Why has California Been Wet and Washington State Cool This Winter

Potential for lowland snow on Sunday--blog around 1:30 PM Saturday

It has been a remarkable winter so far, very different in character from the previous winter seasons.  During the past 90 days, the southwest US, and particularly California and Nevada, have been MUCH wetter than normal.  Some locations have had over 400% of normal precipitation!  Western Washington has been a bit drier than normal.

For temperatures over the same period, Washington and Oregon have been much cooler than normal (particularly east of the Cascade crest), while California close to normal.

So what his interesting north-south pattern and why has it been so persistent?

Well, the proximate cause is easy.  There has been a persistent area of low pressure over the eastern Pacific west of northern California to Washington.  The following figure shows the anomaly (difference from normal) of the height the 500 hPa pressure surface (around 18,000 ft) for the past 90 days.   There is an amazing negative anomaly (purple color) signifying lower than normal heights (or equivalently low pressure).

Storm after storm has brought low heights/pressures along the northern West Coast.

Such a persistent trough or low along the northern West Coast causes the jet stream to be pushed south into California, instead of its normal position coming into the Pacific Northwest  (the jet stream, a current of strong winds tends to follow the outer periphery of the trough).   Troughs are associated with colder than normal air.  So with the jet moving south into California, CA get more precipitation (since the southern of the trough is associated with upward motion), while cold air is found over the Northwest.

But why is the trough found over the northern West Coast?   Good question.  If you look at the pattern of the height anomalies, you will notice a wave-like pattern, with alternating high and low heights.  This pattern is associated with something called Rossby waves in the atmosphere (they are names after a famous meteorologist Carl Gustav Rossby).   Think about throwing a rock into a big pond, with waves radiating away from the rock.

So what is the analog for a rock in the atmosphere?  What is disturbing the atmosphere causing waves to propagate over the Pacific Ocean and north America?

Lots of thunderstorms over the Maritime Continent- places like Borneo, New Guinea, Indonesia, and the adjacent islands.  

Here is a measure of the amount of thunderstorms that we can observe from space--called Outgoing Longwave Radiation  (OLR) for the same 90 day period.  To be more specific, this shows the anomaly (or difference) from normal.  Where values of OLR are low, there are more high clouds from thunderstorms.   You can see a big negative anomaly over the Maritime continent and SE Asia.

Thunderstorm are like rocks to the atmosphere, greatly perturbing the wind, temperature, and humidity fields.   That in turn generates atmospheric waves that can cause localized weather anomalies like we have seen on the West Coast.

But why is there a big thunderstorm anomaly with lots of thunderstorms over SE Asia and the Maritime Continent?    That is probably due to the La Nina of last winter, which wass associated with stronger trade winds that push warm water into the western Pacific.   And why is there La Nina?  Because of a natural oscillation in the tropical Pacific.

Enough questions!  But perhaps there is one more.  Will it snow over the lowlands on Sunday?   The latest model runs suggest some light snow is possible (see snowfall prediction for the 24 ending 4 PM Sunday).  Temperatures are marginal, but where there is some elevation and greater precipitation rates, snow may reach to near sea level.  More on Saturday.

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The Northwest Weather Workshop

And don't forget...if you want to attend the big weather meeting of the year...the Northwest Weather Workshop on March 3-4 in Seattle...you have to register before.  The agenda and more information (including how to register) is found here: https://www.atmos.washington.edu/pnww/