A cold storm system taking shape this weekend over eastern Russia is likely to impact the entire West Coast of the U.S. by next week, but the full extent of the impacts remains uncertain.

As some 20,000 people arrive in San Francisco for the Asia Pacific Economic Cooperation leaders summit this weekend, weather conditions should be pretty nice to start. Saturday and Sunday look to be pleasant fall days with some sun and highs in the upper 60s — a fine introduction to SF in November if there ever was one.

Come Tuesday or Wednesday, though, the chances of rain are high, and that rain could stick around through Thursday.

The Chronicle's meteorology team explains that "A supercharged jet stream is slated to develop across the Pacific Ocean over the weekend," and "Weather models predict a strong storm will develop off the Siberian coast on Saturday."

The shape and strength of that storm will need to be assessed before we know how much rain we should expect next week. The Chronicle puts the rain chances at fairly high on Wednesday, with anywhere from an inch to four inches falling across California over the course of this thing.

The Weather Channel has a 60% chance of rain arriving in SF Tuesday.

This storm system could also mean some early-season snow up in the Sierra, which will be a boon for the ski resorts that all now have opening dates in the coming weeks.

Meanwhile, enjoy the sunny fall weather, which is expected to bring us an afternoon high of 66 today, and 63 on Friday — Thursday is forecast to be slightly chillier and more overcast.

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