Sunday, February 22, 2015

The Ridge Begins to Shift

This has been the winter of the ridge, an area of high pressure in the in the lower to mid atmosphere that has brought us consistently warm temperatures.   Downstream of our persistent ridge there has been a trough over the eastern U.S. that given them unending cold and snow.

But the ridge is shifting, with substantial implications for our weather in the Northwest.

Let me remind you of Ridge 101.  The plot below shows a situation with a ridge over the western U.S.  The color shades are the heights of a pressure surface (in this case 500 hPa).  Where the higher heights push northward, there is a ridge, pushed southward--a trough.   When the ridge is over us (like in this figure) we are generally dry.  During most of the year, we are also warmer than normal when the ridge is overhead (but during the middle of winter sometimes we can be engulfed by fog at low levels).  East of the ridge there is northwesterly flow, bringing cold air from the north.   If a disturbance is embedded in this NW flow, we can have precipitation--yes, even snow.  On the western side of the ridge, there is southwesterly flow:  warmer than normal temps and usually wet.  This is when we get the atmospheric rivers, with heavy precipitation.


This winter we have gone between having the ridge over us (dry, and often a bit warmer than normal) to brief situations when the ridge slipped a bit eastward, give us warm, wet conditions.   As a result, we have had an overall warm winter, with normal precipitation.   Rarely, has the ridge slipped west of us, putting us in the cooler flow.   

But that is going to change.

Let's see how things have evolved and will evolve this week by looking a series of upper level (500 hPa maps).

First, last Monday at 10 PM PST.  Big ridge over the coast, but most of the northerly flow is to the east of the region. (winds are parallel to the lines, where lines are closer together, the winds are stronger, higher heights to right of the winds).  The jet stream is associated with the large gradients in the heights, with the jet stream being the conduit for weather disturbances.

 Saturday at 4 PM.  Looks familiar?  But there has been a subtle westward shift of portions of the ridge and northerly flow is working its way into eastern Washington.   Temperatures have cooled a bit...and there was even some frost in my neighborhood this morning.

 Fast forward to the prediction for Wednesday morning.  The ridge has broadened and shifted to the west.  Disturbances (troughs) are getting around the ridge on its northern flank.

 And by Friday AM, the ridge has strengthened offshore and we are now on the cold side of the jet stream

So we are not only moving into cooler air, but are now vulnerable to feeling the effects of weather disturbances embedded in the northwesterly flow.    This is good for mountain snow lovers.

On Thursday/Friday a disturbance WILL reach our region.   Here is an upper level map for Friday at  one AM;  you can see the upper level trough over us and the big ridge offshore.


Thw 48 hr total precipitation ending 4 PM on Friday is shown below.  Modest amounts, particularly over the western side of the Cascades (1-2 inches)


And there will be snow, as shown by the 48h snowfall totals over the same period (6-8 inches in the Cascades) and even eastern Oregon gets whitened up.  Not a game changer, but helpful.


It is not unusual for a late winter weather regime change, particularly for years with dry/warm mid-winters.  It doubt we will make up for the abysmal snowpack, but we may be able to mitigate the situation at the end.

Rider Oasis Questionnaire

A group of bicycle enthusiasts are developing the idea to place convenient kiosks around the city with items of interest to cyclists.  They have a questionnaire available on their website.  Please take a look and give them feedback if you have a minute...thanks.

The Pacific Northwest Weather Workshop

Interested in attending the big local weather workshop of the region?  The Pacific Northwest Weather Workshop will be held in Seattle at the NOAA facility on February 27-28th.   Everyone is invited and the majority of talks are accessible to laypeople.  To attend you have to register or they won't let you in the gate.  There will be a major session on the Oso landslide.  There is a registration fee that covers refreshments and food, and special student pricing.  If interested, check out this website.

13 comments:

Brian Blackmore said...

There seemed to be about 2" of overnight snow on Pratt Mt Saturday morning, and maybe 3-4" on Defiance. It was just enough to prevent me from making Bandera in the same trip, where I heard a report of at least a foot standing.

The prospect of having to carry snowshoes for the last day of the month... I'm about to pass out from the blissful wishing.

Placeholder said...

Cliff, are there any implications for California? I checked the reservoir data there, and the levels are one-third higher than a year ago. I realize that reservoir levels are only part of the story, with mountain snow depth and moisture content, and soil moisture, being principal components.

To my amateur eye, though, it looks like the drought's back might be broken and the recovery in progress. Some questions:

1. Am I too optimistic about the big picture?

2. Are reservoir levels a leading indicator?

http://cdec.water.ca.gov/cgi-progs/reservoirs/RES

Separately, I am kicking myself for not having signed up in time for your March talk. I hope you'll find a way to reproduce its essentials on your blog. Even though I'm pretty strongly in the skeptic camp on the anthropogenic global warming hypothesis, I have a great deal of respect for and interest in your work and your views. We don't hire professors and others of your caliber to tell us what we want to hear.

Hindu said...

A welcome change, as long as it doesn't go too far to the cool side for us ag folks in E WA. Everything looks early.

Love this blog

JewelyaZ said...

Climate change doubters might want to look at these NASA images. There are about 300 of them, paired images over a few months to, in a few cases, 150 years, to show how the Earth has changed. It's not ALL bad, and arguably, it's not all human-changed, but it's a thought-provoking set of images nonetheless. Images of Change

Traveller said...

I took the survey and gave a few helpful suggestions.

Benjamin said...

I just saw this on the popular IFLS Facebook feed.

http://www.iflscience.com/environment/melting-arctic-and-weird-weather-plot-thickens

Curious what your take is on this considering some of your recent posts.

Cliff Mass said...

Benjamin,
It is more of that unfounded claim that is in contradiction to observations, modeling, and theory. Disappointing..cliff

Placeholder said...

@JewelyaZ, thanks for the link to the pretty pictures. They don't prove anything, but they were fun to look at.

Jason Turner said...

Cliff,

Our local fellows are certainly playing it safe as we move into the 72 w/o QPF amounts. Is this due to some lingering model inconsistency, or are we simply hedging our bets? To my eyes, late Thursday into Friday looks like it could give parts of the Cascades a moderate, and much-needed punch of snowfall. Thoughts?

You're an amazing resource to our region, Mr. Mass, and I've enjoyed following your blog over the last several years. Cheers.

Mark said...

Are We Entering a New Period of Rapid Global Warming?

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2923

brucemac said...

Too little too late. I had held out hope for this shift late Jan early Feb like we saw last year, but those hopes were obviously for not. What a total bummer of a ski season. I'm as tired of seeing/hearing about the cold temps and snowfall back east as they are with dealing with it. Hopefully next season is closer to "average" and the ski resorts make things right for 2014/2015 season pass holders.

Colleen said...

"Too little, too late" ~ same thought here. At this point with all the early blooms and outdoor soccer season (school sports) upon us, I'm ready to put this so-called winter behind us and fully embrace spring!

MMenk said...

@Mark, the article you linked starts with a claim that if there'd have been no 1998 super el nino, there'd be no 18+-year "pause" the global temperature increases. That claim is flatly untrue.