####018008009#### FXUS65 KRIW 160213 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 813 PM MDT Thu May 15 2025 .KEY MESSAGES... - A few lingering showers and thunderstorms this evening with activity quieting down by the early morning hours Friday. - Somewhat warmer Friday and Saturday with less coverage of showers and thunderstorms. - A Pacific weather system will bring a 50 to 60% chance of widespread rain and mountain snow from Sunday through Monday. && .UPDATE... Issued at 1224 PM MDT Thu May 15 2025 As of 1200L, the radar remains fairly quiet with only some light showers across western WY. Scattered showers have been steadily increasing in coverage across most areas west of the Divide and across northern WY over the last hour, and these are expected to continue to increase in coverage across the rest of the region through the early afternoon. Precipitation totals are much lower than yesterday, with likely only a few hundredths rather than tenths of precipitation out of the heavier showers. Thunderstorms will be much more isolated today as well, at only 10 to 20% coverage/probability. Though gusty winds and small hail are a possibility with any stronger storms, but those stronger storms are much less likely than in previous days (5 to 10% chance). Friday will see even less coverage of showers and storms with best chances (20 to 30% chance of showers) across the northwest. && .DISCUSSION... Issued at 326 AM MDT Thu May 15 2025 We have had some changes over the past few days. Storm movement was originally south to north on Tuesday, more westerly yesterday, and now we have transitioned to a northwest flow pattern. But one thing has remained constant, the chances of showers during the wettest time of the year across the Cowboy State. And this will continue through much of the next 7 days, although at varying coverages and intensities. The culprit for today's precipitation is a shortwave moving in from the northwest. We already have a few showers out there this morning, and these will increase in coverage as we head into the afternoon. And some of these showers could come with thunder, commonly known as thunderstorms. We do have some decent CAPE this afternoon, topping out around 500 J/Kg, although the lifted indices are not the best I've seen, generally around minus 1. So, I would put the chance of a thunderstorm at any location around 1 out of 4, with a greater chance of showers. And like previous days, the best chance of showers will be in northern Wyoming, with tapering chances further south. Precipitable waters are not terribly high though, generally topping at around 0.40 inches. And, with the storms moving at a decent clip, the threat of flooding looks small. So, the main threat would be gusty wind. I could also see some small hail, given the low wet bulb zero levels and cool temperatures, averaging around 10 degrees below normal. Wind will also be a concern today. The main area of concern looks to be northern Johnson County and the northern Big Horn Basin, with the favored northwest flow pattern behind the shortwave. Low level wind does not look sufficient for widespread high wind though, with 700 millibar wind topping out at around 40 knots when I would like to see 50 knots. Ensembles are giving around a 2 in 3 chance of gusts past 40 mph though, especially at Buffalo. So, it looks like a toupee alert day for many locations. Most showers should end by late this evening. And, there will be some snow in the mountains, with 700 millibar temperatures between minus 1 and minus 3, putting snow levels between 7000 and 8000 feet. Impacts would be minimal since the strong May sun should melt snow off of the roads. Shortwave ridging then moves across the state on Friday. This should cap the atmosphere a bit more and allow for somewhat warmer temperatures. We will still see some showers, but coverage will be less with most locations likely staying dry most of the time. Again, (noticing a pattern here), the best coverage will be across the north with southern Wyoming mainly dry. Saturday looks like the warmest day of the period as flow turns southwesterly ahead of the next weather system. The deepest moisture will stay west of the area on this day, but enough will move eastward and there will be a few showers and thunderstorms late in the day and in the evening. But, this day again looks like most areas will be dry most of the time so if you want do to something this weekend, Saturday definitely looks like the best day to do it. Then on Sunday into Monday, a big atmospheric bowling ball, AKA an upper level low, then moves into the Rockies and brings another period of wet weather. These lows are notoriously fickle with movement so details about timing of precipitation and amounts are still very much in flux. We do know a few things though. One, we have the concern of mountain snow, and potentially a decent amount of it. Models are indicating 700 millibar temperatures may fall as low as minus 3 in portions of the area, which could drop snow levels as low as 7000 feet on Monday. Probabilistic model guidance gives around a 1 in 2 chance of greater than a foot of snow from Sunday morning through Tuesday morning above 9000 feet across portions of the mountains. This is especially so in the ranges East of the Divide, where there would be some wrap around moisture and upslope on Monday. Not a certainly by any means, but something we have to watch. This could be a wet system, with a greater than 2 in 3 chance of over a half an inch or rain across the northern two thirds of the area through the period with favored northeasterly upslope locations having a greater than 1 in 2 chance of at least an inch of precipitation through 6 am on Tuesday. As a result, we will also have to watch for rising rivers. Melting snowpack would not be a concern though, given the cool temperatures though. Things get more muddled past Monday, as there are different solutions in regards to how fast the low will move away to the east. Most guidance shows ridging building behind the system, bringing drier and warmer weather, but when this will occur is still uncertain. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 341 PM MDT Thu May 15 2025 Shower and/or thunderstorm chances (30%) continue through sunset (at the latest) at all terminals except KPNA and KBPI. At KPNA and KBPI, showers or thunderstorms should remain over the mountains. Shower and/or thunderstorm chances are a PROB30 group for the terminals given the scattered nature and uncertainty in timing and direct impacts. Heavier showers in thunderstorms could drop visibilities and ceilings to MVFR flight conditions; however, confidence in this occurring is not high enough to include in TAF. Otherwise, thunderstorm chances diminish after sunset at all terminals. At KRIW and KLND there remains around a 60% chance in light rain between sunset and 05/06Z with a 30% chance at MVFR ceilings with the light rain. Shower chances (30%) increase again after 20Z Friday at KJAC and KCOD. Chances in MVFR ceilings have dropped to around 30% at KRKS overnight so both KRKS and KCOD have a 30% chance at overnight MVFR ceilings. This potential is communicated with a SCT030 group. Gusty winds should diminish at all terminals by sunset. KCPR can expect gusty winds the entire TAF period. 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... None. && $$ UPDATE...Hensley DISCUSSION...Hattings AVIATION...Gerhardt