Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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484 FXUS65 KRIW 211119 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 519 AM MDT Tue May 21 2024 .KEY MESSAGES... - Rain and mountain snow continue today, ending from west to east this afternoon and evening. - Temperatures will average below normal through most of the week. - An active pattern will continue with additional chances for showers, mainly on Thursday and later in the holiday weekend. && .DISCUSSION... Issued at 250 AM MDT Tue May 21 2024 It has been a rather wet overnight shift at our office tonight, with around a quarter of an inch of rain so far as I write this. Rain continues to be ongoing east of the Divide as well and should continue through the morning hours. By afternoon, the best forcing moves away to the east and things become more scattered and showery. All in all, the previous forecast looked fairly reasonable so we made few changes to it. We did think about taking down the advisory for the Absarokas, but will let it ride for now. As for travel impacts, the main impacts today still look to be the Bighorn Range, where most locations about 8500 feet have at least a 1 in 2 chance of seeing an additional 6 inches or more of snow. Even here, impacts should decrease through the day as the snow becomes lighter and the high May sun angle melts snow off of the roads. As for potential for flooding, rainfall rates have been mainly light to moderate. This, combined with several days of dry weather proceeding yesterday and no snowmelt in the higher elevations, should keep that threat to a minimum. Not zero, but small. And there could be a few snowflakes in the lower elevations. We had a few at the office earlier tonight. This precipitation will end from west to east, with all areas dry later this evening. Temperatures will remain chilly, averaging 10 to 20 degrees below normal. And this brings another concern, the potential for a freeze. There is still some question since some clouds may hang around and keep temperatures up. For now, we will keep the watch up and punt to the day shift for the final decision. Transitory ridging will bring a milder and dry day for many areas Wednesday, the exception being the northwest which could see a few showers and storms in the afternoon. Breezes will also increase for the usual areas pre frontal, mainly from Rock Springs to Casper. The culprit for this, an upper level low moving in from the Pacific Northwest will move near the Montana and Wyoming border from Thursday into Thursday night. This will bring another chance of precipitation, mainly to northern Wyoming although many areas could see a shower or thunderstorm. Wednesday and Thursday is looking a little more interesting as well. Models are indicating 700 millibar temperatures falling as low as minus 8C to minus 9C across the west, which could drop snow levels to the valley floors. Amounts are still in question though. The best forcing and moisture will likely remain over Montana, with most of the area only seeing a glancing blow. Some locations may see advisory level amounts though. Especially those favored by westerly flow, mainly the Tetons and western Bighorns. It is too early for highlights, but something to watch. Flat ridging should then bring drier weather for Friday and much of Saturday as well. Then, a third in the series of lows will approach the area for Sunday with another chance for showers from Sunday and possibly into Memorial Day as well. There is more model spread on this, so details are impossible to flesh out at this time. Ridging may then finally bring a few days of dry weather early next week. But emphasis on may, not will. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 0515 AM MDT Tue May 21 2024 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals A slow moving weather system continues to impact the region through the next 18 hours. Low clouds, and possible LIFR ceilings over far western portions this morning has led to light snow near KJAC. SKC skies have developed over KBPI/KPNA and KRKS. Most of the precipitation will stay over the mountains today, with a chance for rain east of KRKS later this afternoon. Westerly/northwesterly wind will increase this afternoon and gusts will be 20-25 knots through the afternoon hours, quickly fading in the evening. Overall conditions will remain VFR, but a drop to MVFR is likely (80%) if a heavier shower moves directly over a terminal. Mountaintop obscuration will be prevalent through the afternoon. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals A slow moving weather system will continue to pivot over the region over the next 24 hours. Rain will be more focused over Johnson and Natrona counties through the morning, with additional showers developing off the mountains this afternoon. VLIFR/LIFR ceilings are expected at KCPR through 18Z, improving to MVFR afterwards. Similar conditions will be possible at KLND through 14Z, as the rain shifts eastward this morning. MVFR to VFR conditions will be place elsewhere this morning, with VFR conditions more widespread this afternoon. SKC skies will be possible at KRIW/KLND after 06Z, which could allow for fog development toward 12Z. Mountain obscuration will persist through the afternoon, with improving conditions toward 06Z. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 6 PM MDT this evening for WYZ002- 015. Freeze Watch from late tonight through Wednesday morning for WYZ003>006. Winter Storm Warning until 6 PM MDT this evening for WYZ008- 009. && $$ DISCUSSION...Hattings AVIATION...LaVoie