Heavy rainfall and snowfall in Southern California
As an anomalously-deep 500 hPa low began to move inland over Southern California during the 23 January – 24 January 2021 period, GOES-17 (GOES-West) Air Mass RGB images (above) showed a compact Potential Vorticity (PV) anomaly approaching the coast — and the RAP40 model indicated that the “dynamic tropopause” (defined here as the pressure of the PV1.5 surface) was descending to the 675 hPa pressure level at 18 UTC.A west-to-east oriented cross section of RAP40 model fields along Line A-A’ (below) depicted the descending dynamic tropopause at 19 UTC.
GOES-17 Mid-level Water Vapor (6.9 µm) images (below) showed the increasing reports of rain and snow that resulted as the PV Anomaly moved inland and provided additional forcing for ascent. Near the coast, thunderstorms were reported at Fulton and Long Beach around 03 UTC. Storm total precipitation amounts included rainfall of 1.40 inch and snowfall of 12-18 inches. GOES-17 Water Vapor images at 2301 UTC and 0246 UTC (below) revealed sporadic lightning activity (indicated by small clusters of GLM Groups).===== 24 January Update =====
On the following day, as clouds began to clear the areal extent of resulting fresh snow cover (darker shades of red) was seen in GOES-17 Day Snow-Fog RGB images (above). Even parts of the high desert — north and east of the mountain ranges — received some snowfall (for example, 2-3 inches were reported at Hesperia).Suomi NPP VIIRS True Color RGB and False Color RGB images (below) showed the snow cover (shades of cyan) at 2036 UTC.