Latest ZFP from WFO RIW

		
National Weather Service Local Forecast for Star Valley Ranch
Updated: Fri Jul 18  3:28 pm MT

This Afternoon... Scattered showers and thunderstorms.  Mostly
cloudy, with a steady temperature around 78. Light and variable wind
becoming southeast 5 to 10 mph.  Chance of precipitation is 40%.

Tonight... Scattered showers and thunderstorms, mainly before
7pm.  Mostly cloudy, then gradually becoming mostly clear, with a low
around 50. South southeast wind 5 to 9 mph becoming calm.  Chance of
precipitation is 30%.

Saturday... Sunny, with a high near 85. Southeast wind 7 to 13
mph becoming west in the afternoon. Winds could gust as high as 21
mph. 

Saturday Night... Mostly clear, with a low around 51. West wind
6 to 11 mph becoming southeast after midnight. 

Sunday... Sunny, with a high near 88. Southeast wind 6 to 13
mph becoming west in the afternoon. Winds could gust as high as 21
mph. 

Sunday Night... Partly cloudy, with a low around 53. West
southwest wind 5 to 10 mph becoming southeast in the evening. 

Monday... Mostly sunny, with a high near 88. Southeast wind 6
to 14 mph becoming west southwest in the afternoon. Winds could gust
as high as 22 mph. 

Monday Night... Partly cloudy, with a low around 53.

Tuesday... Sunny, with a high near 87.

Tuesday Night... Mostly clear, with a low around 52.

Wednesday... Sunny, with a high near 87.

Wednesday Night... Mostly clear, with a low around 51.

Thursday... Sunny, with a high near 88.

		

Latest AFD from WFO RIW

		
439 
FXUS65 KRIW 182145
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
345 PM MDT Fri Jul 18 2025

.KEY MESSAGES...

- Afternoon and evening shower and thunderstorm chances (40-70%)
  spread southwest to northeast across the area this afternoon. 
  A few stronger thunderstorms could be capable of gusty winds 
  up to 60 mph and isolated small hail.

- Shower and thunderstorm potential on Saturday with a focus
  across southwestern Wyoming.

- Hot with limited shower and thunderstorm chances Sunday into
  next week, thus elevated fire weather conditions are
  anticipated much of next week.

&&

.UPDATE...
Issued at 1243 PM MDT Fri Jul 18 2025

A shortwave moving through the region will bring increased moisture 
and instability today for the region. SPC continues with a Marginal 
risk of severe weather across nearly the entire state today, with 
strong winds being the primary threat. Weak convection has already 
begun to develop across southwest WY late this morning, as expected. 
Dewpoint depressions are expected to increase to around 40 to 50 
degrees by late afternoon, particularly across southwest WY and 
portions of central WY, which will enhance the gusty outflows and 
dry microburst concern this afternoon. Gusts 40 to 50 mph should be 
common with isolated gusts 60+ mph. 

East of the Divide, instability will gradually increase with capping 
across central WY quickly eroding away as temps warm this afternoon. 
CAPE >1500J/kg with 20 to 25kts of SFC-3km shear across central WY 
will allow the potential for some stronger storms. Will be a much 
later show for areas east of the Divide with convection not 
spreading across the Divide until around 3 to 4 pm this afternoon. 
Isolated super-cellular type development is not out of the question 
across portions of central WY, particularly east of the Bighorns, 
and thus, there is a isolated hail threat across Johnson County. 
Small hail will be possible across much of the region, but the 
threat for larger (severe) hail should generally be confined to 
Johnson County and points east. 

The forecast remains on track for Saturday and beyond, with 
scattered showers and thunderstorms across southwest WY Saturday 
afternoon still expected during the afternoon. Beyond that, the 
forecast remains dry with elevated to critical fire weather being 
the primary concern.

&&

.DISCUSSION...
Issued at 244 AM MDT Fri Jul 18 2025

Current (08Z) satellite water vapor imagery shows west-southwest 
flow aloft as an upper-level high builds over the south-central 
CONUS. Embedded within this west-southwesterly flow is a developing 
disturbance over eastern Nevada along with a fetch of increased 
Pacific moisture aloft. This disturbance is forecast to trek to the 
northeast through the day today and provide support aloft for shower 
and thunderstorm chances (40-70%) this afternoon and evening.

Before the disturbance makes its way into southwest Wyoming, dry air 
at the surface combined with 700-mb temperatures between 12-15C 
mixing to the surface will allow for near seasonal temperatures 
(upper 80s to low 90s) east of the Divide today. Temperatures west 
of the Divide will likely be near to slightly below normal (low 80s) 
as morning cloud cover will hinder sufficient daytime heating. High 
temperatures could be a few degrees cooler than what is currently 
forecast east of the Divide if cloud cover ahead of the disturbance 
moves in earlier in the morning. Nonetheless, the dry air and breezy 
winds across eastern Sweetwater County could create elevated to 
critical fire weather conditions for a few hours ahead of any 
precipitation. However, given the small spatial and limited temporal 
range of potential critical fire weather conditions, no highlights 
are expected at this time.

The aforementioned upper-level disturbance is forecast to reach 
southwest Wyoming around 18Z (noon local) with convection initially 
firing off the Uinta Mountains and trekking north. By early 
afternoon, increased low-level moisture convergence, owing to 
divergence aloft, will help focus convection over west and southwest 
Wyoming. Shower and thunderstorms will shift east of the Divide 
through the afternoon and into the early evening as the disturbance 
treks across Wyoming. Because the low levels will likely remain 
relatively dry with peak dewpoint depressions between 40-50 degrees 
and inverted-V soundings, especially across the central Wyoming and 
the Wind Corridor, showers or thunderstorms could produce up to 50 
mph gusts with any collapsing showers or thunderstorms. 
Additionally, areas east of the Divide, primarily the Bighorn Basin 
and Johnson County, would have the highest probably of seeing a 
strong to severe thunderstorm during the afternoon and early 
evening. This is due to these locations staying more cloud free for 
longer allowing for daytime heating to destabilize the atmosphere 
more. East of the Divide, mean SBCAPE values range from 600-1000 
J/kg and 0-6km shear values are between 30-40kts. This supports the 
primary threats being gusty winds up to 60 mph and small hail. 
Thunderstorm chances decrease shortly after sunset, however, a few 
lingering showers could continue past midnight.

This pattern largely remains in place again on Saturday with west-
southwest flow aloft and an embedded disturbance trekking across the 
area. The main difference on Saturday is that moisture will be more 
limited and east of the forecast area, thanks to today's 
disturbance. So, convective initiation will mostly occur off the 
higher elevations of southern and western Wyoming and trek northeast 
with less coverage compared to today.

Ensemble guidance is in consensus that dry zonal flow will take hold 
over the region Sunday and Monday as a broad trough circulates over 
the Pacific Northwest. Embedded disturbances trek across the region 
both days, however, with limited moisture, shower and thunderstorm 
chances will likely be focused across higher elevations of northern 
Wyoming. The trough finally begins to push east as an associated 
upper-level low digs into the Great Basin by Tuesday. An associated 
upper jet then becomes centered over Montana. This leaves the 
forecast area in deep southwesterly flow and within the right 
entrance region of the upper-level jet. Despite being in a favorable 
location for showers and thunderstorms, the greatest moisture 
currently looks to be confined over Montana, thanks to a building 
high over the southern CONUS, so only isolated convection is 
forecast across northern Wyoming at this time. With the deep 
southwesterly flow due to a building high, especially across the 
Wind Corridor, and dry air, elevated to critical fire weather 
conditions are looking favorable much of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 345 PM MDT Fri Jul 18 2025

VFR conditions expected at all TAF sites for the entirety of the
period. Storm activity will be diminishing through the evening
hours still expecting at CPR/RIW/LND through 01Z, COD/RKS 
through 02Z, WRL through 06Z, and JAC through 03Z. The storms
through JAC will affect BPI/PNA between 03-06Z before
diminishing after sunset across the Winds. Main threats
initiallywill be gusty outflows through 03Z before weakening 
even with the later storms through 06Z at the aforementioned 
locations. Otherwise, ceilings lifting to the upper levels 
overnight remaining so through much of Saturday. Some more 
storm activity expected Saturday afternoon but much less in 
coverage. RKS the most likely to see any further south, with a 
PROB30 group but no mention anywhere else. Very low confidence 
in any making its way far enough north to affect BPI/PNA/CPR 
remaining dry for all other locations. 

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

&&

.FIRE WEATHER...
Issued at 543 AM MDT Fri Jul 18 2025

Elevated to critical fire weather conditions are possible ahead of 
any showers or thunderstorms across portions eastern Sweetwater and 
southeast Fremont Counties (this includes portions of Fire Zones 279 
and 289). Minimum relative humidity values could fall just under 15% 
with occasional gusts above 25 mph between 1200L-1600L. However, 
given the limited spacial and temporal extent of potential critical 
fire weather conditions, no highlights are expected at this time. 
Early afternoon dry showers or thunderstorms could produce 40-50 
gusty outflows across the aforementioned locations.

&&

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

&&

$$

UPDATE...Hensley
DISCUSSION...Gerhardt
AVIATION...Lowe
FIRE WEATHER...Gerhardt