Analyses of Surface Information and High Resolution Satellite Imagery


Analyses of satellite imagery and surface observations suggest that the tornadoes could have resulted from interactions of a sea breeze front with the outflow boundary from previous convection. A line of low level cumulus marking the leading edge of a north-to-south moving sea breeze front was a persistent feature on satellite imagery during the late morning and afternoon hours (one example given below).

GOES 9 Visible Image at 2200 UTC showing fixed leading edge of sea breeze front in relation to initial thunderstorm development

The motion of the sea breeze front past KNUQ could be noted in the hourly observations given below, and was marked by a wind shift from southerly to northerly, a decrease in temperature and an increase in dew point in the hour between 2000 and 2100 UTC (1300 to 1400 PDT). As noted in section on Buoyancy, the air north of the sea breeze front was characterized by moderate values of CAPE.

KNUQ Meteogram from 2050 UTC to 0050 UTC showing wind shift, temperature and dewpoint temperature rise at about 2100 UTC associated with southward moving sea breeze front. Arrival of thunderstorm outflow at about 0000 UTC marked by another wind shift and temperature and dewpoint temperature decreases. (Click on image for clearer version)

The sea breeze front then encountered an outflow boundary emanating from a thunderstorm in the foothills of the Diablo Range northeast of San Jose. This thunderstorm was the first to form in the afternoon hours. The passage of the outflow boundary through the San Jose area could also be noted in the hourly observations at KSJC (below) and was marked at around 2200 UTC (1400 PDT) by a wind shift from northerly to easterly, temperature and dew point temperature decreases, observations of rain, hail, and lightning passing from southeast to northwest.

KSJC Meteogram from 2050 UTC to 0050 UTC showing wind shift, temperature and dewpoint temperature decreases at about 2200 UTC associated with westward moving outflow boundary. Arrival of thunderstorm noted in portions of METARS shown for 2245 through 2347 UTC by cloud-to-ground lightning initially east of and, finally, northwest of KSJC, rain and hail (graupel). (Click on image for clearer version)

The Sunnyvale storm initially formed as a new development on the outflow boundary on the initial storm's west flank. This new storm then appeared to travel along the sea-breeze front boundary (below). Such a motion would have the effect of tilting horizontal anticyclonic vorticity solenoidally-generated on the sea-breeze front into the updraft. Thus, the initial tornadogenesis in this case could have been a combination of supercell and landspout processes.

GOES 9 Visible Image at 2346 UTC, showing motion of storm northwest along boundary and how tilting and stretching of vorticity along the sea-breeze front could have generated a landspout. "T" locates tornado at time of image.