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When Summer Weather Happens


Notes:

The High pressure system directs most of the weather to the north. That does not mean that the Bay area is immune to weather. We may catch the trailing edge of a particularly large system passing to the north. For example some cirrus clouds trailing a system to the north visited the bay area on July 2 and 3, 2004.

Another possibility is that the “monsoon” (flow from the Gulf of California into Arizona) or West Coast Hurricanes may be strong enough to spill into the bay area. These are notable since they bring several hot and muggy days.


from “CALIFORNIA AND OREGON HUMIDITY AND COASTAL FOG” in references

Marine Layer Thickness: This corresponds well with 700 Mb synoptic
maps, which show a clear correlation of a weak high over
California during fog events and a trough passing over
the state during clearing, such that fog is about 30% less
likely to occur when a trough is in the vicinity. These
relationships are demonstrated using summer-long
averages and case studies, which are reproduced in
Lundquist (1999).

For forecasting the likelihood of fog on a state-wide
basis, 700 Mb (≈3000m) synoptic maps provide the most useful
tool. Overall, time periods with a 700 Mb high over
California have a 30% greater chance of fog than time
periods with a 700 Mb trough in the vicinity.