‘May gray’ to ‘June gloom’: Scientists explain weird Bay Area weather

After a historically wet winter and then a momentary hot spell, summer fog is already blanketing the coastal Bay Area, a dreary pattern that is expected to persist through the next week. And beyond.

“It’s covering the bay every morning, and some of it has gotten into the delta,” said meteorologist Jan Null of Golden Gate Weather Services in Half Moon Bay, which enjoyed only one fog-free sunset all month.

“This is a year where that summer pattern is setting up early,” he said.

To be sure, Memorial Day weekend is always a melancholy holiday, honoring the tragic loss of young lives alongside picnics and barbeques. So our weather is striking an appropriate tone.

Beachgoers at Point Reyes National Seashore sat bundled in jackets and sheltered between dunes. Only a few people braved bathing suits, and all were children.

The best weather drama recently has been happening in the Sierra Nevada, where towering cumulus clouds are delivering thunderstorms and lightning.

Because of all the snow, there’s a tremendous supply of water vapor, according to National Weather Services meteorologist Rick Canepa. When heated by the sun, this water and pockets of warm air start to rise — updrafts that are propelled by mountain ridges and peaks. All that atmospheric instability creates storms.

Meteorological summer starts on June 1, not the June 21 date associated with the vernal equinox, called astronomical summer.

But the persistent low white clouds have already worn out their welcome.

This spring was 2.3 degrees cooler than average in San Jose during the months of March, April and May, said Canepa. The city’s temperature averaged 57 degrees, compared to the historic average of 59.3 degrees.

If March felt cold, it wasn’t your imagination. That month was 4.2 degrees below normal.

Oakland’s spring was even chillier. The city’s average temperature was 55.5 degrees, more than three degrees cooler than the historic average of 58.7 degrees. It was the third coolest spring on record since 1970.

This was the coldest spring in Half Moon Bay since 1999, said Canepa.

Anonymous Twitter personality @KarlTheFog memorialized our brief springtime in a poem: “Roses are red. Fog is gray. Yesterday was summer. But it’s winter today.”

As usual, the fog has been most persistent at lower coastal elevations, thinning in the middle of the day and at inland and elevated locales.

Driving under cool cloudy skies, Matthew Dodder of the Santa Clara County Audubon Society reached the top of the Santa Cruz Mountains this weekend to find sun and warmth — as well as an exuberant spring chorus of purple martins, olive-sided flycatchers, western wood-pewees, blue-gray gnatcatchers and other special birds.

Back east, one of the earliest springs in recent memory has thrown nature’s timing out of whack, with leaves appearing 23 days early in Baltimore and 11 days early in Atlanta, according to the USA National Phenology Network, which tracks seasonal change. But leaves sprouted three days late in San Francisco, and six days late in Los Angeles.

North of us, it’s been steamy, with record temperatures recorded in the Pacific Northwest and western Canada.

Here in the coastal Bay Area, we’re getting a jump-start on June Gloom.

To be sure, fog has its fans. Sales of souvenir sweatshirts to freezing tourists help put money back into our local economy. The moisture is sponged up by thirsty redwood trees and lichens. Without fog, film noir classics like “The Maltese Falcon” and “Vertigo” might lack a sense of mystery and dread.

What’s going on?

May is usually a transitional month — fog-free in the beginning and then growing grayer, said Null.

Fog is caused by differences in ocean water temperatures and air temperatures. Called the “marine layer,” it forms when warm, moist air passes over a cold body of water.

In late spring, the water temperature of the ocean remains quite cold and cools the lowest layer of the atmosphere, resulting in a stark contrast between it and the warmer layer above it. This creates a temperature inversion and traps the cold air, which eventually cools to its saturation point to form clouds.

Recently, the ocean has been unusually cool. Several days ago, temperatures of 49 degrees were measured at a buoy off the Sonoma County coast near Bodega Bay.

And areas of low pressure in the upper atmosphere, called “troughs,” deepen this marine layer. The deepest marine layers can create fog far inland.

Normally, a pattern of low and high pressure systems dances around the middle latitudes of the globe. Everything keeps moving.

“But we’ve had these troughs over us the entire month,” Null said. “Not much is moving.”

Eventually, the troughs will exit and a high pressure system will arrive, brightening our days. As the air temperature heats up, cloud droplets evaporate until the sun shines through, burning off the marine layer. The ocean will warm.

The National Weather Service’s Climate Prediction Center is forecasting a 33% to 40% chance of above normal temperatures in most of California this summer.

But Fogust is right around the corner. To be later followed by Fogtober.