Winter is coming.

This year's much vaunted El Niño is now, by many counts, the strongest on record*. Sea surface temperature anomalies in what is known as "Nino 3.4" region are running about 3 degrees Celsius above normal. The November monthly anomaly in this region according to the official "ERSST" dataset was +2.35C, which exceeds the largest monthly anomaly from 1997-98 (the previous strongest El Niño). Below is a map showing Sea Surface Temperature anomalies in the Pacific. The extensive region of dark oranges in the Eastern Pacific is the current El Niño:

Current forecasts call for this El Nino to remain at peak through December before a gradual weakening begins in January. 

So it's gonna rain here, right?

The short answer is "probably, yes". The long answer is that, yes, a very (record) strong El Niño in the Pacific creates very favorable conditions for a wet winter in California. However, it doesn't mean that suddenly one day the "El Niño rains" will "start". The sharp temperature gradient between the equator and the poles is likely to drag the median position of the jet stream further south than during an "average" winter:

This southern positioning of the jet streams means more storms are likely to plow through California, rather than into the Pacific Northwest. Also, a jet that is positioned further south is more likely to tap into tropical and subtropical moisture, creating what we now call "atmospheric rivers" (which you might know colloquially as the "pineapple express"). 

So when does it start?

We have had a very dry start to winter (and a very dry fall, minus the epic rains in September). But, the climate experts tell us that this is "to be expected." I tend to agree, though I won't lie - I'm getting nervous too. But, all forecasts are for the El Niño effects to really ramp up toward the end of this month into January, February and March. The long-range models are already forecasting a major shift in the next 10 days or so. If these come true, we could be in for a series of soakers around Christmas!

But wait! It's also gonna rain on Sunday

While the full force of El Niño hasn't really kicked in for SoCal yet, we are expecting our first "significant" storm of the season Sunday night. 1"+ is forecast for the bay area, and the trailing edge of a potent front will sweep through Los Angeles overnight Sunday bringing anywhere between 0.1 and 0.5" of rain. Here's a model forecast of radar reflectivity for Sunday at 10PM:

We can probably expect steady rain for about 3 hours beginning around 8 or 9pm on Sunday (earlier in Ventura, later in OC, even later in SD) :)

BUT. EVEN BETTER. The medium-range GFS model is forecasting an even stronger frontal system for next Sunday. It's still a long way off, but here is a map of 6-hour forecast rain totals for 4pm to 10pm Sunday December 20:

 You should never trust models this far out. But what this tells us is that the pattern *might* be changing. The details of next weekend's storm will come into clearer focus as the week progresses, but for now, it really does look like....

Winter is coming.

Mika ToscaComment