Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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189 FXUS65 KRIW 170744 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 144 AM MDT Tue Sep 17 2024 .KEY MESSAGES... - Widely scattered rain showers and thunderstorms this morning west of the Divide that will spread east in the afternoon. - A cold front pushes through west to east from late morning through mid afternoon with gusty west to north of westerly winds behind it, some of which could be high wind warning criteria. - Cooler temperatures for the rest of the week and into the weekend with continued precipitation chances throughout as well. && .DISCUSSION... Issued at 122 AM MDT Tue Sep 17 2024 IR currently depicts some dry air intrusion across the CWA ahead of the main upper level low pushing across Nevada. Any rain shower activity has dissipated and pushed into Montana out of the Bighorn Basin. An upper level shortwave will begin to push out of northern Utah/eastern Idaho by mid morning that will spark some widely scattered rain showers and isolated thunderstorms west of the Divide. Another area of note will be from Sweetwater County up to Natrona with an area of surface convergence ahead of the cold front. This will all spread north and east into the afternoon hours becoming more numerous with daytime heating. Hi-Res models are picking up on leeside cyclogenesis along the triple point off the Bighorns that will strengthen these aforementioned thunderstorms along the I-25 corridor by 3-4PM before exiting to the east in a short hour or two. Main concerns will be with damaging winds as the main threat with the support a strong mid level jet over 50kts that will aid in mixing to the surface in this area of Johnson County east of the interstate. Otherwise, the bulk of the main energy will be well east of the CWA where more ample CAPE and steeper lapse rates will be. The main concern outside of precipitation activity will be with FROPA and wind shifting out of the west to north of west. Timing is more aligned with the models improving confidence of pushing out of Idaho by late morning and progressing east through the afternoon hours exiting east by 4-5PM. On the backside of the low, moisture will wrap back into northwestern portions of the state with the occlusion emphasizing the stronger winds with CAA. Wind gusts up to 50-60 mph are possible in some areas of the Cody Foothills (funneling) and into eastern parts of Fremont County and Natrona County as well. Other areas will see winds up to 40-50 mph, staying below any high wind thresholds. Precipitation will be for the northern third of the CWA with mountain snow likely for elevations of the Winds, Tetons, and Absarokas over around 8kft as the cold air pushes in behind the aforementioned cold front. Nothing significant in accumulations but a decent amount as the first bigger snow of the season. Lower elevations will see rain showers progress east into the Bighorn Basin after midnight and into Wednesday morning. Precipitation will linger in these areas into the afternoon as winds decrease by late morning and into the afternoon with a diminishing gradient behind the occlusion. A more stratiform type event through the afternoon and evening with the cooler air making its way across the CWA. Afternoon high temperatures will be in the 50s and 60s for points west of the Divide (and some even cooler), with 60s to low 70s to the east for the remainder of the week and into the weekend. Induced upper level ridging ahead of another deepening low making its way down the west coast for Thursday the the CWA remaining mostly dry for most. The EC has the low cutting off well to the south pushing across the Four Corner Region late week with the bulk of the greater impacts out of the state. Regardless, continued cooler temperatures and unsettled weather with daily precipitation chances to include snow in the higher elevations into the weekend and the extended forecast. && .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. && .RIW WATCHES/WARNINGS/ADVISORIES... High Wind Watch from late tonight through Wednesday morning for WYZ003-019-020. && $$ DISCUSSION...Lowe AVIATION...Rowe