Saturday, May 16, 2015

Weather Role Reversal

It is always fun when the normal weather patterns reverse.

This week for instance, the normal precipitation pattern reversed over Washington State, with eastern Washington getting hit hard, while only light showers damped the west.  Here is the percentage of normal precipitation over the past week.  Pretty dramatic!  Some locations around Yakima and Tri-Cities had over 800% of normal rain.  We are talking about 25% of their ANNUAL precipitation in one day!  The Olympics, the home of the rain forests?  Bone dry.


This role reversal was associated with a reversal of the wind direction and storm locations, with a low center moving northward east of the Cascades and easterly flow causing upslope over the eastern slopes of the Cascades.

But role reversals have been occurring over a larger scale as well.  For much of the winter and early spring, the eastern U.S. has been anomalously cold, while the western U.S. has been unusually warm (the reason for the famous snow drought that has caused Governor Inslee to make a statewide drought declaration).  But what about last week?

The roles have reversed.  As shown by the graphic below (max temps, for the last week), it has been warmer than normal over the northeast and colder than normal in the West.


Why?  Because the large scale atmospheric circulation pattern has changed dramatically.  For much of the last year there has been enhanced ridging (high pressure) in the west and troughing (low pressure) in the east.  Like this.



But last week the pattern reversed, with a broad trough over the western U.S.  Big difference!


This change of circulations has also produced a role reversal along the West Coast.  For most of the winter and early spring, the Northwest has been relatively wet while California has been dry.  Oregon has been much drier than Washington.   But with the deep West Coast trough, the jet stream has been heading into California and they have been much wetter than normal.  Heavy rain hit San Diego and the folks freaked out there (see graphic)


And this atmospheric pattern will reverse the situation between us and Oregon, which is going to get hit VERY hard with moderate to heavy precipitation during the next week.  Here is the forecast precipitation for the next 72h.  Lost of rain from eastern Oregon into the Willamette, as well as northern California.  Oregon ducks will be happy.


The next 72h?   Lots of rain over eastern Oregon, but spreading into SW Washington.  California gets plenty.  We are talking about substantial drought relief for many, considering such sustained heavy precipitation is unusual for this time of the year in the West.

Why is this happening?  Some wags might smirk that it results from all the drought declarations, which are like red flags in front of the meteorological weather gods.  But there is another possibility: an extraordinarily resurgent El Nino that is revving up in the tropical Pacific in a way that is highly unusual for this time of the year.
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Sunday Morning Update:  The eastern slopes of the north Cascades have been hit by substantial rain, with some locations getting over an inch of rain during the last day.  Here aer the 24h numbers ending 9 AM Sunday.  There are just the areas that often experience wildfires.


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Announcement:  On June 3, I will be on the Seattle Channel's Civic Cocktail with Governor Inslee.  If you would like to be in the audience, you can get tickets at: http://www.seattlechannel.org/CivicCocktail.  They charge, but you get appetizers with their no-host bar.

4 comments:

Curt Nelson said...

Cliff, you are the Weather God...

Can you help out a rookie and give the forecast for Memorial Day weekend in Seattle?

Keep up the good work and all the best.

Buddy said...

Exactly Cliff. I left a comment last time not saying this is caused by El NiƱo but this pattern reminds me of it.

Stubborn blocking high in Western Canada and storms undercutting, diving low pressures off the California coast, combining with the subtropical jet for crazy weather in the southern plains and gulf coast states. It's been non stop there.

Also, all of this moisture is helping to push fire season a little later. If it wasn't for a strong sun and colder upper atmosphere, one can see why this would be a drought pattern in January but a wet one in May. Urgh...don't even want to think about it.

Lynn Cs said...

Is this expected to last all summer? I have not been here long, only 10 years, but I moved here for the rain. I have watched there be less rain and more sun ever since. The last 5 years it has been slowly getting worse.

Rain would be great right now and yes, I know you do not control it.:)

Ansel said...

Here is plea to the weather gods: PLEASE give us an all-clear Memorial Day weekend for a change! It has been many years since we had one. Early indications don't look too promising but... I want to go sailing in the sun and not have to wear four layers to stave off hypothermia. And to swim in the lake.

I think it is about time. Comments?