Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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693 FXUS65 KRIW 172154 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 354 PM MDT Fri May 17 2024 .KEY MESSAGES... - Elevated to critical fire weather conditions today with humidity below 20 percent and widespread wind gusts of 35 to 50 mph. - Isolated thunderstorms are possible (10% to 20% chance) across northern Wyoming this afternoon, a few of which could be strong with gusty outflow winds. - Elevated fire weather conditions continue on Saturday and Sunday. - A large and wet weather system will bring rain and mountain snow on Monday and Tuesday. Mountain passes are expected to have winter driving conditions. && .DISCUSSION... Issued at 1255 PM MDT Fri May 17 2024 A hearty open shortwave trough is quickly moving through the Northern Rockies this afternoon and evening, bringing active spring weather. The tight pressure gradient on the south side of this system has been the cause of the already gusty winds that have already begun to mix down into the lower elevations. Still expecting gusts in the 35 to 50 mph range for much of the afternoon and evening today, quickly decreasing after sunset. In addition to winds, unseasonably warm temperatures will be felt this afternoon, with high temperatures about 15 degrees above normal. The warm surface air, along with cooler air aloft will lead to 200 to 400 J/kg of CAPE across the Bighorn Mountains and the adjoining high plains of Johnson County and the larger Powder River Basin. A few strong thunderstorms featuring hail and strong outflow winds are possible here due to enhanced shear aloft with the approaching shortwave. A majority of the strong storms will stay further north across southern Montana, but can`t rule out a rogue strong cell over Johnson County this afternoon. A third threat this afternoon and evening is the elevated fire weather danger. The strong winds along with a dry air mass will be conducive for fire growth. That being said, fuels are quite moist this time of year during spring greenup, which will be a mitigating factor. A Rangeland Fire Danger Statement covers this threat. Fire danger decrease quickly after sunset. The rest of the next 7 days remains fairly active as well. The jet stream will remain overhead on Saturday, causing breezy winds to continue. These will not be as strong as those felt today and not quite as widespread, but gusts in the 25 to 35 mph range will be common. On Sunday, the jet stream sags southward and a subtle embedded wave will enhance surface winds to levels higher than Saturday, but not quite as windy as today. There is modest lift with this wave, so scattered showers are in the forecast, focused over the mountainous terrain. Monday into Tuesday brings the next period of widespread precipitation as a slow-moving cut-off low rotates through the larger trough. Model trends have been to slow this system down, and produce more rain with Gulf of Mexico-sourced inflow into the system. Snow levels on Tuesday will be fairly low, around 7000 ft in the morning, so expect snow over mountain passes as well as potential winter weather highlights for some areas. Wednesday through Friday looks to feature another upper low, this time over the Pacific Northwest, pumping moisture into the western Wyoming mountain ranges. All these weather systems will continue to add high elevation snowpack, and increase the potential for river flooding issues later this spring. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 353 PM MDT Fri May 17 2024 Gusty winds up to 40 knots decrease through 08Z. A mainly dry cold front moving through brings a north wind shift to some sites tonight. This may bring a couple of gusty winds to affected locations. Winds begin to increase again Saturday morning. Though it won`t be a windy as today, a breezy afternoon, with gusts 25 to 30 knots are possible for many locations. High clouds begin to increase towards the end of the TAF period ahead of the next weather system. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued at 1255 PM MDT Fri May 17 2024 Widespread elevated to critical fire weather conditions are occurring this afternoon as a surface pressure gradient causes strong winds ahead of a weak cold front. Sustained west winds of 25 to 35 mph with gusts of 40 to 50 mph will continue through sunset. Humidity values are 20 to 30 percent currently, and likely (90% chance) to drop to 15 to 20 percent across most of the area. A slightly drier airmass moves in for Saturday, bringing humidity values down a few more percent. Wind will not be as strong as today, but will still be breezy. Warm and dry conditions continue Sunday before cooler and wetter conditions arrive Monday. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...VandenBoogart AVIATION...Wittmann FIRE WEATHER...VandenBoogart