Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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203 FXUS65 KRIW 170415 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1015 PM MDT Mon Sep 16 2024 .KEY MESSAGES... - Widely scattered showers and a few thunderstorms will occur this afternoon, as a storm system over CA moves closer to the region. - A cold front pushes through Tuesday with more scattered rain showers and isolated thunderstorms. Severe wind gusts are possible across eastern portions of the CWA. - Cooler temperatures and continued unsettled weather through the rest of the week and into the weekend with another system. && .DISCUSSION... Issued at 1212 PM MDT Mon Sep 16 2024 The closed upper low has moved onshore over northern CA this morning and will gradually make its way over the Great Basin tonight. This will result in a diffluent southerly flow over the Cowboy State today. Elevated to near-critical fire weather conditions will continue again today as winds increase across southern portions of the CWA, pushing into central portions through the afternoon. RH values will not be as low as Sunday, but near-critical values will occur across areas east of the Divide. Showers and thunderstorms will be split to western portions and eastern portions (Johnson/Natrona counties) this afternoon and evening. Additional showers and thunderstorms could move over Sweetwater County at varying times this afternoon and evening, but are expected to be short-lived. An isolated shower could also occur over the central basins, but confidence is low. The main impact from all this activity will be strong outflow boundaries once again. Dewpoint depressions of 40-50 degrees remain in place, so wind gusts over 50 mph will be likely. An isolated stronger gust of 60+ mph cannot be ruled out either. Isolated showers will remain possible over far western portions through the night tonight. The closed low will continue its eastward progression through the night tonight, as previously mentioned above, reaching the ID/UT border by 18Z Tuesday. Precipitation chances will increase across the west as a result, with an additional area of showers and thunderstorms developing over the Wind Corridor (the eastern half of Sweetwater County to Natrona County) as a result of the left front quadrant moving of the area. This activity will quickly spread into Johnson County by early afternoon, with surrounding areas (Wind River and Bighorn Basins, Bighorn Mountains) being impacted as well. The associated cold front will reach western portions between 12Z and 18Z Tuesday. There are now indications of a leeside low developing over Sheridan County/southern MT between 20Z and 00Z Wednesday, which will also coincide with any severe potential for the thunderstorms across eastern portions. A lack of moisture looks to be the main culprit for this. For now, it looks like most of this potential will be pushed out of the CWA and in UNR and CYS areas. 700mb temperatures will begin to drop Tuesday morning, averaging around 2C through the rest of the day, leading to snow levels around 9000 ft once again. The sfc pressure gradient will tighten irt to the upper low as well, resulting in southwest winds gusting 35-50 mph from the eastern half of Sweetwater County to Johnson County, including the Wind River Basin. Cooler temperatures will occur Tuesday night, as the cold front exits to the east and the upper low tracks into MT. Snow levels over northwestern mountains could drop to 7500 ft late in the afternoon into Tuesday night, due to a pocket of sub-0C air from the core of the upper low. Overall, this pattern is similar that just occurred last Wednesday/Thursday except for the low becoming negatively-tilted. West-southwest winds in the wake of the storm will lead to downsloping winds gusting 50-60 mph off the east slopes of the Wind River, Absaroka and Bighorn Mountains Tuesday night into Wednesday morning. The winds, though not as strong, will spread across southern Fremont/Natrona counties through the overnight hours, with gusts of 50-55 mph impacting these areas including the Casper area. As this system exits Tuesday night into Wednesday, another system is soon to follow. This one pushes further southward, over southern NV on Thursday. Models have come into better agreement on placement and speed, as it slowly traverses the Four Corners Friday and clipping southern portions of the Cowboy State Saturday as it exits over the Central Plains. Winds will remain gusty Wednesday in the wake of Tuesday`s storm and looks to be the windiest day through the rest of the week. Temperatures will remain seasonal through the rest of the week as well. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1015 PM MDT Mon Sep 16 2024 The large upper-low will move to the northeast into MT through the next 24 hours. As it cyclonically rotates it will bring waves of rain showers and thunderstorms through the area Tuesday morning into Tuesday evening. PROB30 groups are in the forecast for the time period of highest likelihood for prevailing TSRA and MVFR/IFR conditions. A strong cold front will sweep through Tuesday morning/afternoon from west to east, with strong wind ahead of and behind the front. A wind shift from southerly to westerly will occur once the front passes through. Much drier air will surge into the area behind the cold front across the southern half of the area, so this will shut off shower activity for most terminals. Terminals that will be closer to the center of the low will have lingering rain showers into Tuesday evening, which includes KBPI/KPNA/KJAC/KCOD. A gap flow situation is possible (30- 40%) at KCOD Tuesday evening into Tuesday night as the center of the low will be situated just north of KCOD in MT. Westerly flow through the Shoshone Canyon will funnel the already strong wind toward KCOD, which may result in very strong wind Tuesday night. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued at 1210 PM MDT Mon Sep 16 2024 Elevated to near-critical fire weather conditions continue today with southerly gusts up to 25-30 mph across the southern half of the forecast area. RH values will be slightly higher than Sunday, but will still range from the upper teens to lower 20s with isolated locations reaching critical thresholds. Showers and thunderstorms will be widespread across the area this afternoon and evening, with strong outflow gusts being the main threat. Isolated showers will remain possible over western portions tonight. Showers and thunderstorms will increase across the area late Tuesday morning into early afternoon with a chance for wetting rain, as a cold front moves across the area. Western portions will have the best chances. Strong winds will occur Tuesday night into Wednesday across zones 276, 280, 288, and 289. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...LaVoie AVIATION...Rowe FIRE WEATHER...Lowe