


Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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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