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.