![]() A Flash Flood Warning was issued for the Glass Fire burn scar due to a period of intense rainfall rates. ![]() Flood Warnings and Flood Advisories were issued throughout the day from the North Bay, down the San Mateo coastline for flood risks in urban locations and small streams. At higher elevations, winds were observed between 50 to 60 mph with peak gusts between 70 to 80 mph, with gusts at KSFO measured up to 50 mph. Winds out of the southwest increased causing a Wind Advisory to be issued for the entire Bay Area. As the sun rose on the 24th, the water content increased exponentially bringing moderate to heavy rainfall around the Bay Area. Light showers already were showing up in the North Bay late on the evening of October 23rd. But as the event grew closer, it became more and more apparent that the Bay Area was to prepare for a substantial rain event.Īs the parent low pressure system moved toward Seattle and its central pressure rapidly intensified, undergoing bomb-cyclogenesis. There was reason to doubt initial model runs given that this type of an event is unusually early for the region. By the 20th, models were hinting at the chance for a high to moderate "atmospheric river", or AR, event with precipitable water values several standard deviations above the norm. However, these model runs depicted a longer stream of moisture that was tapped into the sub-tropics. As these systems moved toward the California Coastline, it brought light rain in the form of cold fronts along with atypical above average accumulations to the North Bay.īy October 18th, long-range models were indicating yet another low that would approach the Pacific Northwest bringing a chance for rain to the Bay Area. As the 2021/2022 water season began, the pattern shifted bringing a series of upper level troughs through mid-October toward the Pacific Northwest. The 2020/2021 Water year ended unceremoniously in the Summer of 2021 with a "status quo" report with little rain during the rainy season leaving dry fuels around the state keeping the region on edge during Fire Season. Those differences are compounded by a statewide gradient of decreasing rainfall from north to south, since most storms come from the northern Pacific and shed water as they move south.Overview - Atmospheric River - October 24-25, 2021Įarly Season Storm Brought Historic Heavy Rains and Strong Winds to the Bay Area The bottom line is that Mount Tamalpais and the Santa Cruz Mountains may get more than 45 inches, Petaluma and San Francisco 23 inches, and Livermore and Tracy 15 or fewer. Once all that rain comes down, there’s not much left on the other side of the Coast Range, where rain shadows leave cities like San Jose with only 16 to 20 inches of rain. This is known as orographic precipitation, and it means that Kentfield (25 miles east of the light-house) gets 48 inches of rain per year from the same storms. When air cools, it can’t hold as much moisture, so the water falls as rain. As storm clouds move inland and begin to rise over the coastal mountains, the air cools. Average annual rainfall there is only 15 inches. The distribution and amount of rain is similarly all over the proverbial map.Īt the Point Reyes Lighthouse, sticking way out into the ocean, you might expect rain to be plentiful. Folks at the beach are freezing as the fog lingers in San Francisco’s Sunset District while others are sweltering in the bright sun in Livermore. The Bay Area is well known for its many microclimates. ![]() Q: Which spot in the Bay Area gets the most yearly average rainfall and which spot gets the least? Why? Ī: Nearly all the precipitation we receive comes between November and April in the form of rain from moisture-bearing clouds off the Pacific.
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