CONFIDENCE GROWS FOR TRAIN OF WET STORMS INTO CALFORNIA

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THE BIG PICTURE

Three days away from the first major storm, confidence continues to grow that lots of rain is headed our way across the North Bay and most of California.

If the current thinking holds, the storms inbound to California would finally resemble a typical strong El Nino pattern along the West Coast. We’ll break the forecast into THREE parts, short term, jet stream, and long term.

SHORT TERM FORECAST

The models have been remarkably consistent that our first storm, a cutoff low pressure system, should arrive during the day on Sunday the 17th, with possibly heavy rain late Sunday into Monday. The rain will likely continue in waves into Tuesday.

The current thinking is that another cutoff low will approach California on the heels of the first, on Wednesday the 20th, but this one may take a southerly track. We get brushed by rain, but the heaviest rainfall may be directed into Southern California.

It’s important to note that the GFS model tracks the storm on the 20th further north, giving us more rain. In any case, here is the WPC blended outlook (averaging a few different models) for rain through the 21st.

Keep in mind this averages the (very) wet Canadian model with the GFS and much drier European model (the one that sends the 2nd system to the south). In any case, all models show 2+ inches of rain across the North Bay by the 21st.

BIG TIME JET STREAM SUPPORT

A key factor at work is a strong low lattitude jet stream, focused at California. This will be bringing us storms through the 21st, and will likely strengthen later into the last week of December.

Here is the jet stream today on Dec 14th, as the cutoff low is forming in a trough to our west.

By the 17th, this cutoff low is sending rain into California. And, to the west off the coast of Asia a strengthening jet is extending east toward California. All major ensemble models agree on this general pattern.

Here is the jet stream forecast a few days later, on the 20th, as a second cutoff low pressure center forms off the California coast and brings us rain. Notice the strong jet stream is forecast to extend even further east across the Pacific.(toward California)

LONG RANGE FORECAST

If you’ve followed along this far, let’s take a look even further ahead.

The latest models runs are in agreement on an extended “zonal” jet stream, pushing storms into the West Coast in the last week of December. This is the jet stream forecast for December 25th on the GFS model. The European model and ensembles are very similar.

Once again we’ll caution this is all in the long range and much can change. But the model agreement is also significant.

The GFS model shows storms spinning up off the coast of California around the 25th and 27th, and then again around the 29th. And looking at the Pacific, more storms appear to be lined up.

By the 29th the jet stream (as currently forecast) is still pointed at the West Coast, and shows the storm on the 29th spinning up near the CA coast.

Once again, this is far out in the future, but the ensemble models tend to support all of this. They look at many possible outcomes and average the result. This is the European ensemble outlook for the jet stream by late in the month, and the GFS ensemble is remarkably similar.

And total rainfall amounts? Well, we can only guess at those numbers, as small changes in the pattern over the next two weeks can have massive impacts on local rainfall.

The ensemble models, once again looking at many possible outcomes and averaging the result, look wet through December 28th, but certainly not enough for widespread flooding. This could change of course. This is the European. The GFS is similar.

Stay tuned for more details as the pattern develops over the next two weeks. While it certainly looks like a series of storms are likely for the last 8-10 days of the month, we’ll continue to provide updates and perspective in future posts.