Like an invisible wall, a high-pressure ridge is stretching along the entire West Coast, from northern Washington to Southern California, blocking Pacific storms and keeping conditions dry.

Meteorologists, who can only predict weather with some accuracy about 10 to 14 days out, are forecasting the ridge will remain in place, likely preventing any rain until at least Dec. 15.

After that, they don't know what will happen, but they're watching the system closely as its trajectory plays into whether the Northwest, Northern California and Southern California see wet or dry winters.

"This sort of pattern is reminiscent of what we see during drought years," says Daniel Swain, a climate scientist at UCLA. "If it really does just last two or three weeks, it's not a big deal. The real question is wether it will continue to come back."

Only time will tell whether the ridge persists and returns, but Swain says there's some indication that this season could be marked by high-pressure systems on the West Coast based on the conditions in the tropical Pacific, including the presence of La Niña as well as other factors.

"We've know for a long time that cool water temperatures in the eastern tropical Pacific Ocean associated with La Niña can produce a ridge in the Gulf of Alaska, which often tilts the odds in favor of winter high pressure near California and drier than average conditions especially in Southern California," Swain said. "But there's new, emerging evidence that the tropical west Pacific is just as important — and that unusual warmth there can produce a rain-blocking high pressure pattern right over California.

"This year, we have both conditions in play: cool eastern and warm western tropical Pacific. That would suggest an increased likelihood of winter ridging this year, and an increased chance of drier than average conditions especially across the southern half of the state."

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The Northwest saw rain as recently as Saturday, but there's no storm system in the forecast for the next 10 days.

Two-week dry stretches are uncommon in Oregon and Washington during the wet season, which usually sees rainfall every couple days. The last time Seattle saw a dry spell in December of the magnitude forecasters are expecting this month was 1999. Portland has seen only 10-plus consecutive dry days in December 10 times since 1943.

"The last time a 10-day streak happened was 2009," the National Weather Service office in Portland posted on Twitter.

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Northern California has already experienced dry conditions for a week. The last rain of note fell on San Francisco last Monday when two-10ths of an inch were recorded.

"If we do not have any additional rainfall through the 15th, it will be the eighth driest start on record for December," says Charles Bell, a meteorologist with the National Weather Service's Bay Area office. "There have only been two Decembers on record when San Francisco did not see any rainfall—1989 and 1876."

While one dry stretch of two to three weeks is typical for Northern California during the rainy season, it's not normal for the region to see dry stretches that extend beyond three weeks or multiple dry stretches.

But Swain says with these high-pressure systems, which he calls "ridiculously resilient ridges" because they're stable and persist, long dry periods can occur.

"During the five-year drought, the ridiculously resilient ridge was a defining characteristic from an atmospheric perspective," Swain says. "We saw a three-week stretch and and then another and another, and they'd be broken by moderate storms. It'll be interesting to see in a couple weeks if this is the return of the ridiculously resilient ridge. California is really susceptible to these high-pressure ridges, but it's not to say the Pacific Northwest isn't completely invulnerable."