MOMENTUM BUILDING FOR BIG CHRISTMAS STORMS (BUT RESIST THE HYPE)

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There has been a lot of talk online of a big storm series and deluge coming to California around the third week of December and onward. While there is some evidence to back this up (and we’ve talked about it in prior posts), the weather forecast models are only numerical predictions, and can change dramatically from one day to the next.

That said, confidence is indeed building that a wet period is on the way to Northern California. But I would caution against calling it a certainty, at least until we’re less than a week away from the predicted storm cycle.

Here’s what we know for now.

ALL OF THE MAJOR MODELS POINT TOWARD A STRONG JET STREAM

The major weather models are in agreement that a lower lattitude jet stream (into California and not Canada) will strengthen after about December 15th. By around Christmas, the ensemble models, which average lots of different outcomes, agree that the so called “jet extension” may be pointed right at California, and extend all the way back to Japan. This can be a prelude to a series of wet storms.

Here are the European and GFS ensemble mean outlooks for December 25th.

There’s been A LOT of hype online about this bringing a deluge to California and the West Coast in general. This may happen, but it is also very likely that conditions and outlooks will change, perhaps significantly, in the next week or two.

Some online prognosticators are already predicting a surge of moisture and warm air into the U.S. for late December (due to the strong zonal jet stream into California). This is already influencing predictions for winter energy use and gas prices across the nation.

While some models definitely support this, it’s a bit early to say this will happen.

So, take all of this (below) with a grain of salt.

What we do know is that we normally WOULD expect a strengthening southern extension of the jet stream in an El Nino year like this one. So that supports this scenerio. However there are other factors at work here. Various seasonal patterns are not yet lined up to support this, and so the so called “teleconnections” are still a bit off. We won’t get into the weeds about each one, but those factors could very well modify this expected outcome over the next couple of weeks. We’ll have to see.

The long range models ARE predicting wetter than average conditions for the 2nd half of December, beginning around the 18th. But, they are low resolution LONG RANGE outlooks.

SHORTER TERM OUTLOOK

In the shorter term, there is lots to talk about as well, including rain.

At least two cutoff low pressure centers are expected to impact our weather in the North Bay, the first one in less than a week. The models agree this one will come ashore around this Sunday the 17th with POTENTIALLY moderate to heavy rain. The rain may continue into late Monday.

Here’s the WPC blended forecast for rain, which averages a few weather models. The European and GFS are a bit drier, so don’t count on these totals as they will likely change.

After that, another cutoff low is forecast to spin up in the low pressure trough off the California coast, but this one may miss us to the south around December 21st.

And, after that? If you’ve read this far you know there is a lot of uncertainty AND hype. We MAY get a very strong jet stream that will direct a series of storms into California from around the 25th to the end of the month. Or, the strength and/or orientation of the jet stream steering winds may change over the coming two weeks.

The “deterministic” forecasts only go out 15 days (GFS) and are unreliable. But the GFS is CURRENTLY forecasting a sizeable storm to hit Northern and Central California around Christmas Day and continue for a couple of days. And, there COULD be more after that.

One final word on that. The more dependable ensemble models (that rely on an average of lots of possible outcomes) are, currently, also forecasting a rainy period around December 25th-27th.

This makes the storm pictured above a bit more plausible.

CONCLUSION

While there is a lot of hype out there about a huge series of storms and a massively strong jet stream developing that will push them into California in late December, we know better than to jump on the bandwagon too early.

While some version of that wetter scenerio will probably happen, let’s wait to get out the lifeboats until the pattern remains solid in the 7 day forecast window. For now, be prepared for a major pattern change, clear out the rain gutters, but resist the hype. We’ll provide timely updates and the best information and perspective as the “event” draws nearer.

Stay tuned!