Rainshadow on the Wrong Side of the Olympics

As shown below, the Olympic rainshadow is normally on the northeast side of the Olympics, while heavy precipitation is found on the south and southwest side of the barrier.

This configuration is due to the typical southwesterly (from the southwest) winds approaching the barrier during the cool season, resulting in upward motion (and precipitation) on the windward (SW) side and sinking motion (and little precipitation) on the leeward (NE) side.

But something unexpected happened on Tuesday morning.   

The weather radar indicated precipitation in the rainshadow area, with nothing apparent on the southwest side (see below, there is some ground clutter on the SW side from the radar beam hitting some peaks).

A cloud band was quite evident to the north of the Olympics that morning.

Rain gauges picked up modest rainfall north of the Olympics, with precipitation on the dry-sky lovers in Sequim and vicinity.  Surely unwelcome for those hitting the links that morning.

The origin of this backwards situation?   The low-level winds had reversed, with northerly wind approaching the Olympics from the north (see surface wind below).

This situation was associated with high pressure to the north and low pressure to the south (see below), with the wind blowing from high to low pressure.

We will be dry until the middle of next week, after which the atmospheric spigot turns on in a major way.

La Nina is Collapsing: Implications for Late Winter and Spring

As predicted, La Niña is collapsing, which should be good news for those worried about water supplies.

La Niña is associated with cooler-than-normal surface ocean temperatures in the central tropic Pacific:  what is known as the Nino 3.4 region (see below).

Here is a plot of the temperatures in the area (actually, the difference from normal). Most of this winter, these temperatures were .6 to .9 C below normal: a weak La Niña.  But during the past month, these ocean temperatures have started to warm, resulting in "neutral" conditions.

A plot of the sub-surface temperatures across the central tropic Pacific shows the warming (more red colors) and loss of cold surface waters (less blue colors)

The latest NOAA forecast of temperatures in the Nino 3.4 area shows rapid warming, with the cool surface waters gone by April.  Neutral conditions will be in place then.

So why should you care? 

 La Niña tends to produce big ridges of high pressure in the Gulf of Alaska, with occasional breakthroughs of moisture to the south (see schematic below).   We saw a lot of this configuration after January 1; however, the pattern shifted a bit to our south.    

In January, the coastal high pressure produced dry conditions over the NW, and the precipitation was mainly directed into central and southern CA.   A modest shift in the pattern has made a big difference for the Northwest.

But as the La Nina weakened this month, the ridge weakened, and low pressure replaced it, producing the cooler/wetter conditions of the past weeks.   

You can see this transition in the figure below, which shows upper-level maps for January 4 to a few days ago.   Red indicated higher than normal neights (pressures)

January 7-21 had a huge ridge located off of southern BC....no wonder we were dry!

January 11-Feb 9 was transitional, with the ridge weakening.

Feb 6-21 was VERY different, with troughing (low pressure over the West Coast).

With La Niña disappearing, there is no reason to expect any locked-in upper-level patterns.

 Thus, no reason to expect a return of the high-pressure ridging that kept us dry and warm.