An Investigation of a Thundersnow Occurrence in Metropolitan Phoenix, AZ on February 20th, 2013

Thundersnow is linked to increased snowfall intensity and accumulation totals. During relatively warm conditions, increasing intensity of frozen precipitation can lead to a deepening of the near-isothermal 0^oC layer through the cooling effects of melting and sublimation, allowing frozen precipitation to reach lower altitudes. Although rare, thundersnow has been observed and studied across various regions of the U.S., although most attention has been given to the Midwest, Great Salt Lake and Great Lakes regions. On February 20, 2013 an upper level low with 500-hPa temperatures below 243 K tracked slowly eastward across the Southwestern U.S. and brought with it a wide swath of precipitation mostly in the form of rain over the low deserts. In the late afternoon, localized snow and graupel accumulations occurred in the Phoenix Metropolitan Area (PMA), with coincident reports of thunder. NCDC's Severe Weather Data Inventory, which gathers data from the National Lightning Detection Network, confirms lightning strikes in the area. This is an unusual event in that convective snowfall and snow accumulation at lower elevations of the PMA are extremely rare. Initial analysis of 3km High Resolution Rapid Refresh (HRRR) output suggests an isolated region of instability over the PMA (e.g., surface CAPE greater than 500J kg^-1 per the HRRR simulations). This suggests that enough of an updraft was present to initiate charge separation. Radar reflectivity indicates that intense precipitation moved from southwest to northeast across the PMA from 2210-2255 UTC with a near stationary area of local maximum reflectivity from 2230-2240 UTC. Further analyses will consider the role of surrounding terrain in the development of convection and the role of latent cooling from precipitation phase change in the accumulation of snow and graupel.