Area Forecast Discussion
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
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161 FXUS65 KCYS 050545 AFDCYS Area Forecast Discussion National Weather Service Cheyenne WY 1145 PM MDT Sat May 4 2024 .KEY MESSAGES... - Mainly dry and warmer conditions are anticipated for the weekend. - Strong winds (60+ mph) return to southeast Wyoming and western Nebraska Monday with a prolonged period of elevated winds continuing for wind prone locations through midweek. && .SHORT TERM /THROUGH SUNDAY NIGHT/... Issued at 153 PM MDT Sat May 4 2024 Ridging continues to develop over much of Wyoming and western Nebraska today with sunny skies dominating the forecast. Temperatures, while chilly, will only end up about 2-4 degrees below average in most places as heights rise and full sun allows for maximized solar heating and mixing evidenced in forecast soundings. To the east of the Laramie Range, a switch to south- southeasterly surface flow will begin to user in mild air. Overnight lows will tend to run a bit higher than guidance averages given the breezy surface flow east of the mountains. Overall expect mainly clear skies into Sunday morning. By Sunday afternoon, a potent low-pressure system will barrel through the central Rockies. Ahead of this system, fast south- southwesterly flow in the mid and upper levels will spread eastward through the Great Basin and intermountain region. Forecast ensemble guidance and blended output depicts a broad synoptically-driven high wind event affecting all of south- central Wyoming as well as northwest Colorado. Ensemble statistical output places surface gusts in the higher end of climatology, favoring a possible high wind event for portions of western Carbon County through the afternoon and evening hours. While recent national blend output in similar events has ended up higher than reality, have opted to issue a High Wind Watch for the aforementioned zones on Sunday afternoon given the strong signal in global forecast guidance. East of the Laramie Range in the high plains, moisture return in southeasterly flow out ahead of the advancing low pressure system will transport upper 40s to near 50 dewpoints into far eastern Wyoming. As height falls start inching toward the high plains by 0z, some longer-range CAM guidance suggests the possibility for convection to fire in the 0-2z timeframe. High- res guidance remains quite dispersive with potential outcomes on Sunday evening. Stronger forcing and a sharper dryline could be enough to overcome rather strong capping evidenced by a large warm-nose at 700mb in forecast soundings. This particular outcome is quite evident in the past several runs of the NAM Nest, which has consistently suggested a cluster of fast- moving organized convection in the Carbon/Niobrara County area, with more isolated single-cells as far south as the Platte/Goshen County region. The HRRR, given its dry boundary-layer bias, has naturally been at the opposite end of the spectrum indicating only a few isolated high-based areas of shower activity in a deeply mixed environment. Other CAM guidance lies somewhere between these two extremes with varying degrees of meager dryline convergence in the NSSL, ARW, and RRFS. Ultimately the convection forecast on Sunday evening will come down to how quickly we can overlap the height falls from the west with moisture from the east - a slightly faster trough and quicker moisture advection would certainly be enough to realize the more bullish CAM solutions. && .LONG TERM /MONDAY THROUGH SATURDAY/... Issued at 153 PM MDT Sat May 4 2024 An active, windy pattern continues to take shape starting early this week and continuing through mid-week. A potent, upper-level, negatively tilted trough will dig into the region early Monday with strong cyclogenesis expected across southwestern South Dakota by Monday afternoon. As this cyclone develops, strong westerly flow is expected across the region from 250mb down to the surface, suggesting a widespread high wind event is possible once again for southeast Wyoming and western Nebraska. 700mb westerly winds are progged to peak around 75kt throughout the day Monday as the surface low develops and rapidly strengthens as it moves northeasterly across the Dakotas. NAEFS Mean Zonal Wind at 700mb suggests that the 700mb winds will be in the 99.5th percentile (or greater) for this period of time, showing how unusually strong the 700mb jet is expected to be for this time of year. Along with this strong 700mb flow, steep lapse rates are expected to help mix the wind down to the surface further supporting very strong gusts for many locations. Strong downward omega fields are also evident in the GFS, with maxed out values expected across the Laramie Range Monday afternoon. These downward omega fields continue to foster support for a very strong downslope wind event Monday through early Wednesday. In addition to this, Craig to Casper 850mb height gradients have soared to the 80- 113m range throughout the day Monday and into the overnight hours. Similarly, 700mb Craig to Casper gradients have increased to the 85- 90m range. Most of the time, the Craig to Casper height gradients can be used as a rough estimate of peak winds gusts near Arlington. With these unusually high values, winds could peak in the 70-80mph range at Arlington through the Monday/Tuesday period. Finally, in- house guidance continues to remain confident in a prolonged high wind event with 80-90% probability for high winds across the winds prones and into the adjacent plains. This setup will need to continue to be monitored and a High Wind Warning will likely be needed for many locations to start the work week. In good news, winds are expected to decrease by early Wednesday morning, with an additional brief lull possible Tuesday morning. The remaining long term forecast features unsettled, cooler weather as the upper-level low rotates over the Northern Plains before slowly ejecting east by Thursday afternoon. Behind this low, a secondary, flat trough is expected to impact the region, once again favoring daily precipitation chances across much of southeast Wyoming and western Nebraska. Long range models begin to differ significantly after Thursday, with the GFS suggesting another cut- off low rotating just west of the CWA, while the ECMWF moves the whole system off the east before another system approaches the West Coast. Both models still favor precipitation chances and cooler, wetter weather, so kept PoPs between 30 and 60% for the remaining long term forecast. Expect cloudy, cool, and unsettled for the late week after the strong winds decrease early Wednesday morning. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 1135 PM MDT Sat May 4 2024 A potent weather disturbance will move into eastern Utah Sunday evening, sending an associated cold front into southeast Wyoming at that time. Wyoming TAFS...At Rawlins, cloudiness will increase with ceilings lowering from 10000 feet around midday Sunday to 5000 feet Sunday evening, with periods of showers developing after 02Z, reducing visibilities to 4 to 6 miles. Winds will gust to 45 knots after 15Z. At Laramie and Cheyenne, cloudiness will increase with ceilings lowering to 10000 to 15000 feet after 16Z. Winds will gust to 35 knots after 16Z. Nebraska TAFS...Cloudiness will increase with ceilings lowering to 5000 to 10000 feet Sunday afternoon and evening. Low level wind shear occurs until 19Z at Scottsbluff, and after 03Z at Alliance, Scottsbluff and Sidney. Winds will gust to 50 knots. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...High Wind Watch from 9 AM MDT Sunday through Sunday evening for WYZ104-109-111. NE...None. && $$ SHORT TERM...MAC LONG TERM...AM AVIATION...RUBIN