Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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152 FXUS65 KRIW 210853 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 253 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 though 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 into 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 and 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 8 to minus 9 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 thresh 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 /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 959 PM MDT Mon May 20 2024 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals A slow moving weather system will pivot through the region over the next 24 hours, which will bring rain chances to KRKS/KPNA Tuesday afternoon. At KJAC/KBPI mostly dry conditions are expected, but an isolated rain shower is possible (20-30%) Tuesday afternoon. Westerly/northwesterly wind will increase Tuesday afternoon and gusts will be 20-25 knots through the afternoon hours, quickly fading Tuesday 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 tuesday afternoon. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals A slow moving weather system will pivot through the region over the next 24 hours, which will bring rain to all terminals tonight into Tuesday morning. Steady rain tonight will give way to rain showers late Tuesday morning through Tuesday afternoon at KRIW/KLND. A dry period is likely (70%) at KCOD from 15Z-20Z/Tues before more rain showers develop in the afternoon. A steady rain will hang on at KCPR the longest, until 23Z, and then rain showers hang on for a few hours before rain ends Tuesday evening. Wind increases at KCPR late in the afternoon and will remain breezy for a few hours. At KWRL rain ends at 20Z/Tues. As rain occurs MVFR/IFR conditions will prevail at terminals. Mountain obscuration will develop and persist through Tuesday afternoon. 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...Rowe