Train Echoes, Flash Flooding and Near Hurricane Strength Winds in the SF Bay Area and Sacramento Valley, 1/22/97

Wind Information courtesy of National Weather Service

Convection can interact with its environment do develop new convection relative to existing cells. Flash flooding can occur when conditions favor redvelopment of convective cells over the same area (known as "training" and the appearance on weather radar "train echoes."

Any boundary (frontal or outflow, for example) is the source for intense storm-scale lift when boundary-relative flow is moist and relative inflow is strong. This can occur when ambient flow is strong but the boundary is moving slowly.

On January 22, 1997, a slow moving frontal boundary and wind shift line lay across north-central California (not shown yet, in preparation). The Oakland morning sounding/hodograph indicated some buoyancy (not shown yet, in preparation) and very strong shear (not shown yet, in preparation).

Flash flood producing rains occurred over the region affected by the cells, with nearly 6 inches in the coastal hills and 2-3 inches in the lowland sections of the northern San Francisco Bay Region. A tornado occurred in southern Marin County at around 20 UTC.

Winds rose into the hurricane force range by late morning. San Francisco State Univeristy recorded a peak gust of 73 mph 1t 11:38 AM with other gusts exceeding 50 mph, 60 mph and 70 mph. The National Weather Service reported other winds near hurricance strength in the San Francisco Bay Region.

Motion of cells in the morning was on the hodograph and positive shear in the 0-6 km layer exceeded 5 X 10-3. Surface winds were inflow, at right angles to the boundary.

The enhanced infrared GOES-9 satellite loop covers a roughly 9 hour period while the surface boundary slowly passed through northern California. Backbuilding was evident in the imagery, with three separate "explosions" of convection traversing the same area, particularly, in the southern Sacramento Valley.

IR GOES-9 Sat Imagery, 15 UTC Jan. 22 to 01 UTC Jan 23

This is a loop of the 1/2 degree tilt, reflectivity data for KDAX Doppler near Sacramento. A similar loop for KMUX (in the SF Bay Area) is being prepared. Only images at 1 hour intervals in roughly the same period as shown above are given. I am in the process of getting intermediate time images as well.

At 1/2 degree tilt, KDAX will not "see" precipitation along the coast very well. However, this loop illustrates the persistant training over the southern Sacramento Valley.

KDAX BREF1, 19 UTC Jan. 22 to 01 UTC Jan 23

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