Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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702 FXUS65 KCYS 022031 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 231 PM MDT Tue Jul 2 2024 .KEY MESSAGES... - Thunderstorms are possible on Wednesday afternoon, especially across east central Wyoming & the western Nebraska Panhandle. Any storms which develop will have the potential to become severe with all hazards possible. - A cool and pleasant Independence Day is expected in the wake of a strong cold front set to sweep across the area on Wednesday night. && .SHORT TERM /THROUGH THURSDAY NIGHT/... Issued at 225 PM MDT Tue Jul 2 2024 Current Visible Satellite loop and surface observations this afternoon show dissipating showers and thunderstorms with patchy fair weather CU across the mountains and eastern plains. The first upper level trough axis has pushed eastward into western Nebraska at this hour with drier air moving into the bulk of the forecast area with dewpoints dropping into the 20s and 30s across most of southeast Wyoming. Surface moisture remains in place for the northern panhandle of Nebraska, but upper level subsidence has greatly reduced convective activity across the area. Continued the trend of lowering POP and prob thunder late this afternoon through this evening with clear skies expected tonight and cool overnight lows. Surface winds are forecast to gradually shift into the east for western Nebraska with surface moisture increasing through midnight. This will set the stage for the potential of severe weather Wednesday as another strong cold front digs south out of western Canada. All models indicate the next upper level shortwave trough and associated cold front digging south across Montana and Wyoming on Wednesday. Will continue to monitor moisture transport closely, but as of right now, the best chance for strong and severe thunderstorms will be well east of Interstate 25...and more specifically along and east of a line from Chadron to Kimball Nebraska. Thunderstorms may get started pretty early depending on the speed of the cold front, with some high res guidance showing convective initiation as early as 11 am to noon. Thankfully, there should be a pretty small area that is under this severe weather threat in our forecast area with mainly Dawes, Box Butte, Morrill, and Cheyenne County in the most likely region, but this may change if moisture transport occurs further west towards I-25. All models of severe weather are possible if any discrete supercells develop, including tornadoes. High res guidance and the NAM continue to show solid 0-1km helicity of 200 to 300 within the area of peak 0-3 km max CAPE of 3000 to 3500 j/kg in the boundary layer. Definitely large hail and strong winds are possible threats, but a few tornadoes can not be ruled out as well late Wednesday morning through mid Thursday afternoon. There is additional concern for the North Platte River valley from Scottsbluff to Bridgeport/Broadwater and vicinity with models showing some backing of the surface winds late Wednesday morning and strong veering of winds aloft. Expect this activity to push east of these four counties as early as 200 to 300 PM mountain time, so a pretty quick event for our western Nebraska counties. However, the upper level shortwave is forecast to push across the northern zones (Douglas, Lusk, northern NE Panhandle) late Wednesday evening. There may be a second round of strong to severe nocturnal thunderstorms mainly north of Interstate 80 Wednesday night through sunrise Thursday. Otherwise, after a warm day on Wednesday, much cooler temperatures are forecast to settle across the area late Wednesday and Wednesday night with highs back into the 70s for all areas Thursday afternoon. With the cooler and more stable airmass, thunderstorm coverage will be more isolated on Thursday with mainly shower activity. Will also have to watch the potential for fog early Thursday morning due to the thunderstorm activity up north. Overnight lows Wednesday and Thursday night will be on the cool side for this time of the year with lows in the 40s to low 50s. A few high valley locations west of I-25 may see readings in the upper 30s just before sunrise Friday. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Issued at 208 PM MDT Tue Jul 2 2024 Near to slightly above average temperatures can be expected for this holiday weekend into early next week. A departing shortwave trough will eject out of the Northern Plains towards the Great Lakes region on Friday morning. We will be under the influence of weak CAA behind this shortwave trough, with 700mb temperatures being slow to rise through Friday afternoon. The northern forecast zones portions of the NE Panhandle will have 700mb temperatures only creep up to +5C to +8C by 0Z Saturday. However, the surface high pressure will keep skies mostly sunny, and daytime highs in the 70s to low 80s appear favorable. Areas in southeast WY, primarily west of I-25 will see 700mb temperatures rise to +10C to +13C for Friday, leading to warmer surface temperatures. We will remain with northwest flow aloft at H5 and H3 as an amplifying upper level ridge will slowly shift to the east by this weekend from the western CONUS. This northwest flow aloft will keep our cwa on the periphery of passing shortwave disturbances. Positively-tilted shortwave disturbances will eject out of the Canadian Rockies toward the northern Plains on Saturday and Sunday. This close proximity to the shortwave disturbances combined with daytime heating will create the potential for isolated rain showers and thunderstorms, especially Saturday night and Sunday. Moisture advection from the Gulf of Mexico should combine with weak atmospheric dynamics during this time. Have kept the mention of slight chance to chance of thunderstorms in the forecast for now, but we could see an isolated strong thunderstorm on Saturday evening and Sunday afternoon. Daytime highs on Saturday and Sunday will be in the 70s and 80s, with the cooler temperatures occurring on Sunday once the weak cool front passes through our cwa. By early next week, the aforementioned upper level ridge begins to advect to its east from the Great Basin area. While it will become somewhat elongated and flattened, 700mb temperatures will creep upwards during the afternoon hours. Model guidance is trending with +10C to +17C for an average spread across our cwa. As a general rule of thumb, +14C is a good threshold to use for daytime highs near or above 90 degrees F at the surface during the afternoon. This will translate to daytime highs in the lower elevations of middle 80s to middle 90s, unless a shift in the pattern comes up in the next several rounds of model guidance updates. Stay tuned to weather updates as we have a gradual warmup trend on its way, making it feel like summer once again across the region for the extended forecast. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1125 AM MDT Tue Jul 2 2024 VFR is expected at all terminals through the forecast period. VCSH and VCTS will be possible after 18Z for a couple of terminals, but VIS impacts should be minimal. Wind gusts of 20-35 knots can be expected from 18Z-01Z for most terminals before tapering off to 12 knots or less overnight. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && $$ SHORT TERM...TJT LONG TERM...BW AVIATION...BW