Latest ZFP from WFO RIW

		
National Weather Service Local Forecast for Star Valley Ranch
Updated: Sun Apr 27  3:44 pm MT

This Afternoon... Showers and possibly a thunderstorm.  High
near 59. Southeast wind 6 to 13 mph becoming northeast.  Chance of
precipitation is 80%. New rainfall amounts between a tenth and quarter
of an inch, except higher amounts possible in thunderstorms. 

Tonight... Showers and thunderstorms likely, mainly before 8pm.
 Cloudy, then gradually becoming partly cloudy, with a low around 30.
East southeast wind 5 to 7 mph becoming calm  after midnight.  Chance
of precipitation is 60%.

Monday... A chance of rain and snow showers before 11am, then
rain showers likely.  Increasing clouds, with a high near 49. Light
and variable wind becoming north northwest 5 to 10 mph in the morning.
 Chance of precipitation is 70%. Little or no snow accumulation
expected. 

Monday Night... Partly cloudy, with a low around 33. North
northwest wind 5 to 9 mph becoming calm  in the evening. 

Tuesday... Partly sunny, with a high near 56. Calm wind
becoming west 5 to 7 mph in the afternoon. 

Tuesday Night... Showers, mainly after midnight.  Low around
40. West southwest wind 5 to 7 mph.  Chance of precipitation is 80%.

Wednesday... Showers likely, with thunderstorms also possible
after noon.  Partly sunny, with a high near 53. West northwest wind 5
to 7 mph.  Chance of precipitation is 70%.

Wednesday Night... A 20 percent chance of showers before
midnight.  Mostly clear, with a low around 33.

Thursday... Sunny, with a high near 61.

Thursday Night... Mostly clear, with a low around 35.

Friday... Sunny, with a high near 67.

Friday Night... A slight chance of showers.  Mostly clear, with
a low around 40.

Saturday... A chance of showers and thunderstorms.  Mostly
sunny, with a high near 66.

		

Latest AFD from WFO RIW

		
022 
FXUS65 KRIW 271934
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
134 PM MDT Sun Apr 27 2025

.KEY MESSAGES...

- Warm temperatures, low relative humidity values and gusty
  winds continue to bring elevated fire weather to much of
  central and southern Wyoming this afternoon. 

- Showers and isolated thunderstorms are possible (60-80%
  chance) across western Wyoming through sunset today. 

- An approaching weather system brings rain and mountain snow to 
  much of the area late tonight and Monday, with the highest 
  amounts in northern Wyoming.

- Widespread rain and mountain snow chances (60-90%) for Tuesday
  night through Wednesday night.

&&

.UPDATE...
Issued at 131 PM MDT Sun Apr 27 2025

The upper-level low, evident on satellite water vapor imagery, has 
begun to inch its way out of the Great Basin and into the 
Intermountain West. In the northeastern quadrant of the low (across 
western WY) is an area of enhanced upper-level divergence and 
increased low level moisture convergence. These factors will favor 
the development of rain and mountain snow across western WY this 
afternoon. Additionally, there is potential for isolated 
thunderstorms with unstable mid-level lapse rates and SBCAPE values 
around 500 J/kg.

Further east, an upper-level jet streak associated with the low is 
positioned over central, southern, and eastern WY, creating gusty 
conditions. Most areas remained sunny through midday (18Z), 
allowing for temperatures to warm well into the 60s and low 70s 
and relative humidity values to drop into the teens and low 20s.
These conditions will continue into the afternoon, creating 
elevated fire weather conditions in the aforementioned 
locations.

Guidance remains in consensus that the upper-low will push east 
across southern WY and northern CO/UT late tonight with its 700-mb 
reflection pushing northeast across northern WY. This will expand 
precipitation (mountain snow and low elevation rain) to the north and 
east late tonight through Monday with an associated cold front 
dropping through the area. As the cold front treks through on 
Monday, precipitation chances exist across central WY Monday 
afternoon. The greatest precipitation amounts are still expected 
across northern WY. Mountain snow amounts remain consistent from the 
prior forecast so no changes expected to winter headlines. Windy 
conditions are forecast across Johnson County and the Bighorn Basin 
on Monday with strong CAA behind the front. No highlights are 
expected at this time as probabilities of reaching high wind 
criteria are <10%. However, an isolated gust above 50 mph cannot be 
ruled out at Buffalo. 

&&

.DISCUSSION...
Issued at 210 AM MDT Sun Apr 27 2025

Things look fairly active over the next few days across western and 
central Wyoming. And we will have a variety of elements, including 
rain, snow, wind and even some fire weather concerns. So, without 
further delay, let us jump into it.

Things are quiet right now, but this won't last long. An upper 
level low now located over the Great Basin will move east and 
transit across southern Wyoming tonight and through Monday. And 
this will be the driver of the active weather. First we will 
focus on the warm weather impacts today, since southwest flow 
ahead of the system will push temperatures to above normal 
levels across the area today. Southwest flow will increase as 
the pressure gradient tightens ahead of the low. With humidity 
expected to fall to around 15 percent, elevated fire weather 
looks likely again this afternoon across many areas east of the
Divide. This is especially the case in the Wind Corridor, from 
Rock Springs through southern Johnson County. The warmer 
temperatures will also bring some instability that could lead to
some thunderstorms across areas further west this afternoon and
evening. As for stronger storms, we can't rule out one or two 
but most should be of the garden variety. The best chance of a 
stronger one would be across northern Wyoming.

Now to the colder part of the system. During the day, 700 millibar 
temperatures remain fairly warm, generally minus 2C or higher 
so this would keep snow levels above 7500 to 8000 feet. This 
will change in the evening as flow turns northward and cooler 
air gets wrapped around the system as the low moves east. Most 
guidance shows 700 millibar temperatures falling generally down 
to minus 4C to minus 5C across most of the area. This should 
keep snow levels around 5500 to 6000 feet and away from most 
populated areas. Another trend in the guidance this morning is 
this will bring the low a bit further north, and that would 
bring drier air further north. As a result, precipitation 
amounts have fallen to the south of the Owl Creek and Bridger 
Mountains. The chance of a quarter inch or rain in places like 
Riverton and Casper has fallen to at most 1 out of 2 as a 
result. The highest amounts would be in Montana, but areas 
roughly north of a Buffalo to Cody line have around a 1 in 2 
chance or greater of picking up a half an inch of precipitation 
or more. Now, on to snowfall amounts. All guidance is pointing 
toward the Bighorns, especially the eastern slopes, seeing the 
heaviest amounts since flow will turn north and upslope there. 
The large majority of the guidance is also showing the period 
from 6 am to noon on Monday with the heaviest precipitation, 
with the lift being assisted by the left front quadrant of a jet
streak. As for highlights, we have gone with advisories. Most 
of the range will only see advisory level amounts, with the 
heaviest amounts in the far north and the higher peaks. The 
locations with greater than a 1 in 2 chance of 12 inches or more
are only over the higher elevations with few impacts. In 
addition, the warm weekend warming the ground and high late 
April sun angle would likely hinder accumulation on roads. We 
have also issued an advisory for the Absarokas. A decent portion
of the area has a greater than 1 in 2 chance of 6 inches or 
more. The only location with greater than a 1 in 2 chance of a 
foot or more here is Beartooth Pass and seeing how the road is 
still closed, impacts would be limited. Other locations, like 
Yellowstone or the Tetons, may have isolated amounts of 6 inches
or more but again, these would largely only be the highest 
elevations. So, no advisories here. And, we have one more 
concern. Some MOS guidance is showing sustained winds of 34 
knots around Buffalo on Monday with the cold advection behind 
the low. There is no consensus here though, so we will hold off 
on any wind highlights. The day shift should take another look 
at it though. Most of the snow should end by sunset Monday with 
only a few leftover evening showers.

Tuesday looks to be a mainly dry and seasonable day as transitory 
ridging moves across the state. Then the next system approaches from 
the northwest for Tuesday night into Wednesday. This system is 
faster moving and as less moisture to work with though. It will 
bring a decent chance of showers, but given the fast movement 
of the system precipitation amounts should remain on the light 
side. Ridging then brings dry and warmer weather for Thursday 
and Friday before the next potential system for next weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1100 AM MDT Sun Apr 27 2025

VFR conditions to start the period at all TAF sites. Increasing
winds at all locations with gusts up to 18-25kts, highest at RKS
up to 30kts and CPR up to 35kts during the afternoon peak
heating hours with the strongest mixing to the surface. The will
diminish towards sunset after 01-02Z with radiational cooling.
FROPA will occur after 02-05Z that will see a more northwesterly
direction but remaining around or less than 10-15kts. 

Precipitation wise, expect thunderstorm development near JAC
between 20-22Z becoming just rain showers there after to include
BPI/PNA with this round ending by 05Z. Behind the front, a more
stratiform precipitation with a mix possible for JAC/COD after
07Z for COD and 12Z for JAC. This should be more light rain at
COD after 12Z spreading to WRL after 14Z Monday morning. At this
point, MVFR ceilings are possible as winds increase post FROPA
seeing gusts up to 25kts into the next TAF cycle for MOnday
afternoon. 

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM to 6 PM MDT Monday for 
WYZ002-008-009.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Lowe