Anomalous Low Elevation Snowfall Event in Southern California of 30–31 December 2014

Accumulating snowfall occurred at elevations as low as 300 meters over parts of southern California, especially in southwestern Riverside County near the communities of Temecula, Murrieta and Wildomar, when a closed upper-tropospheric low moved overhead 30-31 December 2014. Local snowfall accumulations between 25 and 50 cm were reported at 700 meters elevation on the Santa Rosa Plateau near Wildomar and Murrieta, which was only slightly less than a 55 cm report on Palomar Mountain to the southeast at 1600 meters. This caused major transportation problems, including a closure along heavily traveled Interstate 15, and damage to trees, including citrus and avocadoes. While closed upper-tropospheric lows move through southern California a few times each winter, they rarely bring snowfall below 800 meters; the last time low-elevation snow occurred in this region was 2004.

This storm system was unusual in that the position of the surface low just off the coast of southern California combined with all-time highest sea-level pressures ever recorded over parts of the northwestern United States. This resulted in an intrusion of cold dry air near the surface, with drying enhanced by downsloping katabatic flow from the northeast beneath a moist layer in southerly flow which extended from 850 to 600 hPa above a sloped frontogenesis zone. The moist layer included layers at and above 700 hPa favorable for formation of dendritic snow crystals. Cooling due to evaporation and sublimation of precipitation between the surface and 850 hPa resulted in a lowering of the snow level to 300 meters where wet bulb temperatures were near 0 degrees C. The snowfall occurred during the night when temperatures were lowest, lasting over eight hours as the dry northeast flow maintained the evaporative cooling beneath the moist layer. The Weather Research and Forecasting Environmental System (WRF-EMS) using the Advanced Research WRF (ARW) will be used to examine this event as it was able to predict the low snow levels because of its ability to model the low wet bulb temperatures below the moist layer in the complex terrain of southern California.