Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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423 FXUS65 KRIW 141750 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1150 AM MDT Sat Sep 14 2024 .KEY MESSAGES... - Warmer and dry today. A breezy afternoon for much of the area brings elevated to critical fire weather conditions. - Warmer Sunday. An approaching weather system brings isolated showers and thunderstorms in the afternoon across western Wyoming. - Eastward spreading rain/mountain snow chances Sunday night through Tuesday. A cold front brings cooler temperatures Tuesday. - Active weather for the end of the workweek, favoring cool and wet weather. && .DISCUSSION... Issued at 112 AM MDT Sat Sep 14 2024 With an approaching upper low still a couple days out and high pressure over the southern United States, Wyoming sits in the middle of this synoptic set-up through the weekend. Today, upper flow is southwesterly, allowing some warmer air to makes its way in. Highs peak 5 to 10 degrees warmer today compared to yesterday. This means low to middle 70s west of the Continental Divide, with middle 70s to low 80s east. These values are near, to a bit above, average for mid- September. The main weather story for today is fire weather concerns. Given the southwest flow and decent pressure gradient, winds begin to increase late morning. The least windy locations look to be the central basins, like the Wind River and Bighorn Basins. Elsewhere, gusts 15 to 25 mph occur. The windiest locations are those favored in southwest flow, such as the Wind Corridor from roughly Rock Springs to Casper. Models are also indicating some windier locations for southwest Wyoming, like in Lincoln County, extending a bit into the Green River Basin. For these windier locations mentioned, gusts of 25 to 35 mph are likely (60%-90%). Where these windier locations are, relative humidities (RH) will also be to critical levels, generally 10 to 15 percent. The 00Z HREF also highlights the Wind Corridor and southern Lincoln County with 60% to near 100% chances of Red Flag Conditions today. Will continue the Red Flag Warnings for this afternoon and evening, with the addition for likely (70%) Red Flag conditions in southern Lincoln County. The slight lack in confidence for this area comes from two factors. The first is if critical RHs are going to be widespread enough. The other is if winds of 25 mph or more are going to occur for a prolonged period of time. Current thinking is these conditions will occur, but it may end up being a marginal day. Sunday, flow turns more southwest to south ahead of an approaching upper-level low moving into the Pacific Northwest. Winds should look similar to the previous day, with emphasis over the Wind Corridor. Afternoon RHs bring another day of elevated fire weather conditions. Will hold off on any highlights for now as winds are looking a bit too low, and RHs improve very slightly across southern Wyoming. Most of the day remains dry, though some leading moisture could spark a shower or thunderstorm over the west, though chances are about 20% or less through much of the afternoon. Late Sunday evening through Monday morning, better moisture filters in. Current models keep precipitation chances along and west of the Divide through Monday morning as the upper low takes more of a due south direction. Chances spread east of the Divide Monday evening as the low begins tracking eastward. The low then tracks northeast through Tuesday, the center of it clipping northwest Wyoming. This should produce the best precipitation chances over western and especially northwest Wyoming. This being said, everywhere looks to have at least a 20% chance of showers or thunderstorms, those 20% chances being more for eastern Sweetwater and southeastern Natrona Counties - locations furthest from the low`s center. Cooler temperatures also arrive Tuesday as the low`s cold front passes through. Given 700mb temperatures around 0 to 4 degrees Celsius, snow levels as low as 8500 feet could occur, but should generally be 9000 feet or higher. Low elevation highs ranging from the 50s to middle 70s occur for the middle and end of the workweek. Past this system moving out Wednesday, long range models diverge a little bit on the details, but show another system moving through. This low travels further south, moving through or just south of Wyoming in the Thursday to Saturday timeframe. In other words, continued active weather for the second half of the week as well, with a favoring for cooler and wetter weather. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1150 AM MDT Sat Sep 14 2024 Besides high cirrus clouds, mostly clear skies are forecast this afternoon through tonight. Several locations will see typical afternoon breezes develop, with winds quickly decreasing just before sunset. The drier air in place tonight should keep patchy fog out of the KJAC area for Sunday morning. Sunday afternoon will feature a slight increase in winds for sites west of the Continental Divide. Scattered showers and a few weak thunderstorms could bring (20% chance of) brief strong and erratic winds gusts for KJAC, KBPI, KPNA, and KRKS. && .FIRE WEATHER... Issued at 112 AM MDT Sat Sep 14 2024 Elevated to critical fire weather conditions this afternoon and evening. A Red Flag Warning is in effect for fire zones 277, 280, 289, and 300 from noon until 7pm MDT today. RHs 10 to 15 percent occur for these zones, as well as most lower elevation locations east of the Divide and across zone 279. Afternoon winds gusting 15 to 25 mph will be common. For the zones with a Red Flag Warning, favored southwest wind brings gusts more in the 25 to 35 mph range. Breezy and dry again Sunday, with elevated fire weather conditions. Some showers and thunderstorms begin moving into western Wyoming in the afternoon. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning until 7 PM MDT this evening for WYZ277-280- 289-300. && $$ DISCUSSION...Wittmann AVIATION...VandenBoogart FIRE WEATHER...Wittmann