Forecast challenges and impacts of severe downslope wind events

The coupled mountain wave, downslope wind and rotor circulation system pose significant impacts to the aviation and public service communities. The observation and understanding of these events was greatly enhanced following the operational forecast support that the NWS Las Vegas provided to the Sierra Rotors Project in 2004 and the Terrain-Induced Rotor Experiment (T-REX) in 2006. These research experiments took place in the lee of the southern Sierra Nevada along the Owens Valley in California. Due to the prevailing synoptic flow which blows perpendicular to the terrain, severe downslope wind events (surface winds speeds of 26 m s-1 or greater) are common in the Owens Valley, particularly from November through March. The frequency and strength of such events was not fully understood due to the lack of surface observations in the Owens Valley prior to the installation of the 16-station surface mesonet in Independence, CA in 2004. On average 6 to 10 severe downslope wind events occur in the Owens Valley annually.

When stability is high and/or a critical level is located just above ridge top level, light to moderate wind speeds (12-25 m s-1) perpendicular to mountain crest can produce severe downslope winds (1.5 to 2.5 times that of ridge top level) along the eastern foothills as the flow is reflected and accelerated down the eastern slopes. These mountain waves, downslope winds and in particular rotors, can have significant consequences for the aviation community by producing clear air turbulence in the form of breaking waves aloft and extreme wind shear in the lower troposphere. The NWS Las Vegas has begun to provide forecast discussions on the potential and/or occurrence of these events in both the public and aviation sections of the Area Forecast Discussion (AFD).

Severe downslope wind events also pose problems to the general population but are most problematic for ground transportation. Due to the north-south orientation of the Sierra Nevada mountain chain, the Owens Valley, and U.S. 95, downslope flow produces dangerous crosswinds for ground transportation along U.S. 95 and in particular to high profile vehicles. Multiple tractor trailers are blown over each year while traveling along U.S. 95 and the highway is closed on numerous occasions to high profile vehicles. Several strong rotor circulations were evident during February of 2007 via satellite imagery and surface observations. During the afternoon of 25 February and 26 February 2007, severe downslope winds were observed by the Independence mesonet in the sensors located in the foothills of the Sierra and in the sites along the western and central portions of the Owens Valley, while observations along the eastern portion of the valley reported a substantial easterly component of the surface wind. Additionally, there were multiple reports of severe to extreme turbulence by general and commercial aviation. Both visible and infrared satellite imagery depicted a dramatic rotor cloud along a large portion of the Owens Valley on both afternoons.

The NWS Forecast Office in Las Vegas began running a 4-km version of the WRF-NMM over the southern Sierra and Owens Valley of California to aid in understanding these atmospheric processes and to help with the production of operational forecasts, warnings and advisories. The experience gained from the Owens Valley is now being applied to the Las Vegas Valley. The population of Las Vegas and surrounding communities continues to grow at a rapid pace. The population is now moving into areas where downslope wind events are more common. We are now running a second 4-km WRF-NMM domain over the Spring Mountains in southern Nevada to improve forecast support of these high impact downslope wind events in the Las Vegas Valley.