Orographically-Inverted Snow: Forecast Methodology and Decision Support Services Implications

The Wasatch Front metropolitan area of northern Utah is frequented by winter storms with varying impacts to surface transportation. During a typical event, the nearby Wasatch Mountains to the east receive more snow with increasing elevation, sometimes several multiples of what is measured along the populated valley floor. However, recent events have been documented by the National Weather Service Weather Forecast Office (WFO) in Salt Lake City (SLC) wherein lower elevations including the valley floor receive equal or even greater snowfall amounts than the adjacent higher terrain. During these events, the profile of snowfall with elevation is inverted in comparison to what would typically be expected by orographic forcing, hence the name orographically-inverted snow. These events have significant impacts to mitigation strategies for surface transportation. This presentation will detail research performed by WFO SLC on three such storms, including forecast tools to aid in diagnosing future events.

The Froude Number is investigated as a potential tool in diagnosing the potential for blocked flow, and the extent to which the orographic precipitation maximum is shifted to the mid-slopes of the mountains, benches, and even the lower valleys. It is found that limitations of the Froude Number for this application include assumptions made for barrier depth and uniformity of the wind profile. A more hands-on diagnostic scheme is shown using model profiles at a nearby point, and an alternative means to calculate the Froude Number which is more applicable to a tall mountain barrier like the Wasatch is shown. Lastly, potential targeted decision support service opportunities are demonstrated.