495 FXUS65 KRIW 020422 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1022 PM MDT Sun Jun 1 2025 .KEY MESSAGES... - Showers and strong thunderstorms will continue to wane across eastern portions this evening. Additional showers will be possible later through the night. - Minor river and small stream flooding remains possible through Monday due to increased mountain snowmelt. - Cooler and unsettled Monday through Wednesday. Light snow is expected in the mountains of central and northern Wyoming. && .UPDATE... Issued at 114 PM MDT Sun Jun 1 2025 Forecast remains on track this afternoon, with thunderstorms the main concern in the near-term. We are beginning to see the building cumulus fields over the mountains translate to radar returns, and expect this trend to continue as a shortwave clips northwestern Wyoming. The strongest storms will be across the Yellowstone area and off the Bighorns into Johnson/Natrona counties, where the most favorable instability and shear will set up. Strong outflow wind and a hail threat will exist with these storms. However, scattered showers and thunderstorms are also expected across most of the remaining area; given the large surface T/Td spreads of 50 to 60 degrees, wind gusts of the same magnitude are possible with any cells this afternoon. QPF for the main trough passage early this week is becoming more focused, albeit lower, with the highest amounts expected across the northern third of the area. The associated cold front will lead to gusty winds on Monday and widespread cooler temperatures through the week. && .DISCUSSION... Issued at 311 AM MDT Sun Jun 1 2025 Welcome to summer! Well, meteorological summer, (the Solstice is on June 20th for astronomical summer). And in true Wyoming fashion, we will have more than one season in this forecast period, summer, spring and, for some of the mountains, winter. First, I want to apologize for the lateness of the discussion, as we had a really good display of the Northern Lights, with Kp indices of 8 and some really brilliant colors. There could be a repeat performance tonight, but there will likely be some complications in seeing them. Details will be below. It will feel like summer today. Southwest flow will increase ahead of an advancing cold front and upper level low and bring some very warm temperatures. We fell short of 90 degrees yesterday, but there is a better chance today. Again, the best chances will be across the Bighorn Basin and places like Casper and Riverton. But there is another side of this summer part of the forecast. And this is showers and thunderstorms. The approaching front will bring some lift and increasing moisture will lead to scattered showers and thunderstorms. Models have trended drier though, with the best chance of convection across northwestern Wyoming (about a 1 in 2 chance) tapering to 1 out of 6 further south and east. We continue to have a marginal chance of severe weather across the northwest. Strong wind will be the main threat, but there will be enough low level moisture for some hail as well. Further east, the low levels will remain very dry through the day. Dewpoint depressions will be as high as 50 to 55 degrees this afternoon and model soundings are showing very obvious inverted Vs. This means high based showers and thunderstorms and the chance of wind gusts past 50 mph with any precipitation, even innocent looking showers. Yes, this looks like it could be a "Little Green Blob" day across much of the area east of the Divide. And, with the warm weather, we have some creeks and rivers approaching Action stage so we could see some Flood Advisories at times today as the high elevation snow melt continues. Now for the possible Northern Lights viewing. This could be a very good display, with a G4 watch in effect (the Space Weather Prediction Center doesn`t issue G5 watches). It looks like we will have some clouds around through much of the area through the night. If you want to take a look, the eastern portions of the area, especially Natrona, eastern Fremont and eastern Sweetwater counties look like the best locations. They would likely not be visible until at least 10:30 pm given the long June days. The front will move through the area tonight and bring an end to the well above normal temperatures. The chance for a widespread heavy rain event is looking less likely as well. The question was when would the systems phase. And now it looks like it will occur east of our area, hence the slight risk there. As for precipitation amounts in our neck of the woods, the chance for a half inch or more from Monday morning through Wednesday night tapered to at most 1 out of 3 for areas east of the Divide except for the mountains. We are still confident of a few things. One, it will be a much cooler pattern through at least Wednesday, with the coolest temperatures expected on Tuesday when some of the lower elevations will struggle to get out of the 50s, up to 20 degrees below normal. There will be a series of shortwaves moving across the area through at least Wednesday, bringing some rounds of showers and isolated thunderstorms. There are different solutions though for timing of the waves, so pinpointing a time remains difficult. Now for the wintery part of the system. Much cooler air will move in, with the coldest period being Monday night and Tuesday. Models are now showing 700 millibar temperatures as low as minus 3C, which could lower snow levels down to 7500 to even 7000 feet. There remains a greater than 1 in 2 chance of over 3 inches of snow in the higher elevations of the Bighorns, Wind River Range and Absarokas. The chance of 6 inches or more is generally less than 2 out 5 and only at the highest elevations where impacts would be minimal. Flow then becomes largely zonal Thursday and into the beginning of next week, which should bring a return to near normal temperatures. We will continue to have a few waves moving through the flow as well, bringing chances of showers and storms. Again, timing is uncertain but the rule for later in the week is unsettled with near normal temperatures. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1020 PM MDT Sun Jun 1 2025 Isolated showers continue across portions of the state for the start of the TAF period. These showers may eventually near KJAC, KPNA, KBPI, KLND, and KRIW around 09Z through 13Z Monday. These showers may briefly lower conditions down to MVFR if they track over top any terminals. Remaining terminals for the start of the TAF period will see a mid level SCT to BKN cloud deck which will gradually rise by the mid morning hours Monday. Another round of showers and thunderstorms is possible during the afternoon and evening Monday. Shower coverage is likely to be less widespread with most terminals seeing a 30% or less chance. The best chances look to be at KCPR, with a prevailing light rain period during the afternoon. Showers will likely be possible near the end of the TAF period into the start of the next period. However, due to low chances most terminals do not see any precipitation groups to end the period. Winds increase during the late morning and early afternoon Monday with most terminals seeing breezy winds of 15 to 25 knots at times. These winds gradually diminish by the evening hours becoming light through the end of the period. Mountain obscuration will be likely especially to start the TAF period with some improvement possible during the day Monday before returning by Monday evening. 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. && $$ UPDATE...Myers DISCUSSION...Hattings AVIATION...Dziewaltowski/LaVoie