Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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458 FXUS65 KRIW 180405 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1005 PM MDT Mon Jun 17 2024 .KEY MESSAGES... - An abrupt wind shift and the arrival of cooler air follow passage of a cold front sweeping east across the region through Monday evening. Light snow falls in northwest Wyoming later this evening and overnight. - The potential for strong to severe storms exists over mainly northern Johnson County between 3PM and 6PM MDT. - Tuesday morning is chilly across the western valleys and basins as overnight lows fall to 27-32F. - After an unseasonably cool Tuesday, temperatures begin to climb Wednesday through the end of the weekend. The best chance for widespread showers and storms is Friday. && .DISCUSSION... Issued at 125 PM MDT Mon Jun 17 2024 As of early Monday afternoon, satellite clearly shows an approaching shortwave near the ID/MT border along the Bitterroot Range. A jet streak has rounded the base of the associated trough and is making its way toward southwest Wyoming. This SW-NE oriented jet streak will aid precipitation production late this afternoon and evening as it traverses the forecast area. Additionally, a surface cold front moving through far west Wyoming continues east and works in concert with the jet streak to boost coverage of showers and thunderstorms east of the Continental Divide late Monday afternoon and evening. Gusty west to southwest wind veers to the northwest behind the cold front. This will led to persistent 20 to 35 mph northwest wind through Monday evening in favored locations such as northern portions of Johnson County and the Bighorn Basin, along with the Wind River Basin. A prevailing south-southeast surface wind has increased dew points into the mid 40s across Johnson and Natrona Counties as of 1PM Monday. CAPE is forecast to reach 500-800 J/kg between 2PM and 4PM and CAMS depict convection strengthening during this time as CIN slowly erodes. The strongest storms are expected over northern Johnson County ahead of the front, with the deepest convection moving east and coming to a close between 6PM and 7PM Monday. SPC has included a slight risk for this region. Large hail will be the primary hazard with the strongest storms. To the west, orographic northwest flow will favor a period of precipitation this evening and overnight across northwest Wyoming. The flow regime favors the heaviest snow in the higher elevations of the Teton Range, while lower elevations above 7000 ft MSL in Yellowstone could see snow accumulations up to around one inch. The best chance for snow accumulation is from 8PM Monday to 2AM Tuesday. As cooler air filters into the region, Tuesday morning lows will fall to 27-32F. Cloud cover may elude locations south of an Alpine to Bondurant to Pinedale line, where the coldest temperatures are most likely. It would not be surprising to see overnight lows in the mid 20s in a few locations, like Bondurant and south Lincoln County if the westerly wind diminishes under a clear sky by sunrise Tuesday. Tuesday keeps a moist westerly flow across the northern-third of the forecast area. Precipitation lingers across this region through the morning and becomes more widespread as instability increases Tuesday afternoon. Temperatures will be unseasonably cool Tuesday with most locations in the upper 50s to upper 60s, about 8 to 15 degrees below normal. The trough departs to the northeast Wednesday and temperatures begin to rebound and most areas return to dry conditions. The exception could be the I-25 corridor late in the day. A weak trough over California Thursday ejects eastward and crosses the forecast area Friday. The Friday convection provides the best chance for widespread showers and thunderstorms during the Thursday to Sunday period. Temperatures warm Thursday and are somewhat hindered by cloud cover and showers Friday. Nonetheless, seasonal readings are anticipated both days. Zonal flow in the wake of this wave allows for upward trending temperatures and drier weather Saturday. Ridging then builds over the Northern Rockies Sunday leading to above normal temperatures and dry conditions to end the weekend. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1004 PM MDT Mon Jun 17 2024 Rain and even some snow showers are ongoing across much of the area late this evening behind a cold front that has pushed east. This activity will decrease over the next few hours, leaving VFR conditions prevailing through most of Tuesday morning. This has also brought gusty winds, which will linger at KCPR and KRKS all night. By Tuesday afternoon showers will return to the northern half of the area as moisture wraps back in to Wyoming. The best chance for terminal impacts will be at KCOD, but certainly can`t rule out brief impacts at KJAC and KWRL. This activity will end around sunset Tuesday. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION... AVIATION...Myers