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.

June Gloom Comes Early

It is the transition that Northwest residents both expect and regret:   the intrusion of low-level stratus clouds into western Washington and Oregon. 

Usually occurring in late May, this year the transition came a bit early, with cloudy skies the rule today and yesterday over the western lowlands.

Below is the visible satellite image this morning.  The low clouds are stopped by the higher Cascades, and the Olympics are an island that extends above the clouds.

As an aside, the heavily irrigated eastern Washington is a sea of green.

The Seattle Panocam was well into the murk this morning (see below), and at my home in north Seattle, there was drizzle from the low clouds.

Yesterday was similar, and the high temperatures at many locations were below normal....the short heat wave is over!

As normal during our late spring June Gloom periods, the entire eastern Pacific is full of low clouds (the visible satellite image this morning is shown below)

The cool, moist marine air is only a few thousand feet deep, as shown by the temperature and dewpoint sounding at Forks, on the Washington Coast (red is temperature, green is dewpoint).  

When temperature and dewpoint are the same (in the lower portion of the sounding), the air is saturated...which means clouds!

Also note the strong inversion...temperature warming with height--above the cool air.  This inversion acts as a stable layer that prevents mixing of the low-level cool air into the atmosphere above.

Why do low clouds form in spring over the eastern Pacific?

Because of high pressure building aloft.  The weather map showing 500 hPa pressure (about 18,000 ft) for 11 AM this morning is shown below.

And at the surface:

Low-level high-pressure offshore pushes cool marine air into the westerm lowlands.  High pressure aloft creates upper-level sinking, which warms the air aloft.  This warm air creates the inversion that prevents the cool marine air from mixing with the dry air aloft.

We get stuck in this pattern for days or weeks this time of the year.   A good time for meteorologists to go on vacation.