HUGE MODEL FLIP: NORTH BAY RAIN LIKELY TO RETURN AFTER (A WARM) CHRISTMAS

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SUMMARY

In the past few days all of the major forecast models have trended in one direction, now predicting a wet end to the year across Northern California. This big change comes after almost all earlier forecasts had us dry through the end of the year.

In fact, the latest model runs show moderate to heavy rains a real possibility across Northern California between December 27th and January 1st. Will it happen? Read on for details.

NOW UNTIL CHRISTMAS DAY

Even as extremely cold air plunges into much of the U.S. east of the Rockies, we will enjoy a warming trend right through Christmas Day.

The changes will be dramatic. To start the week we’re still cooler than normal across NorCal, while frigid air begins to plunge south over the upper plains.

By the middle of the week, while we warm up rapidly, the middle of the country is tightly locked under an extreme arctic blast, with freezing temperatures reaching the Gulf.

By next weekend, temperatures across all of the West should be well above normal as the artic blast spreads east. The North Bay should see upper 60s for highs with a chance of reaching 70 degrees on Christmas Day.

LONG RANGE FORECAST. CHRISTMAS DAY TO JANUARY 4TH

Changes come rapidly after Christmas. High pressure over the West disappears as a strong jet stream sets up across the Pacific.

This kind of “zonal” jet stream can carry lots of moisture. A series of storms are forecast to ride along, with the energy pointed directly at the West Coast. This graphic shows the first rains reaching NorCal late on December 27th.

This far out the models differ on how far south the storm track will dig. And that will be critically important. We could get a lot of rain, or just a sprinkling.

The GFS model is more aggressive at present, bringing us multiple storms with potentially significant rain totals by the 4th of January. The European model keeps the storm track further north, with some rain (but less) reaching us.

The VERY GOOD news is that all of the ensemble models (averaging lots of runs and possibilities) show us wetter than normal over the last 5 days of December.

At this point it looks like these storms will not be especially cold ones, since they are approaching us from the west, and not the north.

REALLY GEEKY STUFF

For those who really like to dig deeper, the Eastern Pacific Oscillation pattern, which can help predict troughs or ridges (stormy or fair weather) along the West Coast looks set to plunge into negative territory in the 2nd week of January. That could foreshadow another stormy period. We’ll see.

But for now, we’ll focus on the weather from now through early January. More updates to come.