Northern California could see 8 consecutive days of rain. Here’s when it starts

Something happened in San Francisco on Tuesday that hasn’t happened in almost a month. It rained.

On Wednesday morning, Bay Area weather stations recorded the first measurable precipitation in 27 days. It’s an impressive streak, especially considering it took place in what are typically two of the wettest months of the year

It was the longest November-December dry stretch in nearly 40 years. But it probably won’t be remembered for long.

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Rain and snow, potentially significant amounts of both, are again in the forecast for the Bay Area and much of Northern California. Tuesday’s light rain was simply the announcement that the Pacific storm door is open again. After nearly four weeks without precipitation, San Francisco could see eight straight days of measurable rain starting Friday and leading up to Christmas.

That makes Thursday the likely last dry day before the holiday.

You’ll feel the change even before the rain arrives, as humidity rises steadily on Thursday. The air will feel milder and slightly humid, a stark contrast to a December that has been colder than normal so far.

That increase in moisture is a classic signal of an atmospheric river—and that’s exactly what’s developing. Initially, the moisture plume targets the Pacific Northwest, a region that does not need more rain. But by Thursday it will slide south, bringing rain to Del Norte and Humboldt counties by Friday morning and the Bay Area by Friday afternoon.

Friday afternoon looks wet around the Bay Area, something to keep in mind for holiday travel. There should be breaks in the rain, but Saturday night into Monday morning currently looks like the most active window. Sunday seems to be the worst day for travel.

That brings up an important point about the stretch that follows. While it may technically rain every day until Christmas, it won’t rain non-stop. There will be ebb and flow, depending on the location of the atmospheric river.

This is a good source, a real Pineapple Express, stretching from the Pacific Coast past Hawaii and originating less than 1,000 miles from the International Date Line. It enters an abnormally warm Pacific Ocean, providing it with sufficient moisture and making it less likely to break down quickly.

That’s why, after the first peaks of wet stretches Sunday through Tuesday, a second, potentially stronger storm system could arrive around Christmas Eve.

What remains uncertain is exactly where the core of the moisture plume settles. Those details often don’t come into focus until storms are just around the corner.

However, we are not flying blind. Forecast models can resolve the general location of the atmospheric river, and we can rely on how similar configurations have behaved in the past.

For now, the two main forecasting models do not agree. The European model shows a wetter solution, about 2 to 4 inches of rain through Tuesday in the Bay Area, with more possible around Christmas. The American model is considerably drier.

The two models should reach a consensus in the coming days, although it is worth noting that the American model has performed quite poorly in recent months.

Bay Area breakdown Thursday

San Francisco: The day will start with an abundant amount of low clouds and fog in the city. Drizzling may occur in Twin Peaks and other high elevations. Otherwise, it’s a mostly cloudy day. Temperatures will be cooler than Wednesday, with highs in the mid to low 50s. But humidity levels close to 100% will compensate for the drop in air temperature and it won’t feel particularly cold. A little afternoon sun helps a bit, too. Clouds and fog return overnight, with lows in the 50s.

North Bay: Parts of the North Bay will be hard pressed for sunshine, especially along the Highway 101 corridor. Areas of patchy dense fog could slow the morning commute a bit before lifting in the afternoon. It will remain mostly cloudy, however, with the best chance of sunshine east of Napa. Temperatures will be in the upper 50s, with dew points not far behind. It remains cloudy and mild overnight with light showers moving in from the north in the pre-dawn hours.

East Bay: Clouds and fog will be most persistent in the morning along the stretch of I-80/580 through Berkeley. It is likely to remain mostly cloudy along that stretch for much of the day, with some partial calm in the Tri-Valley and Livermore valleys. Temperatures will be in the 60s across the board and it will feel slightly humid. Overnight, clouds and fog thicken closer to the bay, with temperatures in the 40s and 50s.

Peninsula: Some areas of dense fog will persist around parts of the 101 Freeway in the morning. Parts of the Peninsula will see afternoon sun as a light gulf breeze should keep the marine cloud layer at bay. Places that are more likely to be locked in cloud — like South San Francisco — will see temperatures in the upper 50s, while places with more sun potential — like Redwood City — will see highs in the 60s. Then it will be mostly cloudy and foggy again overnight, with lows in the upper 40s to around 50 degrees.

South Bay: Low clouds and areas of fog will persist for much of the morning in the South Bay. By afternoon, the sun will peek through these clouds in most places. Temperatures will be mild with highs in the low to mid 60s. The region should be cloudy with patchy fog overnight with lows in the 50s.

This article was originally published at Northern California could see 8 consecutive days of rain. Here’s when it starts.