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As our mid-winter dry spell continues, we can take heart that December and January dry periods are more the norm than the exception across California.
Over the past 70 years, central California has consistently seen dry spells like our current one, averaging 19 days in length. If you’re interested, here are the numbers for the past 20 plus years, with the length of the dry spell, its starting date, total rainfall for the year, and the winter weather pattern. (We are currently in a La Nina year) This is based on San Francisco’s rainfall, which averages just over 20 inches a year. So, anything over 21 inches is a wet year. (Image 1)
If you saw our video from Lake Sonoma a few days ago, you know that our weather pattern is stuck for the next two weeks. High pressure will keep storms away from the West Coast, and in fact most of the Western U.S. The GFS is the most optimistic, showing some weak storms beginning to push in during the final days of the month (Image 2)
When weather patterns are stagnant, it can be helpful to look at the ensemble forecasts. These are a series of model runs which start at the same place, but make slightly different assumptions about how atmospheric conditions will evolve. The European ensemble forecast for rain across the west averages 50 separate forecasts. Here is the current average of all of them. (Image 3)  Similar to the GFS above. Pretty dry, with a hint of moisture arriving late in the month.
When you look at the separate model runs, a bit more detail emerges. About 40% are completely dry through the 27th of January, but about 60% show some rain, and a few are wet. (Images 4 and 5)
Our best information comes from the ensemble outlooks for rain, and also the ensemble outlooks for pressure. Most show high pressure beginning to move to the northwest and away from the West Coast during the last few days of January. (Image 6)
Whether that opens up the storm door a bit to an undercutting storm track, or storms moving down from the north, is uncertain at this point. (Image 7)
The long range outlooks are pretty useless beyond 10-15 days. However, the European hints at an opening up of the pattern, with at least an average amount of rain arriving during the first week of February. (Image 8)
For now, we wait and hope. The good news is that most of the state is still running above average (greens) thanks to our October AR and the relatively wet December. (Image 9)
For what it’s worth, the Euro Weeklies outlook (not a forecast) for possible rain through the 9th of February. Way, way in advance, so take with a grain of salt… (Image 10)
We’ll have more details on what’s to come in January and February as the weather pattern evolves.

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