement was
originally south to north on Tuesday, more westerly yesterday, and
now we have transitioned to a northwest flow pattern. But one thing
has remained constant, the chances of showers during the wettest
time of the year across the Cowboy State. And this will continue
through much of the next 7 days, although at varying coverages and
intensities.

The culprit for today`s precipitation is a shortwave moving in from
the northwest. We already have a few showers out there this morning,
and these will increase in coverage as we head into the afternoon.
And some of these showers could come with thunder, commonly known as
thunderstorms. We do have some decent CAPE this afternoon, topping
out around 500 J/Kg, although the lifted indices are not the best
I`ve seen, generally around minus 1. So, I would put the chance of a
thunderstorm at any location around 1 out of 4, with a greater
chance of showers. And like previous days, the best chance of
showers will be in northern Wyoming, with tapering chances further
south. Precipitable waters are not terribly high though, generally
topping at around 0.40 inches. And, with the storms moving at a
decent clip, the threat of flooding looks small. So, the main threat
would be gusty wind. I could also see some small hail, given the low
wet bulb zero levels and cool temperatures, averaging around 10
degrees below normal. Wind will also be a concern today. The main
area of concern looks to be northern Johnson County and the northern
Big Horn Basin, with the favored northwest flow pattern behind the
shortwave. Low level wind does not look sufficient for widespread
high wind though, with 700 millibar wind topping out at around 40
knots when I would like to see 50 knots. Ensembles are giving around
a 2 in 3 chance of gusts past 40 mph though, especially at Buffalo.
So, it looks like a toupee alert day for many locations. Most
showers should end by late this evening. And, there will be some
snow in the mountains, with 700 millibar temperatures between minus
1 and minus 3, putting snow levels between 7000 and 8000 feet.
Impacts would be minimal since the strong May sun should melt snow
off of the roads.

Shortwave ridging then moves across the state on Friday. This should
cap the atmosphere a bit more and allow for somewhat warmer
temperatures. We will still see some showers, but coverage will be
less with most locations likely staying dry most of the time. Again,
(noticing a pattern here), the best coverage will be across the
north with southern Wyoming mainly dry.

Saturday looks like the warmest day of the period as flow turns
southwesterly ahead of the next weather system. The deepest moisture
will stay west of the area on this day, but enough will move
eastward and there will be a few showers and thunderstorms late in
the day and in the evening. But, this day again looks like most
areas will be dry most of the time so if you want do to something
this weekend, Saturday definitely looks like the best day to do it.
Then on Sunday into Monday, a big atmospheric bowling ball, AKA an
upper level low, then moves into the Rockies and brings another
period of wet weather. These lows are notoriously fickle with
movement so details about timing of precipitation and amounts are
still very much in flux. We do know a few things though. One, we
have the concern of mountain snow, and potentially a decent
amount of it. Models are indicating 700 millibar temperatures
may fall as low as minus 3 in portions of the area, which could
drop snow levels as low as 7000 feet on Monday. Probabilistic
model guidance gives around a 1 in 2 chance of greater than a
foot of snow from Sunday morning through Tuesday morning above
9000 feet across portions of the mountains. This is especially
so in the ranges East of the Divide, where there would be some
wrap around moisture and upslope on Monday. Not a certainly by
any means, but something we have to watch. This could be a wet
system, with a greater than 2 in 3 chance of over a half an inch
or rain across the northern two thirds of the area through the
period with favored northeasterly upslope locations having a
greater than 1 in 2 chance of at least an inch of precipitation
through 6 am on Tuesday. As a result, we will also have to watch
for rising rivers. Melting snowpack would not be a concern
though, given the cool temperatures though.

Things get more muddled past Monday, as there are different
solutions in regards to how fast the low will move away to the east.
Most guidance shows ridging building behind the system, bringing
drier and warmer weather, but when this will occur is still
uncertain.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1030 PM MDT Thu May 15 2025

VFR conditions expected through the overnight hours. Any
precipitation should be east and south of RIW/CPR to start the
period, carrying only VCSH at CPR. Otherwise, broken low to mid
level ceilings at all locations with light winds less than 10kts
into the Friday morning hours. Ceilings will lift or scatter out
with daytime heating and mixing after around 18-19Z as winds
increase at BPI/PNA/RKS and strongest at CPR up to 25kts. A
weaker and less scattered round of rain showers will only affect
JAC/COD between 19/20-01/02Z with conditions likely to remain
VFR (>60%). Otherwise, improved conditions in the wake of these
rain showers lifting and scattering out as well after the end of
the period and overnight into Saturday morning.

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...Hensley
DISCUSSION...Hattings
AVIATION...Lowe