BIG RAINS COMING: DETAILS AND TIMING FOR THE NORTH BAY

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The shift to a warm and very wet weather pattern is already underway out in the Pacific Ocean, and the impacts for California will begin on Thursday and last for a week or more. Here is the latest information, timing and amount of rain to expect.

On Wednesday the ingredients were already coming together for a “pineapple express” pattern, with a moisture stream from the southwest merging, or “phasing” with low pressure dropping down off the West Coast.

As forecast, the strengthening southern branch of the jet stream is extending eastward toward California. This will direct and intensify a series of storms into California.

The first major plume of moisture, or atmospheric river, will reach us during the day Thursday.

Here you can see the low pressure center to our northwest helping to draw moisture toward California.

Additional waves of energy from this storm will keep showers going well into Saturday. Our rain by Sunday could be 2-5 inches across the North Bay with the highest amounts in the hills. The entire Bay Area down through the Central Coast look to be in the crosshairs.

The latest atmospheric river forecast shows the first AR to reach us Thursday into Friday topping out from moderate to strong in intensity. The second one, arriving late Monday, may be weaker. This is the European ensemble outlook.

By Sunday, as the first storm and plume is moving out, the second one will be approaching for arrival on Monday the 13th.

Rainfall will last into Tuesday. Here is the European outlook for the storm Monday-Tuesday. Possibly a less robust moisture feed. But you can see another storm wave to the west that is forecast (on the European model) to arrive Wednesday.

Here’s a look at possible rainfall by Wednesday. This is a blended forecast that averages out the various model outcomes. Perhaps a total of 5-10 inches of rainfall across the North Bay from the 2 storms.

After that? Well the long range models begin to diverge. The European has another storm for Wednesday the 15th.

While the GFS model has a break until Saturday the 18th.

Although they differ on timing, the major models agree that the jet stream could continue to be extended across the Pacific towards California into the 3rd week of March, directing storms toward us.

And the ensemble models hint that could keep us wetter than normal into the 2nd half of the month of March. However they also suggest that the storms will weaken somewhat into the 3rd week.

Finally, there will likely be plenty of urban and small stream flooding during, and shortly after periods of peak rainfall. Commutes will likely be affected. And, because the ground is saturated, the mainstem Russian River may exceed monitor stage in Guerneville (and even reach flood stage at locations like Hopland). The most likely time for that to happen is late Friday into early Saturday. There is even a chance of reaching minor flood stage at Guerneville, especially with more rain expected on Monday the 13th.

And athough there’s less certainty, the Russian River may stay near monitor stage over the next 10 days if additional storms arrive as forecast.

So, get ready for a lot of rain over the next week or more across the North Bay, this time relatively warm rains with highs in the high 50s to low 60s, and lows mostly in the 40s through the period.

We’ll provide additional updates as changing conditions warrant. Stay tuned!