FXUS65 KRIW 190444 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1044 PM MDT Mon May 18 2026 .KEY MESSAGES... - Cloud cover will gradually decrease from the northwest overnight leading to widespread sub-freezing temperatures east of the Continental Divide early Tuesday morning. Fresh snow cover in some areas could allow temperatures to drop into the teens. Freeze Warnings are in effect Monday night. - Conditions will be dry and temperatures rebound Tuesday and Wednesday, but daytime highs remain well below normal. The arrival of another weather system Thursday cools temperatures a few degrees and provides another round of precipitation. - Early indications point to a much warmer and dry Memorial Day holiday weekend with widespread 75 to 85 degree daytime highs. && .UPDATE... Issued at 1235 PM MDT Mon May 18 2026 The snow and wind across the area has been mostly as forecasted. Greatest impacts were around South Pass and I-80, with winter travel impacts going into this afternoon. Conditions should slowly improve through the evening as snow decreases and winds diminish. As was the trend in the day leading to the start of the event, northern locations have not seen as much (or any) snow, so have ended the advisories there early. As the system exits, very cold temperatures remain in place, with lows tonight in the upper teens to mid-20s for a majority of the area. Cooler temperatures remain in place Tuesday and Tuesday night, with a very gradual warm up into the end of the week. && .DISCUSSION... Issued at 240 AM MDT Mon May 18 2026 Rain changed over to snow in the Wind River Basin and across Sweetwater County just a few hours ago after 11pm Sunday night. Much of Natrona County is around 37F degrees as of 08Z (2am), with precipitation still falling as rain. The forecast remains on track this morning, with the threat of snow and wind impacting areas from South Pass southward into Sweetwater County. The 700mb low from this storm system will be passing close to the CO/WY border this morning, resulting in a tight temperature and pressure gradient. 700mb northeast winds of 50-60kt will occur from the Green Mountains/Rattlesnake Range into Sweetwater County. This will result in wind gusts of 60 to 70 mph occurring this morning over locations like South Pass down to the Rock Springs/Green River area. Windy conditions (wind gusts up to 35 mph) will be widespread across the CWA this morning as the storm system moves over CO this morning. Snowfall amounts of 2 to 6 inches is still expected across portions of Natrona County, the Lander Foothills and the eastern half of Sweetwater County. Travel conditions could be difficult this morning due to the blowing snow and icy roads. Impacts from snow accumulations on roads is likely to only last through 15Z/16Z (9am or 10am) as it will be more difficult for the snow to stick due to the high sun angle. These impacts will begin to decrease through the morning, with areas like Lander and Casper improving first as snow begins to end toward midday. Impacts along I-80 will last the longest due to the proximity of the 700mb low, but is expected to wane between 18Z and 21Z as snowfall ends. Winds will be decreasing across the area after 18Z as well, with the stronger winds of 35 to 40 mph continuing from South Pass southward into Sweetwater County and decreasing through the afternoon. Needless to say, temperatures will be anywhere from 5 to 25 degrees colder today and about 15 to 30 degrees below normal. Winds will continue to decrease this evening, as the storm system quickly exits over the Central Plains and light winds (10kt or less) expected for most areas by 06Z Tuesday. Fog development could occur as late as sunrise Tuesday morning as clouds are expected to remain in place across central and southern portions of the CWA through much of the night tonight. Dry and warming conditions are expected Tuesday and Wednesday, with another Canadian storm system impacting the area on Thursday. This system doesn't look to be as cold as today's storm and snow levels staying around 7000ft at this time. Model guidance remain split at this time, but confidence is medium for a mainly dry holiday weekend for at least 2 of the 3 days. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1040 PM MDT Mon May 18 2026 West of the Divide...KJAC/KRKS/KPNA/KBPI Terminals Shortwave energy within a longwave trough traversing Wyoming Monday evening has been responsible for lingering IFR/MVFR light snow at KPNA and KBPI. Current satellite and model trends indicate this activity ends around 08Z/Tuesday with low MVFR cloud decks to persist afterward. There is still a chance of LIFR/IFR fog and low clouds at both terminals, particularly KBPI, until 15Z/Tuesday. There may be similar IFR/MVFR at KRKS during that time, but confidence is low given current model trends. All terminals are VFR after 16Z/Tuesday through the remainder of the period. Enough mid-level moisture and instability is present for cumulus to form over the mountains and drift into the valleys/basins late Tuesday afternoon. A few showers could occur around KJAC, but confidence is low for any impacts. Mountain tops frequently obscured through 16Z/Tuesday. East of the Divide...KCPR/KCOD/KRIW/KLND/KWRL Terminals Shortwave energy within a longwave trough traversing Wyoming Monday evening has been responsible for intermittent light snow at KCPR. Given current satellite and model trends, this activity should end before 07Z/Tuesday. There could be a brief period of fog and IFR ceilings at KCPR overnight before increasing southwest wind arrives around 11Z/Tuesday. Other terminals to be VFR through the period, although fog may be in the vicinity of KRIW and KLND shortly after sunrise Tuesday. Enough mid-level moisture and instability is present for cumulus to form over the mountains and drift into the valleys/basins late Tuesday afternoon. A few showers could occur around KCOD and perhaps KLND, but confidence is low for any impacts. Mountain tops frequently obscured through 12Z/Tuesday. Please see the Aviation Weather Center and/or CWSU ZLC and ZDV for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... Freeze Warning until 9 AM MDT Tuesday for WYZ003>006-010-011- 016>020. && $$ UPDATE...Wittmann DISCUSSION...LaVoie AVIATION...CNJ/LaVoie