Substantial Precipitation Will Soon Return to the Pacific Northwest

For those worried about Pacific Northwest drought, I have some news that should give them substantial comfort: substantial rain and snow will return to the Pacific, accompanied by cool temperatures.  

And not a hint of any heatwaves.  In fact, much cooler temperatures are expected.

So get your rain jacket out of the closet, get ready to plant some seeds,  and think about delaying that late-May hike.

Let's start with the best, the European Center model forecast for precipitation over the area (below).

The expected precipitation through Wednesday afternoon starts the wetting process:

By Sunday evening, all the mountains will be thoroughly moistened.  

But there is more!  By the morning of Monday, May 25, the totals are impressive, with some mountain locations exceeding 4 inches.  Even rain in eastern Washington.  This is serious precipitation.

Importantly, temperatures will cool substantially over the next week, so there will be substantial fresh snow in the Cascades (below).   Expect the % of normal snowpack to rise substantially from this late-season snow.

So what is going on?  

With the change in season and the transition from La Niña to El Nino, the upper- level pattern has unlocked, and some strong troughs of low pressure will be moving into the Northwest.  

To illustrate,  below is the forecast upper-level map for Friday morning.  An impressive trough of low pressure (blue colors) will build off our coast.

A week later, ANOTHER strong trough is forecast....winter-like in intensity in a similar position.

Other modeling systems (like the NOAA GFS) are doing similar things.   

Extremely favorable and well-timed for Northwest agriculture and local water resources.

Major Forecast Failure

Weather prediction has become hugely more skillful during the past several decades, but there are still some failure modes.

This week in western Washington, we had a master class in forecast failure due to our local weather nemesis:  low clouds.

On Monday afternoon, the National Weather Service forecast for the high temperature in Seattle on Tuesday was 76F.   For Wednesday afternoon, 72F (see below).

The actual highs?  66 F and 57F.  

HUGE errors on the cold side (10F and 15F!)

Instead of a continued warm period, temperatures were well below normal (see a comparison to climatology below).  The brown color shows the normal range. 

The vaunted UW high-resolution ensemble of many model forecasts at Seattle (below) was highly accurate on Monday, but too cool on Tuesday and Wednesday (yellow dots are observations).

In contrast, forecasts of the National Weather Service and the UW model for the Washington Coast and Eastern Washington were excellent.

So what is going on?   

This event (and others like it) demonstrates a great weakness of most weather prediction systems:   an inability to predict or maintain a cool, shallow cloud layer.

Most models tend to produce too much low-level mixing, which destroys shallow, cold, saturated layers.

Let me show you the proof.  

There are visible satellite images for 2 PM on Tuesday and Wednesday.  Lots of low clouds over the western lowlands.  Clouds associated with cool surface temperatures.

In contrast, the predicted clouds, the UW WRF model, or the National Weather Service GFS models had no low clouds over western Washington.

Several of us are working on this problem, but in the meantime, either the National Weather Service forecasters need to more actively intervene in the forecasts, or we need to more effectively apply statistical corrections during these low-cloud periods.

This is a serious problem for Western Washington since low clouds are quite frequent during late spring and early summer.