The Challenges on using Ensemble Data and Providing Probabilistic QPF Guidance in Mountainous Terrain

Qualitative precipitation forecast (QPF) and snowfall remain a challenge in areas of significant terrain, including the Great Basin. QPF is dependent on many factors that include moisture availability, upwards forcing, stability, cloud physics, and orographic forcing. Another component is oro blocking and this is not well understood and unresolved by modeled QPF on the Sierra Nevada lee side. A more significant forecast challenge is QPF and snowfall across the Great Basin, where alternating valleys and prominent mountains exist. Mountain waves dominate the circulations downstream from the Sierra Nevada, creating the “rain shadow effect.” This is evident in the QPF forecast where higher values are aligned with terrain and elevation, but verification reveals more variability. There are instances when the “rain shadow effect” is minimized or nonexistent, resulting in “spillover” precipitation that spreads east across the Great Basin. Ensemble guidance may help improve QPF/snowfall and probabilistic QPF/snowfall but failed to provide forecast uncertainty since many models do not adequately resolve the narrow mountain ranges that exist in the western United States. Providing possible outcomes and confidence levels can be a challenge when operational meteorologists encounter a wide range of outcomes. This presentation reviews the model parameters from an event that occurred during the February 29 through March 1, 2020 timeframe in the hopes of helping meteorologists build confidence in predicting and messaging events with large ensemble spreads at short lead-times.