Meteorological winter in the Northwest usually ends in the third week of February, and it seems unusually dry conditions will prevail.
Perhaps the most ingenious tool for predicting expansion over the next two weeks is the set of multiple forecasts produced by Central Europe. The figure below shows the predicted difference (or anomaly) from normal in rainfall through 4 p.m. Tuesday, February 14.
Much drier than normal (2 inches or more below normal) is predicted for much of the West Coast. This is especially acute for California, where every drop can be obtained.
The National Service GFS aggregator has a similar solution for the same period (see below). Very dry.
What is the source of such Sahara-like conditions in the often wet Pacific Northwest?
A persistent high-pressure mountain peak offshore.
Let me prove it to you. Here are the predicted altitudes above (solid line) at 500 hPa (you can think of this as pressure at about 18,000 ft). Color indicates a difference from normal, with red showing much higher than normal color. A giant high pressure peak will dominate the northeast Pacific).
And this model is a very dry model for the West Coast. Ravines/high pressure areas are associated with subducting air, with subduction strongest east of the ridge. That is exactly where we are.
And why is this huge high-pressure, high-amplitude peak offshore? I suspect La Nina is at least partially at fault. We are in for a mild La Nina winter, with central tropical and eastern Pacific sea surface temperatures essentially cooler than usual. The cool waters of the Pacific Ocean have an effect on the upper atmosphere, creating a train of wave-like turbulence that propagates northwest into the midlands.
NOAA/NWS produced a composite map of the difference from normal of the superior pattern (500 hPa elevation) over La Nina years (see below). The major impact is a high pressure peak (orange and red) over the northern Pacific Ocean.
The exact position of this ridge is very important. If we are concentrated in the central Pacific Ocean (as above), we tend to be cold and wet, with a lot of snow. If we move slightly to the east (like this year), we tend to be cool and dry.
Thankfully, the Northwest is full of reservoirs and snows often as we enter this dry period of mid-winter. California has a thick layer of snow and ice, but its reservoirs remain below normal due to previous dry years. But at this point, there’s still time for more rainfall after these dry weeks, and it looks like La Nina is starting to weaken.
So wait a moment.
