A COOLER, WETTER DECEMBER ALL BUT CERTAIN. FIRST STORM NOW IN SIGHT.

Our pattern change will be gradual and start early next week. First will come showers, mainly in the Sierra, then progressively more active weather will arrive later next week.

NOW THROUGH SUNDAY

Mostly dry, mild weather. Strong high pressure in charge. (Image 1)

MONDAY-TUESDAY

A low pressure trough will plunge southward, to the east of California. The Sierra may be brushed by some rain and snow, but we stay mostly dry. (Images 2 and 3) But, this low pressure trough will usher in big changes.

LATER NEXT WEEK

As you can see, the Monday-Tuesday storm will bring very little moisture to the North Bay. (Image 4) But, all models now agree on positive changes later in the week.

By the 10th, low pressure is forecast to strengthen and push west, over California. (Image 5)

The models vary on details, but are in good agreement on the big picture. Here’s the Euro (Image 6).

This westward movement of the trough will allow storms to track further west, over the ocean and pick up more moisture. The Euro outlook for the 10th is the driest, but still good. (Image 7)

TROUGH MAY STICK AROUND

Better yet, the models have been trending further west, and wetter, with each recent run. And, it looks like the West Coast trough may stick around for awhile. Here’s the GFS ensemble outlook for December 18th. Low pressure still in control of our weather. (Image 8)

What this does is open the door for more storms into California. The pattern setting up, with a ridge of high pressure near the Aleutians, and a trough off the West Coast, is ideal for NorCal rain. The GFS sees that. (Image 9) So does the European ensemble (Image 10)

THE GEEKY STUFF

For those who are interested, seasonal patterns are coming together to support the wetter West Coast scenerio later in December. The Eastern Pacific Oscillation (EPO) is forecast to be in a positive phase (Image 11), supporting a West Coast trough. And, the Pacific North American pattern (PNA) is going negative, also supporting storminess off the West Coast. (Image 12)

When patterns align, meteorologists call it “teleconnection”. This gives us more confidence that a big change will in fact arrive.

Way too early to predict rain amounts, but for those interested in a guesstimate, the GFS is willing to provide one. (Image 13)

It forecasts a healthy amount of rain by the 19th. Certainly don’t take that to the bank, as much can change. But it is now fairly certain that mid to late December will be turning much cooler and wetter for the North Bay and Northern California. More updates to come!