Some Continuing Forecast Challenges Arising from Terrain-Flow Interactions

Despite the steady upward increase in the skill of global operational deterministic and ensemble prediction models as defined by the anomaly coefficient metric applied to 500-hPa geopotential height forecasts out to 10 days, some important and stubborn forecast challenges remain. A number of these forecast challenges can be related to terrain-flow interactions in regions of complex terrain.

Cold season forecast challenges include temperature and precipitation type forecasting near surface boundaries (e.g., coastal fronts) associated with cold-air damming, secondary cyclone redevelopments in the lee of mountain ranges like the Appalachians, how long extensive freezing rain episodes will last in sheltered mountain valleys, mesoscale precipitation bands tied to channeled flow down confluent river valleys surrounded by higher terrain, downstream predictability issues that can occur when upper-level disturbances of Pacific origin move inland through ridge environments along the West Coast of North America, and whether, where, and when damaging high winds will mix down to the surface in cool season high shear, low CAPE situations.

Warm season forecast challenges include predicting when long-lived MCSs and derechos will cross the Appalachians and reach the Atlantic coast, under what conditions terrain flow-interactions can contribute to the formation of tornadic storms, when predecessor rain events will occur adjacent to mountainous terrain ahead of tropical cyclones, to what extent elevated mixed layers will surge eastward from the Rockies and contribute to severe weather environments on the Plains, when severe weather will occur over southern Arizona associated with shear-related MCS development over the Mogollon Rim, under what conditions weak disturbances from the eastern Pacific can survive inland transits across the Rockies and trigger severe storm development in the lee of the mountains.

Several of these cold- and warm-season terrain-flow interaction forecast challenges will be illustrated. Findings from observational investigations will be used to inform possible predictability studies designed to elucidate relevant dynamical and physical processes will be illustrated.