Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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529 FXUS65 KRIW 201057 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 457 AM MDT Mon May 20 2024 .KEY MESSAGES... - Cool and wet weather returns to the area today into Tuesday. - Several inches of snow are likely in the northern mountains tonight and Tuesday. - Below normal temperatures and continued chances of showers continue through the week. && .DISCUSSION... Issued at 330 AM MDT Mon May 20 2024 After a few days of typical May weather, the pattern will take a step back in the time machine and make things look more like April. Colder air has filtered into the area this morning. The main driver of the weather over the next couple of days will be a deep trough now over the Pacific Northwest that will slowly drift eastward and bring a period of wet weather. The steadiest precipitation today will be focused along a right rear / left front jet couplet that will move across the area. This will focus the heaviest precipitation roughly from Sweetwater County through Fremont and into Natrona Counties, but all areas will have a chance of some showers and even an isolated thunderstorm. The thing many will notice are the temperatures, which will average 15 degrees cooler then yesterday and similar readings below normal. This will continue into tonight. The main player then becomes the trough that will close off into an upper level low that will slowly move across northern Wyoming. And that will transition the steadiest and heaviest precipitation to northern Wyoming, with lesser amounts further south, especially on the other side of the Divide. And that transitions another concern, the mountain snow, how much of it and the impacts of it. Models have finally come into somewhat better agreement in putting the greatest QPF across the northern ranges, mainly the Bighorns, Absarokas and to a lesser extent, the eastern Wind River Range with more favorable upslope flow developing. Maximum impacts will travel from west to east, with the heaviest in the Eastern Wind Rivers and Absarokas late tonight into Tuesday morning, and the Bighorns on Tuesday. As for highlights, we have gone with advisories rather than warnings. Why you ask? Well, amounts in the Wind Rivers are in the advisory range. In the Absarokas, there are some locations with warning level amounts, with a few areas in the southern Absarokas having a greater than 2 in 3 chance of 12 inches or more. However, these are in areas with few people, roads or impacts. The Bighorn Range has the best chance, with a decent area having around 1 in 2 chance of 12 inches or more. The difference here is that the heaviest snow will occur during the day on Tuesday. And the combination of the high sun angle and warm ground temperatures will make it difficult for snow to accumulate on roads during the day. So, we went with advisories here as well. As for snow levels, the coldest most 700 millibar temperatures get is around minus 4C, which would put snow levels at around 6500 feet. So, lower elevations look to have mainly rain. A place like Dubois could see a coating though. And I can`t rule out some flakes in the air at night if rates are heavy enough in the lower elevations. QPF amounts look to average between 0.25 and 0.90 inches across most of the area. As for the threat of flooding, it looks low right now. The rain should fall over a long period of time for one. We have also had 4 days of dry weather, so the soil can absorb some rain. And, with the cold temperatures in the mountains, snowmelt will be slim to non existent through the period. So, no flood highlights at this time. Most of the area will have a brief break on Wednesday with transitory ridging over the area, although another approaching wave will bring some additional showers to the west at this time. This wave and low will cross the area on Thursday. Models have trended somewhat further north with it this morning. This would keep the heaviest rain over Montana, but models have not been consistent with it so uncertainty is higher. Another wave may approach for Friday or Saturday, but there is more spread on timing with this. The area will remain in zonal flow to a weak trough through the period, so the trend of below normal temperatures and above normal precipitation should continue at least to the start of the upcoming holiday weekend. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 457 AM MDT Mon May 20 2024 Showers will continue from the southwest corner of WY to Natrona/southern Johnson County to begin the TAF period, skirting the KRIW/KLND and KCPR terminals. These showers will continue through the rest of the morning, becoming widespread across the rest of the CWA through the afternoon. MVFR conditions are expected with these showers. Any shower activity near KJAC will be late this afternoon into the early evening, but confidence is too low to include in the forecast at this time. Precipitation will become more focused east of the Divide after 00Z, with rain and mountain snow and mountain obscurations. IFR/MVFR conditions will occur with the rain, with LIFR/IFR conditions more common after 06Z. Although temperatures will be well below normal for mid-May, snow is not expected. 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 from 6 PM this evening to 6 PM MDT Tuesday for WYZ002-008-009-015. && $$ DISCUSSION...Hattings AVIATION...LaVoie