A Dry End to Meteorological Winter

Meteorological winter in the Northwest typically ends the third week of February, and it looks like unusually dry conditions will dominate.

Perhaps the most skillful tool for extended prediction over the next two weeks is the ensemble of many forecasts produced by the European Center.   The figure below shows the predicted difference (or anomaly) from normal for the precipitation through 4 PM on Tuesday, February 14th).  

MUCH drier than normal (brown colds are 2 inches or more below normal) is predicted for much of the West Coast.  This is particularly serious for California, which needs every drop it can get.

The National Service GFS ensemble system has a similar solution for the same period (see below).  Very dry.

What is the origin of such Sahara-like conditions in the normally soggy Pacific Northwest?

A persistent ridge of high pressure offshore.  

Let me demonstrate that to you.    Here are the predicted upper-level heights (solid lines) at 500 hPa (you can think of this like pressure at roughly 18,000 ft).  The colors indicated differences from normal, with red indicating much higher than typical.   A huge ridge of high pressure will dominate over the northeast Pacific).

And this pattern is a very dry one for the West Coast.   Ridges/high pressure areas are associated with sinking air, with the sinking strongest on the eastern side of the ridge.  That is exactly where we are.

And why is this huge, high-amplitude ridge of high pressure offshore?   I suspect La Nina is at least partly to blame.     We are in a moderate La Nina winter, with the central and eastern tropical Pacific sea surface temperature being substantially cooler than normal.   The cool waters of the Pacific have an effect on the overlying atmosphere, which produces a train of wave-like disturbances that propagate northwestward into the midlatitudes.

NOAA/NWS has created a composite map of the differences from normal of the upper-level pattern (500 hPa heights) for La Nina years (see below).  The big impact is a ridge of high pressure (orange and red colors) over the northern Pacific.   

The exact position of this ridge is critical.  If centered over the central Pacific  (like above), we tend to be cold and moist, with lots of snow.  If shifted a bit eastward (like this year), we tend to be cool and dry.

Thankfully, the Northwest has full reservoirs and normal snow as we go into this mid-winter dry period.   Califonia has a decent snowpack but its reservoirs are still below normal due to the previous dry years.    But at this point, there is still time for more precipitation after these dry weeks pass by, and it appears that La Nina is starting to weaken.  

So stay tuned.