Wednesday, December 11, 2013

An Unusually Dry Fall

A number of people have commented to me about how dry this fall has been...and they are right!  Let me show you the story.

Let's begin by looking at the percentage of normal precipitation from October 12 through yesterday (December 10th, see graphic).   Mighty dry in the western U.S.   Much of Oregon and Washington have received less than half of normal precipitation;  in central and northern CA less than 25% has been common.

Looking more closely at the last month, here is the cumulative precipitation at Quillayute, Seattle, and Spokane (red is observed, blue is normal).  Quillayute is down about 10 inches, Seattle down by roughly 3.5 inches, and Spokane by nearly 2 inches.
The USDA Drought Monitor web site highlights the very dry conditions in California and Nevada (see graphic).  As of Dec. 3, extreme drought covers central CA and northern Nevada. 


Ok, you are convinced it has been dry.  But why?   

The reason has been a very persistent ridge of high pressure over the eastern Pacific.   Here is an average upper air  (500 hPa) map for October 1 through December 8th.  You can see the eastern Pacific ridge quite clearly.
We can subtract this average 500 hPa map from a long-term average climatological map and get what is called an anomaly map, which highlights the differences from normal.  Anyway, here is is.  You can see the anomalous high heights (high pressure) over the eastern Pacific--really clear.
We can create the same type of anomaly map for sea level pressure (see below).  Same story...very anomalous high pressure over the Gulf of Alaska.
Why did we get such an unusual pattern?  I don't have that answer and I suspect no ones else does either.  Not expected during a neutral year,  which is neither La Nina nor El Nino.

The latest NOAA Climate Prediction Center three month precipitation is for normal precipitation (see below). 


Tomorrow we will get the first wet, mild weather system in a long while, with rain developing in the morning.   The atmosphere has warmed up sufficiently that the lowlands will only experience rain.  Enjoy normality.

2014 Washington Weather Calendar

Need a Good Weather Calendar? 
Like to Help UW Atmospheric Sciences students go to conferences?

Then buy the 2014 Washington Weather Calendar!  You can order online (about $15. plus shipping) here.  A few dollars  goes to the UW Chapter of the American Meteorological Society for each calender.  A fine holiday gift for the weather lover. 

Jeff Renner  of KING5 will be signing calendars at the U Book Store in Seattle on Saturday, Dec 14th at 3 PM.  Info here.

9 comments:

Hindu said...

PDO? That W ridge seems to hang around annually during the winter.

Jeffrey Jacka said...

Has the area of high pressure in the gulf of Alaska lifted? Or is it still there?

Eric Moss said...

Can we look back into past seasons where there has been a similar anomaly to try and determine what the weather "may" be like for winter? IE does this type of fall ridge correlate to a stormier, snowier winter, or a continuation of dry benign weather?

Glenn said...

I know you've rejected Francis and Vavrus' paper on slowing Rossby waves. But it certainly seems to fit in well with the prolonged cycles of unchanging weather we've had here in the last few years; ie long dry spells, long wet spells, long warm spells, long cold spells. IIRC, you said they were looking at the wrong altitude. _But_ their hypothesis seems to fit observed phenomona; what would it take to advance it to a theory?

dbostrom said...

Discussion of a couple more papers pertinent to Glenn's question here:

http://arstechnica.com/science/2013/12/melting-arctic-sea-ice-could-be-altering-jet-stream/

Authors and findings seem to be slowly piling up. Maybe there's some coalescence around the notion?

Cliff Mass said...

Glenn,
As noted by the Barnes paper I have noted, the Francis and Vavrus paper has very serious methodological deficiencies. Really no evidence for their hypothesis..cliff

Unknown said...

Yet several other papers have added weight to the Francis/Vavrus conjecture, or variations of it. This is far from settled.

Unknown said...

I can't help wondering if the focus on sea ice loss is a bit misplaced...it's the incredible anomalous warming of the whole arctic region which COULD be affecting the jet stream. The sea ice loss is just one component contributing to this.

dbostrom said...

Unknown, this paper by JA Screen delves into components a little bit:

http://iopscience.iop.org/1748-9326/8/4/044015/pdf/1748-9326_8_4_044015.pdf