Rain chances in SF remain unpromising: 'Just enough to wash the pollen off your car'
News // Weather
There's no rain in the forecast for the San Francisco Bay Area through the end of February.
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3of19Buy PhotoKing tides: This non-scientific term is commonly used to describe exceptionally high tides. "Tides are long-period waves that roll around the planet as the ocean is "pulled" back and forth by the gravitational pull of the moon and the sun as these bodies interact with the Earth in their monthly and yearly orbits," according to NOAA. In the Bay Area, king tides occur in the winter. Paul Chinn/The ChronicleShow MoreShow Less
4of19Diablo winds: The offshore winds blowing hot air from the interior to the coast are commonly called the Diablo winds in the San Francisco Bay Area. (They're known as the Santa Ana winds in Southern California.) They most often kick up in the spring and fall. These are the same winds that howled wildly on the night of Oct. 8, spreading flames and embers across Northern California's Wine Country and amplifying fire conditions. On that night and in days after, gusts reached up to 70 miles per hour. George Rose/Getty ImagesShow MoreShow Less
Atmospheric river: These supercharged storm systems are known for delivering massive amounts of snow and rainfall in a matter of days on the West Coast of the United States. The long plumes of water vapor in the atmosphere are like rivers in the sky—250 to 375 miles wide, on average. On the West Coast, they most often originate in the South Pacific, and, as they travel from the tropics across the ocean, they collect incredible amounts of moisture. The rain is funneled into cities like San Francisco, Sacramento, Portland, and Seattle, where the runoff systems are often unequipped to deal with the days-long deluge. When an atmospheric river hits land, water vapor rises and cools, unleashing precipitation. The amount that falls is dependent upon the strength of the storm and its moisture content. An atmospheric river carrying moisture from as far away as Hawaii to the West Coast is what's known as a "pineapple express."
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6of19Tule fog: This thick ground fog settles in the San Joaquin Valley and the Sacramento Valley when the skies are clear and the temperatures are cool. The season runs late fall through early spring and usually starts after the first rain. Sometimes winds sweep the fog into the San Francisco Bay Area. This differs from our summer fog pattern, which typically moves inland from the Pacific. Anthony DunnShow MoreShow Less
Fallstreak cloud: These stunning formations are also sometimes called hole punch clouds because it literally looks as if something punched the sky with cloud puffs dissipating from a large elliptical gap. The National Weather Service explains fallstreaks occur when the water temperature in a cloud is below freezing, but the tiny water droplets forming the cloud have yet to freeze. "These 'supercooled' water droplets need a 'reason' to freeze, which usually comes in the form of ice crystals," NWS explains. "Planes passing through the cloud layer can bring these ice crystals. Once the ice crystals are introduced, the water droplet quickly freeze, grow and start to fall. A hole is left behind, which will start to expand outward as neighboring droplets start to freeze." See photos from Sept. 14, 2018, when fallstreak clouds were spotted all over the Bay Area.
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8of19Sun dogs: When the sun's rays pass through ice crystals in the atmosphere, mini-suns can appear at its sides, as in this photo from Fairbanks Alaska, Legend has it that the ancient Greeks figured Zeus was out walking his pooches. Actually, Apollo was the sun god, but who wants to ruin a good story?John Wagner/Fairbanks Daily News-MinerShow MoreShow Less
9of19High-pressure ridge: Elongated ridges of high pressure can move into California at any time of the year, and they're commonly associated with dry conditions. Air in a high-pressure area compresses and warms as it descends. The warming prevents the formation of clouds, meaning the sky is normally sunny in high-pressure areas. What's more, when a high-pressure ridge is off the West Coast, it blocks incoming storms from the Pacific Ocean and weakens those that successfully pass through.National Weather Service Show MoreShow Less
Ridiculously resilient ridge: Daniel Swain, a climate scientist at UCLA, coined the term "ridiculously resilient ridge," sometimes shortened to "RRR," to describe the phenomenon when a ridge of high-pressure persists for an extended period of time over the father northeastern Pacific Ocean. The ridge blocks storms from reaching the West Coast, resulting in abnormally warm and dry conditions in California.
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Virga: This weather phenomenon occurs when the clouds are thick and dark but don't drop rain to the ground. It's because a pocket of dry air in the upper atmosphere evaporates the water droplets before they can hit the ground.
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Open storm door: This metaphorical term refers to a weather pattern when low-pressure is overhead, allowing for storms from the Pacific to move into California.
El Niño: The inverse of La Niña, El Niño, is a warming of the equatorial waters in the center and eastern Pacific Ocean. This warm zone pumps heat and moisture into the atmosphere, releasing energy that impacts the high-altitude jet streams circling the planet. While El Niño is associated with above-normal precipitation in California, a number of different outcomes are possible.
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La Niña: The cousin of better-known El Niño is a cooling of equatorial waters in the eastern and central Pacific Ocean and can impact atmospheric conditions worldwide. In California, the La Niña weather pattern is known for bringing above-average precipitation to the far northern reaches of the state near the Oregon border and delivering below-average precipitation to the central part of Northern California and even drier conditions into Southern California. Historical data, however, reveals that a number of outcomes are possible.
15of19Inside slider: This type of storm drops directly down from the north and moves north to south as opposed to the more common Pacific storms that move from the west to east and come off the Pacific Ocean. A storm known as an "inside slider" is bringing icy temperatures and a light dusting of snow to the northern Sierra Nevada Monday. This type of storm drops directly down from the north and moves north to south as opposed to the more common Pacific storms that move from the west to east and come off the Pacific Ocean. "It's a completely different trajectory and usually we have different characteristics with it," explains Dawn Johnson, a meteorologist with the National Weather Service's Reno office. "As a general rule, inside sliders tend to bring in colder air masses. Because these come from an inland trajectory, they don't have that Pacific moisture feed going into them and they tend to be drier."Northstar California ResortShow MoreShow Less
16of19Buy PhotoFlood stage: The level at which a river rises enough to cause sufficient inundation of areas that are not normally covered by water, causing an inconvenience or a threat to life and/or property. When the water rises to this level, it is considered a flood event. In this photo, the Merced River rises to just below Swinging Bridge in Yosemite National Park, which was under a flash flood watch, on Sunday Jan. 8, 2017.Michael Macor/The ChronicleShow MoreShow Less
17of19Buy PhotoDebris flow: A geological phenomenon when water rushes down a mountainside carrying soil and fragmented rock. These often occur in areas that were recently burned by wildfire, as the ground is loose. In this photo, a mud and debris flow led to the closure of Santa Rosa Avenue in Guerneville, Calif. on Wednesday, Jan. 11, 2017. Paul Chinn/The ChronicleShow MoreShow Less
22 degree halo: These rings of light occur around the sun or moon when light is refracted by ice crystals associated with high-level clouds known as cirrostratus.
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19of19Earthquake weather: An infrequent hot and muggy San Francisco day is called earthquake weather after the meteorological conditions on April 18, 1906 and October 17, 1989 (pictured). It's meteor-illogical to assume the next big quake will be on this kind of day, there is just no evidence to support the conditions causing quakes, but it's a good reminder to check the bottled water and canned food supply. Show MoreShow Less
When will it rain again in the San Francisco Bay Area?
As we head into the final days of February, it's looking highly likely the region won't see any measurable rain this month with clear skies and unseasonably are the forecast through Saturday, March 1.
This month will most likely go down as the first time the Bay Area was completely dry in February since 1864.
Meteorologists are forecasting showers at the start of March, but Steve Anderson with the National Weather Service said, "The chances of rain are quickly diminishing. We had forecast 30% chance of rain on Saturday and Sunday, but now it's knocked down to 20% chance on Sunday with 0% chance on Saturday."
If it does rain, Anderson said only a sprinkle — less than a tenth of an inch — is likely. "It'll be just enough to maybe wash the pollen off your car," he said.
ALSO: Photo: See how this year's dwindling Sierra snowpack compares to last year's robust one
The dry spell is the result of high-pressure ridge over the eastern Pacific Ocean that's steering storm activity away from California and toward the Pacific Northwest.
The National Weather Service measures rain averages using the water year, running Oct. 1 through Sept. 30. In this water year, rainfall totals are falling well below historical averages. The rain gauge in San Francisco on average measures 17.05 inches between Oct. 1 and Feb. 19, but this year it has received just 8.83 inches, or 52% of normal. The Santa Rosa Airport is at 61% of normal and San Jose 33% of normal.
Temperatures will be increasing around the Bay Area through the week. On Monday, highs will be in the low-60s on the coast and in the high-60s to the low-70s inland. Afternoon temperatures will inch up a few degrees each day through Thursday.
On Thursday afternoon, San Francisco is forecast to reach 70 degrees and some inland locations will flirt with 80 degrees.
Amy Graff is a digital editor with SFGATE. Email her: agraff@sfgate.com.