San Francisco and the rest of the Bay Area are in for an unseasonal soaking starting Wednesday evening, raising the possibility of rain as late in the week as Sunday during Bay to Breakers frivolity. This kind of storm is known as an “atmospheric river,” but that terminology refers both to storms powerful enough to cause major flooding and damage or mild enough to be considered part of the normal yearly precipitation cycle. Until recently, researchers who study weather had no easy way of classifying these distinctions. According to the National Oceanic and Atmospheric Administration, the term “atmospheric river” is pretty close to literal, referring to huge streams of water ferried through the sky in volumes comparable to waterways: These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. [...] Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks—particularly in the western United States. While atmospheric rivers are responsible for great quantities of rain that can produce flooding, they also contribute to beneficial increases in snowpack. According to the forecast from the National Weather Service, this incoming storm—part of a series which will most likely yield significant rain every day this week and weekend except for Friday—does not bring any particular danger of flooding. But the absence of a potential emergency doesn’t tell us much about how potent it may be. In February, researchers at Scripps Institution of Oceanography at the University of California San Diego published a new scale to measure the intensity of storms just like this one, similar to the widely cited scale used to measure the strength of hurricanes. “The presence or absence of a few AR events can ‘make or break’ precipitation over the course of the water year,” meteorologist F. Martin Ralph wrote in the American Meteorological Society journal, noting that it’s unhelpful for states like California to have to rely on this key weather phenomenon without an easy way to measure severity. Ralph’s scale designates atmospheric rivers as categories 1 through 5. An AR Category 1 is deemed “primarily beneficial,” bringing with it the smallest risk of flood danger and mostly consisting of just helpful precipitation. An AR Category 5 is “primarily hazardous,” the kind of storm event that can last more than 140 hours and cause flooding up to 50 feet in the examples provided in the journal. So how bad is this storm? According to the “atmospheric river outlook” produced by the Scripps Institution this week, “The ARs forecast to make landfall are predicted to produce AR Category 1 conditions for much of coastal Northern California and Oregon” starting today. “The additional AR activity forecast to potentially make landfall between 18 and 22 May 2019 could bring AR Category 2 condition,” the forecast adds, meaning “moderate” severity that’s still mostly beneficial, but this is more likely further south. Note that the outlook also anticipates as much as 3.5 inches of rain in higher elevations and as much as two inches in lower areas like the Bay Area, so while the hazards are not pronounced residents should still probably batten down the hatches. Just in case.<doc-sep>SAN FRANCISCO (CBS SF) — A winter-like storm front began sliding toward the San Francisco Bay from the Pacific Northwest Monday, threatening to dump as much as an inch of rain by midweek. Carolina Walbrun, a meteorologist with the National Weather Service, said a stormy week was in store for the Bay Area beginning early Wednesday. “What’s unusual is the model for these storms, and the timing and the strength of these storms,” Walbrun said. The weather service predicted that light rain may begin in the North Bay on Tuesday night. “Rainfall is then expected to become more widespread and increase in intensity on Wednesday, and especially Wednesday night,” forecasters said. “The heaviest rain will occur on Wednesday night and early Thursday when moderate to locally heavy rain rates are possible. Rain is then expected to change over to showers on Thursday.” Rainfall totals during the second half of this week were forecast to range from a half inch to an inch across most of our area, with isolated amounts up to 1.5 inches or more possible. Another storm front is expected to roll into the region in time for the weekend. Rain totals were forecast to be another 1/2 inch or more. The weather service said if the “active weather pattern” persists on into next week as currently forecast, rainfall totals could become very impressive for this time of the year. The weather service said rainfall totals through the middle of next week range from 2 to 2.5 inches across our area. “While 10-day rainfall totals of this magnitude would not be at all unusual during the winter months, they would definitely be unusual for mid May, the weather service said. Winds could also be a factor, especially late Wednesday and Wednesday night when gusty southerly winds are are expected to develop.<doc-sep>SAN FRANCISCO (CBS SF) — Mother Nature was preparing a spring surprise for the Bay Area Tuesday, a series of winter-like storms that will dump as much as 200 percent of the normal precipitation for May by the time they move on next week. Over the next week, a low pressure system from the Gulf of Alaska will send waves of stormy weather through the Bay Area and over the Sierra. Forecasters said as much as 2 inches could fall in the North Bay and the Santa Cruz Mountains with 5-10 inches of snow in the Sierra. “The characteristics of the passing Wed/Thur system look more like a winter storm than a May storm,” the national weather service said. While the morning commute on Wednesday by be a little wet north of the Golden Gate Bridge, the evening commute could be very challenging with steady rain in the forecast for most of the Bay Area. “The evening commute on Wednesday could be a mess for San Francisco northward,” the weather service warned. The Wednesday-Thursday weather system is just the first predicted for the next week. Rain chances will gradually diminish Thursday night into Friday, forecasters said, as the upper low exits to the east and ridging develops. The drier weather will be short-lived on Friday and early Saturday as another system will bring additional rainfall this weekend. Another break in the action was possible early Monday, forecasters said, before yet another system takes aim at the Bay Area bringing even more rainfall. “The rainfall over the next 5-7 days is definitely above normal,” the national weather service said. “Parts of the region could see more than the monthly normal (rainfall) in just a few days. When it`s all said and done the one outcome that seems most certain is slowing the onset of wildfire season.” In the mountains, a winter storm watch goes into effect from Wednesday evening through Thursday night above 5,000 feet. “Impacts to Sierra passes were not expected until Thursday evening into Friday morning when snow levels fall to near 6000 feet,” the weather service said. “Sierra passes from Donner to Carson Pass could see 2-7″ of snowfall by Friday morning, but the heaviest snowfall totals are expected on mountain top locations above 8500 feet mainly across Mono County where 10-18″ is possible.”<doc-sep>Rain in May? In San Francisco? It’s about as rare as naked joggers doing a Bay-to-Breakers-style race across Cedar Rapids, Iowa, during a December snowstorm. Related Articles Unusual storm this week could make it the rainiest May in eight years ‘Atmospheric river’ storms to soak Bay Area And yet the rain is on the way. And there’s more where that came from — just in time for Bay to Breakers this Sunday. Meteorologists tell us that Wednesday night’s widespread rainfall is part of a sprawling system that could spawn the region’s rainiest May in eight years. They say we could get more rain in one day than we normally get for the whole month. And with as much as 1.5 inches of rain expected in some parts of the Bay Area, the weather pattern stands in sharp contrast to the norms: On average, this paper reported, San Francisco receives 0.70 inches of rain in May, while Oakland receives 0.77 inches and San Jose 0.51 inches. And there’s still more rain in the forecast, with 10-day totals reaching as much as 2 to 2.5 inches, according to the weather service. An unusually wet spring weather system is forecast to impact our area later this week. Preliminary forecast rain totals from Tuesday night thru Friday range from 0.5-1.5 inches. Most of that rain expected Wed. Night and early Thurs. Impressive rainfall amounts for mid May! #CAwx pic.twitter.com/Y9291e1rWd — NWS Bay Area (@NWSBayArea) May 13, 2019 Travelers headed the Bay Area this month, however, could be in for a bit of a shock. If they used any of the gazillion travel sites out there as they planned for their trip to S.F., here’s some advice they might have received: From ThomasCook “What’s the weather like in San Francisco in May? In the month of May, San Francisco is mild and dry, making this a great time of year to explore the city’s many cultural sights.” From TripAdvisor Asked about an “April or May Trip to SF,” user and presumed local resident “Madeline M” replied: “By April we’re usually past any rain.” Another user said: “You should be good in May. It’s usually past the rainy season, and the fog has yet to build up to its summertime levels. We often have a mini ‘hot spell’ at some point during May (only a few days though). I always remember, while growing up here, it being beautiful, right up to getting out for summer vacations in school.” From FreeToursByFoot “HOW MUCH SUN AND RAIN? Most of the days this month will be sunny or partly sunny as May averages 29 days of sunshine. The rest of the days are more likely to be cloudy than rainy.” From Lonely Planet “Virtually no rain in May,” one user wrote. From SFToDo “It is highly unlikely that you will encounter any rain in San Francisco in May. May is a terrific month to visit San Francisco. It probably won’t rain.” From Quora Local Richard Wood says “The winter rains will taper off and be mostly gone by May.” From TheBlondeAbroad “May means sunshine!” From RoughGuides “Spring and autumn usually have the sunniest days, while summer often sees heavy fog roll in through the Golden Gate. … Winter brings most of the city’s rainfall, sometimes in torrential storms. The best time to visit San Francisco is late May or June, when the hills are greenest and covered with wildflowers.” From TripSavvy “Pack a mid-weight jacket, especially for evenings. Bring short-sleeved shirts and lightweight pants, with a warmer layer. For sunny days, a hat is a good idea. … May begins the drier part of the year, with continued sunshine and even less rain than April.”<doc-sep>
Winter is barging back into the San Francisco Bay Area this week with three storms taking aim at the region across seven days. The first could arrive as early as Tuesday night, when a cold front dives down from the northwest and taps into a moisture plume stretching across the Pacific Ocean. The system is forecast to deliver rain Tuesday night through Thursday, with the first sprinkles falling in the North Bay and then gradually working its way across the central Bay Area. By Wednesday night, rainfall will be widespread and the storm is expected to intensify. "The bulk of the precipitation will be Wednesday night," said Drew Peterson, a forecaster with the National Weather Service Bay Area. "For May it is heavy rain relatively speaking, but it's not comparable to the heaviest rain we see in the winter." Rain chances will gradually diminish Thursday night into Friday as the storm heads east. Meteorologists forecast the first system will deliver .5 to 1 full inch in urban areas, with San Francisco forecast to measure .75 inch, Oakland .85 inch and San Jose .5 inch. North Bay valleys could measure up to 1.25 inches and mountaintops up to 2 inches. The Scripps Institution of Oceanography in San Diego categorizes weather patterns and is calling this system a late-season atmospheric river. These storms carry long streams of water vapor through the atmosphere (250 to 375 miles wide on average) and are like rivers in the sky. When an atmospheric river hits landfall, the water vapor rises and cools, unleashing precipitation. The amount that falls is dependent upon the strength of the storm and its moisture content. With this storm, the region could see more than the monthly rainfall total for May in a single day. Most spots in the region record less than an inch of rain on average in May. ALSO: San Francisco has seen nearly twice as much precipitation as Seattle this year After a break from the wet weather on Friday, a second soaker is forecast to sweep the Bay Area on Saturday afternoon. The wettest period is bound to be Saturday night and showers could linger into Sunday morning, impacting the Bay to Breakers race in San Francisco. Another break in the action is likely early Monday, according to the National Weather Service, before a third storm drapes the Bay Area, delivering another dose of rainfall. Since this system is nearly a week out, details are unclear and the timing may change in coming days. "That's where the crystal ball gets fuzzy," said Jan Null, a consulting meteorologist with Golden Gate Weather Services. "I'm not thinking much about that one at this point. Historically, weather models are less accurate at this time of the year than they are in the middle of winter." While a week-long spell of wet weather is atypical in May for the Bay Area, Null says it's unlikely to be record-breaking. By the end of the third storm, San Francisco will likely have recorded about 1.5 inches since May 1 and the city's rainfall record for May is 4 inches in 1925. The last significantly wet May was in 2015 - when a little over an inch was measured. "It's going to be a good wet event, but it's not going to be record-setting either," said Null. "People can turn off their sprinklers for a few days and Little League practices may rain out."