Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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255 FXUS65 KRIW 281918 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 118 PM MDT Fri Jun 28 2024 .KEY MESSAGES... - Slightly below normal temperatures continue for the rest of the day and gusty WNW winds persist through sunset, with very dry air across central WY. Elevated fire weather conditions exists for parts of central and southern WY. - Isolated thunderstorms are possible this afternoon along the WY/CO border, and a cool front will push southward out of MT into northern WY this afternoon and reach central WY this evening. - Near normal conditions with mostly to partly cloudy skies are expected Saturday - Sunday will see windy and gusty southwest winds with very warm and dry air over much of the area. NW WY will see isolated showers and thunderstorms during the late afternoon and evening. - A broad storm system across the northern Rockies will bring much cooler air and a chance of showers and thunderstorms Monday and Tuesday. && .DISCUSSION... Issued at 115 PM MDT Fri Jun 28 2024 As a secondary trough moves through Montana this afternoon, the mid- and upper-level gradient is strengthened across WY, bringing breezy/windy and gusty WNW flow to the forecast area. Temperatures have cooled to below normal though very dry air of humidities below 15 percent has developed across parts of central and southern WY. A moisture boundary along the WY/CO border should allow for isolated thunderstorm development across far southern Sweetwater County, though most models are indicating they should stay south of the border with perhaps some gusty outflow pushing back into WY. As the MT trough moves eastward, a cool front is expected to push southward into northern WY along and east of the divide. Breezy and gusty north winds will move southward during the afternoon, increasing humidities and reach central WY early in the evening. As the central/southern WY winds diminish this evening and the pressure gradient relaxes, the north/northeast winds will then move into Sweetwater County by midnight. Higher RH values in this air mass could lead to localized low clouds in the Bighorn Basin and Johnson County overnight. Saturday will see high pressure and the ridge axis build over the area, leading to breezy west winds across southwest WY, and cooler east/southeast winds east of the divide. Temperatures will return to near normal for late June for a very nice mid- summer day. Mostly clear skies in the morning should become partly cloudy is some areas with clouds moving northward from eastern UT. Sunday will see strong dry southwest flow return to the forecast area and a trough deepens over the PacNW and increases moisture and energy into western MT. As a result, temperatures will climb sharply to be 10-12 degrees above normal for 80s and 90s (even near 100) in the lower elevations. As SW winds increase to 15-25 mph and 25-40 mph gusts with humidities dropping to below 15 percent, fire weather conditions will become elevated. YNP is about the only place to not see these conditions as cloud cover with a slight chance of storms move over NW WY in the afternoon and evening. Models are currently showing the first wave of the broad trough to move strongly into NW WY after midnight early Monday morning, increasing showers and isolated thunderstorms across western and northern WY. Temperatures will cool again on Monday as weak instability bring continued showers and thunderstorms through the day along with breezy westerly wind. The best chances remain across the northern half of the forecast area. A secondary wave in the trough then moves through southern MT early Tuesday, continuing the northern WY showers into the afternoon as well as the cooler temperatures. Southern WY however will see dry westerly flow. Dry zonal flow returns on Wednesday to increase temperatures and west winds. Thursday and Friday appear to have another situation of dry, very warm, and windy across SW WY, and cooler, moist, and north wind breezy east of the divide as a couple of shortwaves drop down from Canada into the northwest Plains. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1125 AM MDT Fri Jun 28 2024 All terminals to be VFR through the forecast period. A shortwave passing north of the state nudges a frontal boundary south through the Bighorn Basin this afternoon and central Wyoming early this evening. Gusty westerly wind 15-30kts prevails ahead of this boundary with similar speeds expected from the north following passage of the boundary. KJAC will not see the strong northerly push, although downvalley light northerly flow develops there by late evening. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued at 1245 PM MDT Fri Jun 28 2024 Elevated fire weather conditions continue this afternoon. Minimum RHs between 10% and 15% for much of Sweetwater, Fremont, and Natrona Counties, as well as parts of southern Lincoln and southern Sublette Counties. A Rangeland Fire Danger Statement (RFD) is in effect this afternoon for those areas. Elsewhere, RHs down to around 20-25% in the lower elevations and east slopes of the Wind River Mountains and Wyoming Range. Gusty west winds have picked up during the late morning for the Wind Corridor from southwest WY into central WY. Afternoon gusts 30 to 40 mph will continue through 1900 to 2100. A cool front brings a north wind shift to the Bighorn Basin and Johnson County this afternoon, with gusts to 30-40 mph possible. This pushes down into Fremont and Natrona Counties this evening, with breezy northeast winds even getting into Sweetwater County around midnight. Dry conditions continue for Saturday, but winds are much lighter. Elevated fire weather conditions return for Sunday, with RHs in the upper single digits to low teens for much of the area. Gusty winds also return, with high temperatures around 10 degrees above average. && .RIW WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...McDonald AVIATION...CNJ FIRE WEATHER...McDonald/Wittmann