Thursday, February 23, 2017

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.


___________________
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/

9 comments:

Westside guy said...

We actually had very wet snow this morning, a mile or so south of Sumner (in the Sumner-Orting valley). Roughly 3/4" quickly accumulated at around 8:00 am. It seemed pretty localized - I noticed in Sumner they didn't even have enough to show on the grass or sidewalks.

Vicki Wise said...

"Western Washington has been a bit drier than normal."

According to the Seattle Times this morning, Friday 2/24, ytd precip thru yesterday was 12.56 inches while the average ytd was 8.45 inches. That looks like a bit wetter than normal.

While much of what is published by the ST is suspect I don't think they would fudge these numbers.

Cliff Mass said...

Vicki... I am talking about an area and not a single station. Take a look at the graphic which is from the Western Region Climate Center...cliff

Rebecca Timson said...

Such a well-written explanatory piece! Thanks, Cliff.

Buddy said...

That same type of anomaly set up 2 & 3 yrs ago over the Great Lakes and the NE in which the media coined polar vortex.

It has been a remarkable winter. I'm 32yrs old and I cannot recall such a cold winter. Yet looking at the stats and records, we might not reach the top 10 coldest. But it's just been persistent. Constant. Never any major arctic outbreak. Just solid cold. Solid snow cover for 3 months now. Longest I have seen snow on the ground. Longer than 96' and 92' where we got way more snow.

The culprit. No westerly winds to eat the snow and warm up the atmosphere. Yakimas' last 50+ was Dec 3rd. It hasn't been above 50 since. We'll probably break 3 straight months. 2 yrs ago we had 45 days above 50 in that same period. The lack of strong west flow/storms, the upper leeward slopes, which benefit from return upslope flow and slop over precip from the west, are down in snow total this year. The upper Yakima Basin is @ 75% right now. But the lower east sites are doing quite well due to the cold and the trajectory of the storms this winter. It's the water at the crest that counts though. The water situation will be just fine this summer. Thank you October and this month.

jeff said...

Warm air holds more moisture. Could it be a symptom of Global warming? More to come? The old-timers here in eastern Washington swear this winter was The Worst!

Elizabeth said...

I love reading your blog. Even though we are waaaaay over here in NW Montana, your forecasts have been pretty spot on for us too. Keep them coming!

Placeholder said...

Warm and dry? Global warming.
Warm and wet? Global warming.
Cold and dry? Global warming.
Cold and wet? Global warming.
Completely average? Gloibal warming.

Sorry, but you're in a cult. And whatever happened to that mantra about it being weather and not climate? I guess that wasn't sexy enough?

JoelGombiner said...

Nice post. What, if any, role do negative SST anomalies in the NE Pacific play in the story?