Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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198 FXUS65 KRIW 160409 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1009 PM MDT Sun Sep 15 2024 .KEY MESSAGES... - Mainly dry today, with a breezy afternoon. Isolated (30% or less) showers and thunderstorms develop for western Wyoming this afternoon. Little rain expected, but strong outflow winds are possible. - A weather system pushes into the area Monday, with rain and mountain snow chances pushing east of the Divide late Monday through Tuesday. - A cold front moves through Tuesday, with cool temperatures and unsettled weather expected for the remainder of the week. && .DISCUSSION... Issued at 1042 AM MDT Sun Sep 15 2024 Weather today will be relatively quiet, as a longwave trough digs along the PACNW. A shortwave is moving northward over Sweetwater County this morning, with isolated showers and thunderstorms occurring over the northern half of the county. Isolated (30% or less) showers and thunderstorms will be mainly confined west of the Divide, but some of these could cross into the Wind River Basin and the Cody Foothills. Not expecting much, if any, rain to be falling with any of this convection this afternoon, as the boundary layer remains rather dry. Hence, strong outflow winds up to 50 mph will be the main threat due to the large dewpoint depressions of about 40 degrees. Showers and thunderstorms will remain possible tonight and become confined to far western portions of the CWA, as a closed low forms and moves onshore over northern CA. These shortwaves moving in from the south will also lead to moisture advection, so a wetting rain could occur in this area tonight. Speaking of dry boundary layers, fire weather conditions will be limited this afternoon. Critical relative humidity (RH) values will return once again east of the Divide, however winds are not expected to reach criteria. The closed upper low will slowly move over northern CA into the Great Basin through the day Monday. This will result in a diffluent southerly flow over the Cowboy State. Elevated to near-critical fire weather conditions will continue as winds increase across southern portions, pushing into central portions through the afternoon. Showers and thunderstorms will occur over southern portions again, as another shortwave arrives along the nose of the supporting jet of the upper low. Some of these will move over central portions through the afternoon as well. The closed low will continue its eastward progression through the night Monday night, reaching the ID/UT border by 18Z Tuesday. Most of the precipitation looks to stay along the eastern periphery of the low, which would keep it over western portions Monday night/Tuesday morning. This trend will likely continue through the day Tuesday, as there is a hint of a dry slow developing east of the Divide and Sweetwater County. The associated cold front will also reach western portions Monday night/Tuesday morning and continue eastward through the day Tuesday. 700mb temperatures will average around 2C across western portions, 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. There will be a chance for severe weather Tuesday afternoon over eastern portions of the CWA, as the low begins to pivot and become negatively-tilted as it moves over northwestern portions of the state. So far, models are not hinting at any moisture being in place in these areas likely due to the possible dry slot. We will certainly be keeping an eye on this for any changes. 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. Overall, this pattern is similar that just occurred last Wednesday/Thursday except for the low becoming negatively-tilted. As this system exits Tuesday night into Wednesday, another system is soon to follow. This one pushes more southward, into the Nevada area on Thursday. There is some model disagreement on exact placement. This low tracks eastward through Saturday. Highs at the end of this coming workweek range from around 60 to around 70. With this next system, temperatures look to fall a bit more for the weekend. Long range models are hinting at this second low to finally move through Saturday, with hints of another trough dropping southward from Canada around the Sunday time frame. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1008 PM MDT Sun Sep 15 2024 An active weather pattern means more chances for rain showers, thunderstorms, and strong outflow boundaries on Monday. Tonight a few rain showers will pass through western WY, with a TEMPO for - SHRA at KJAC from 07-11Z. Other terminals should stay dry and VFR tonight into Monday morning. Southerly flow will increase on Monday as an approaching upper-low moves into the region. Sustained wind will average 10-20 knots, with gusts of 20-30 knots. Convection will develop in the afternoon, with the best chances for terminals west of the Continental Divide and KCPR. All terminals have a good chance (50%) of strong outflows/downdrafts during the afternoon/evening hours. Monday evening into Monday night convection will hang around the low, so KJAC/KBPI/KPNA may have rain showers and thunderstorms continue. Wind will decrease substantially around sunset (01-02Z) for all terminals, but will remain around 7-12 knots through the evening hours. Conditions will remain VFR for all terminals through the TAF period, with the exception being if/when thunderstorms directly impact a terminal. If/when this occurs brief reductions in VIS to MVFR/IFR will result. 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 1042 AM MDT Sun Sep 15 2024 elevated to near-critical fire weather conditions will be in place today through Tuesday. Critical RH values will remain in place across areas east of the Divide this afternoon, while winds will be slightly below criteria. RH values will decrease slightly Monday and again Tuesday, as winds begin to increase much of the Casper dispatch area. A cold front will move across the area Tuesday, cooling temperatures back down to seasonal levels. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...LaVoie AVIATION...Rowe FIRE WEATHER...LaVoie