A Cool Windy Spring for the
San Francisco Bay Area...
and the Prospects for Summer

by Jan Null, CCM
Golden Gate Weather Services

Yes, it has been cooler and windier than usual in the San Francisco Bay Area this Spring.  And an examination of the previous La Niña Springs shows that this is not unusual.

The average temperatures at San Francisco, San Francisco Airport and San Jose all were cooler than normal for the months of April and May 1999 (based on preliminary data).

Additionally, a look at the average wind speeds at San Francisco International Airport showed both months had winds higher than normal.  Though the differences below do not seem large, keep in mind that if we were to look at only the "windy" time of the day (i.e., approx noon to 8 PM) the values would be significantly higher.

The weather patterns during the months of April and May of seasons when there is La Niña (1965, 1971, 1974, 1976, 1989 and 1996) indicate the above results should not be surprising.  The composite graphics were constructed using the online facilities of the NOAA Climate Diagnostics Center.  They represent the difference compared to the long term avereage (i.e., the anomaly).

The historic Sea Surface Temperature (SST) anomaly for Spring during La Nina shows cooler the normal water along much of the West Coast. (click image to enlarge)	This US Navy current SST anomaly chart shows cooler than normal water in the northeast Pacific.  It is unclear how much of the cold anomaly can be directly attributed to La Nina.   Also note the strong upwelling along the immediate coast.  this is a function of the persistent and stronger than usual sea breeze.(click image to enlarge)
Composite April-May Sea Level Pressure Anomalies for La Nina years showing lower pressures over theGreat basin, another key for an enhanced seabreeze. Composite April-May Surface Temperature Anomalies for La Nina years reflects cooler than normal conditions along much of the West Coast and warmer temperatures in the southern Rockies and Great Plains.	JPL NASA image showing a large area of cooler the normal water over much of the northeast Pacific Ocean. (click image to enlarge)  The question remains whether this condition will continue through the summer and what impact it will have on California weather.

...AND THE PROSPECTS FOR THE SUMMER

	A Composite of Sea Surface Temperatures from previous La Nina summers shows a negative anomaly in the coastal waters adjacent to California.
	The June, July, August Composite of Sea Level Pressure for La Nina years shows a weaker onshore gradient than we have seen in the past few months, but still slightly stronger than the long term average.
	Temperatures for the previous La Nina period of June, July and August shows the coastal sections of Northern and Central California being cooler than normal.  At the same time inland areas are indicated to be near seasonal levels.
	The National Center for Environmental Prediction (NCEP) forecast varies somewhat from the longterm climatology.  The chart to the left indicates a greater than normal chance of having above normal temperatures for most of California for the period June, July and August.