005 FXUS65 KRIW 260426 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1026 PM MDT Fri Apr 25 2025 .KEY MESSAGES... - Remaining cool today with 30 to 60% chances for showers and isolated thunderstorms (20% chance) across western WY. - Warmer and mostly dry for the weekend with elevated fire weather conditions expected from Sweetwater County through southern Johnson County both Saturday and Sunday. - A more potent system brings rain and mountain snow from Sunday afternoon through Monday. && .UPDATE... Issued at 101 PM MDT Fri Apr 25 2025 Very few overall changes to the forecast for today. Low and mid- level cloud decks linger east of the Divide through tonight, scattering out Saturday. West of the Divide, similarly to the past few days, skies have cleared and thus will lead to increasing instability through the afternoon, prompting the initiation of convection, helped along by the passing shortwave. Unlike previous days, flow is more southerly, thus showers (50 to 60% chances) and isolated thunderstorms (15 to 25% chances) will stay generally along the western WY, west of a Farson to Crowheart to Cody line. A few CAMs are hinting at one or two storms developing farther east that would track into the Wind River Basin or Natrona County, but confidence on that is low (10 to 20% chance). Saturday and Sunday forecast remains on track with warmer and drier conditions still looking likely. The key message for both weekend days will be the elevated fire weather conditions. RHs will be 10 to 15% across much of the wind corridor from Rock Springs through Johnson County, along with southwest winds gusting 20 to 25 mph Saturday and 30 to 40 mph on Sunday afternoon. Fuels are in green- up, so no fire weather highlights are expected. The other notable forecast update is for the more potent storm system progged for Sunday Night/Monday. NBM QPF has increased with the latest run, especially for northern WY, including the Bighorn Mountains. This will be something to watch as we head into the weekend. && .DISCUSSION... Issued at 201 AM MDT Fri Apr 25 2025 We are now in the wettest time of the year across western and central Wyoming, with this period lasting into June. Early this morning, the weather outside reminds me of a late March day in my native New England after a back door cold front passage, with a chilly easterly breeze, light rain and drizzle, and temperatures in the 30s to around 40. It was my least favorite time of year when I was growing up there many years ago. Spring is also known to be changeable, and this will be no exception over the next several days. We will have a few showers early this morning, but for the most part activity will decrease toward sunrise. Another shortwave will move through the area today and bring more showers, with the emphasis mainly across the west with less coverage further east. And there could be a few thunderstorms, but these will be isolated. This still looks like a cool day east of the Divide, with a lot of cloud cover lingering, although it will be a bit warmer than yesterday. Most of these showers will end after midnight. Saturday will be a marked changed across the area as flow turns southwest and warms temperatures up again. Most lower elevations east of the Divide will climb into the 70s again with above normal temperatures across the area. As for precipitation, there will be some convection across northwestern Wyoming but the chance is small, only around 1 out of 3 at the most. The main concern on Saturday will be elevated fire weather with a gusty southwesterly breeze developing and dew points dropping into the teens. This will largely be from eastern Sweetwater County through southern Johnson County, which are favored in this pattern. A more substantial system in the form of an upper level low will move across southern Wyoming from Sunday night through Monday. The day will start off dry, but showers will spread into the west in the afternoon. This looks like a fairly warm system to start, with 700 millibar temperatures generally minus 2C or warmer, which would keep snow levels at 7500 feet or higher so the valleys should stay mainly rain. Areas east of the Divide, with the exception of the far north, will likely have a dry, windy and warm day. And, like Saturday, elevated fire weather will be a concern across similar areas as on Saturday. Precipitation should become more widespread Sunday night into Monday as the low moves to the east, flow turns north and moisture wraps around the back side of the low. Even with the cooler temperatures, there is not a lot of cold air to work with. Guidance shows 700 millibar temperatures only falling to around minus 4C at the coldest, which would keep snow levels to 6500 feet at the lowest. There could be a decent snowfall in the mountains though. Most probabilistic guidance is focused on the Bighorns with a 1 in 2 chance of 12 inches or more of snow through Monday night. Some highlights may be needed but it is a little too early for that right now. Most models show the heaviest precipitation falling across the northern half of the area, with much of the area east of the Divide having a 1 in 2 chance of a quarter inch of precipitation or more ending Tuesday morning at 6 am. We even have a Excessive Rainfall Outlook with a marginal risk across northeastern portions of the area for Monday. Most guidance shows the rain tapering off Monday night. Tuesday at this point looks mainly dry with transitory ridging over the area. Another approaching system will bring another chance of showers for Wednesday. So, the changeable nature of late April weather continues across western and central Wyoming. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1021 PM MDT Fri Apr 25 2025 Last batch of showers continues to sweep northward along western WY this evening.TAFs for KJAC will continue with TEMPO showers through 08Z, when the end of the band passes. These showers are also sweeping just west of KRIW, so have added tempo group for those passing by in the next hour or so. While these showers should be dissipating as they move north, there`s a small chance these could reach KCOD around 08Z with a tempo lower cig. KCPR remains under pesky llvl cloud deck around 800ft, which will vary btwn SCT and BKN through the night. Other than KCPR, remaining sites will gradually improve overnight as low clouds move to the northeast and only high cirrus remain. These cirrus will thin through the morning Saturday, with VFR through the rest of the period. Southwest winds will increase during the afternoon Saturday, with some gusts of 20 to 25kts at KCPR and KRKS. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Hensley DISCUSSION...Hattings AVIATION...Straub