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Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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747 FXUS65 KRIW 272302 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 502 PM MDT Thu Jun 27 2024 .KEY MESSAGES... - Isolated to scattered showers and thunderstorms are expected this afternoon, and some storms may become strong to produce very strong outflow winds with some large hail. Northern WY and far southern WY should expect the strongest storms. - Friday will be cooler with below normal temperatures in the 60s and 70s, yet winds will be breezy and gusty out of the north and northwest. - Elevated fire weather conditions will return Saturday and Sunday with warming temperatures, drier air, and gusty winds. - Another storm system will impact the area late Sunday into Monday, bringing cooler temperatures and a chance of precipitation to the northern half of the state. && .DISCUSSION... Issued at 1230 AM MDT Thu Jun 27 2024 With the trough axis moving quickly from eastern WA/OR into Idaho today, strengthening and moistening west-southwest flow has moved into western and central WY with the expectation of isolated to scattered showers and thunderstorms through the afternoon and early evening. A very big change from the previous shift is that the Storm Prediction Center has drastically cut back the slight risk of severe storms over the forecast area to now only include the NE corner of Johnson County instead of 90 percent of the CWA. The 12Z models are indicating less instability across the central portions of the forecast area, concentrating the convection from west-central WY toward north- central WY, as well as across the southern half of Sweetwater County toward southern Natrona County. WSW have already picked up across much of the area ahead of the trough and thunderstorms will soon move into Star Valley. High resolution models as well as the NAM/GFS combo are inline with the latest thinking. In areas where strong storms could develop (since we still have a marginal risk of severe storms over much of the forecast area), strong outflow winds over 40 mph are the most likely threat. Next, some storms may produce hail larger than an inch in diameter, and these storm are mostly likely over the Bighorn Basin into Johnson County. For the most part, the triggers for the storms quickly move eastward with the storms out of the area by 800 pm, with lingering convection remaining across far southern Sweetwater County until after midnight. Winds will generally decrease around sunset as well with a northerly push into the northern Bighorn Basin and Johnson County late this afternoon. Friday will see the trough axis over the state and moving eastward with much cooler mid-level and surface temperatures over the area, dropping high temps down to the mid 60s to upper 70s in the lower elevations. The flow pattern will continue breezy and gusty winds across the area from the WNW. During the afternoon, another "cool front" will move into northern WY as a secondary trough moves through WY. The Bighorn Basin and Johnson County will see gusty north winds of 30-40 mph during the afternoon and evening hours, with the north winds pushing southward into central WY during the evening. Models continue to show showers and thunderstorms across southern Sweetwater County Friday afternoon and evening until the north winds push them south. While Saturday starts off cool with mostly clear skies, the ridge axis quickly moves over the state from the WSW and southerly surface to SW and southern WY during the early afternoon. Temperature will warm back to near normal with some mid- and high-level clouds over the norther parts of the state. Sunday will see temperatures increase again to the 90s as S-SW flow increase over the forecast area. The advecting air from the Great Basin will be drier as well with afternoon humidities dropping to the low teens. These elements will create elevated fire weather conditions over much of western and central WY. Another trough from the PacNW into MT will increase clouds over NW WY late Sunday, and bring showers and isolated thunderstorms to NW and northern WY late Sunday into Monday. Temperatures will cool again as showers linger across northern WY into Monday evening. A generally dry and warm westerly flow pattern then sets up for Tuesday through Thursday, elevating fire weather conditions across western, southwest, and southern WY. Models are hinting at cooler and moister air from the northern plains backing into northeast WY to keep some areas east of the divide from otherwise hot and dry conditions. Will wait and see however on the westward movement of this air mass. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 501 PM MDT Thu Jun 27 2024 The tail end of a vigorous shortwave trekking east across Montana has ignited isolated thunderstorms over the northwest-quarter of Wyoming this afternoon. Additional storms are present over Sweetwater County within a plume of mid-level moisture. Recent high- resolution models shows convection ending around 01Z/Friday for all but the extreme south, far north, and the corridor from KCPR to KBYG. KCPR and KRKS have a 30 percent chance or less of being impacted by convection, while other terminals are not impacted. The primary hazard will be gusty outflow wind 40-45kts given high cloud bases and a dry sub-cloud layer. Prevailing westerly surface wind 15- 30kts continues until 02Z-03Z/Friday before the atmosphere decouples with the setting sun. These winds return to most terminals between 16Z-19Z/ Friday. Another shortwave crossing to our north allows these surface winds to become more west-northwest Friday afternoon. Northerly surface wind follows for areas east of the Continental Divide just beyond the valid forecast period. 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. && $$ DISCUSSION...McDonald AVIATION...CNJ