The sun-seeking know to avoid southern California beaches in the late spring. A period known to the locals as May Gray and/or June Gloom often darkens the coastal skies of sunny southern California with a layer of marine stratus. During this time, the coastal clouds may remain all day but often give way to some hazy afternoon sunshine. The number of days during this two month period that are gloomy vary from year to year. Cooler ocean temperatures (La Nina conditions) usually foretell a gloomier period.
The Climate Research Division at the Scripps Institution of Oceanography is currently in the throws of a May Gray / June Gloom contest. Employees are invited to forecast the number of days from May 1 to June 30 that will be gloomy.
A Gray / Gloom day is defined as one with a ceiling (broken [BKN] or overcast [OVC] cloud layer) at or below 2,000 feet above ground level at Lindbergh Field (KSAN) for at least 2 out of the 8 hourly weather observations between 9:00 AM and 4:00 PM.
On a typical May Gray / June Gloom day, the visible satellite imagery will show marine stratus hugging the coast of southern California. June 16, 2004, was such a day and the visible satellite image from that day is show below.
The graph (below) shows a climatology (1950-2005) of average percent of possible sunshine. May and June are the months with the lowest percentages at 59% and 58% (respectively).
SST anomalies may play a role in forecasting the number of gray/gloom days San Diego will experience. Below are maps of the SST anomalies for November to March of the winter preceeding the May Gray / June Gloom season (2010 through 2004 and 1999 and 1998).