The End of Northwest Cold for The Winter and A Short Heat Wave

There comes a point each winter when it is simply not possible to get really cold anymore.

Here in the Pacific Northwest, we are now crossing that line.

The figure below shows observed temperatures at SeaTac Airport so far this year as well as the daily record highs (red lines) and lows (blue lines) for the entire year.  The brown band shows the average daily range throughout the year

Our temperatures so far this year have been quite normal, with warmer and cooler days.  

But now look at the cold records.  In January, record temperatures have dropped below ten degrees on many occasions.  But past the first few days in February, there is an abrupt jump to warmer temperature records, with record lows never getting below 15F.

By early March, we never get below roughly 25F.  

East of the Cascades at Yakima Airport, the situation is similar, except that the temperatures are much colder.  After early February, sub-zero temperatures are history!

The key driver of the transition to warmer temperatures is that the sun is getting stronger and days are getting longer.  You can see this in the plot of solar radiation reaching the top of the atmosphere over the year (below).  

The sun now is about the same as it was in early to mid-November.

 Talking about warmth, according to the highly accurate weather.com prediction, tomorrow through Friday should rise to near 60F at SeaTac.

But if you are willing to take a short drive, you may be able to experience 70F!

Where? In the western foothills of the Cascade!

High pressure has built over our region and will be in place overnight (see the upper level map--at 500 hPa pressure..about 18,000 ft for tomorrow morning).    High pressure results in warm temperatures aloft.

Tomorrow, low-level easterly (from the east) winds will develop that produce sinking on the western slopes of the Cascades, producing additional warming by compression.

As a result, there will be a band of uber-warm temperatures on the slopes in areas such as North Bend.  A UW model temperature prediction at 1 PM shows the warm zone.

A similar, if not stronger, warm zone will be apparent on Thursday (see below), with powerful warming on the western side of the Olympics tomorrow.

There are a lot of weather enthusiasts/weather chasers in our area.  Will one of you take a hike up Tiger Mountain or Mount Si tomorrow, making temperature measurements every few hundred feet?

I will add it to this blog if you give me the information!

High Pressure Danger: Fog and Roadway Collisions

This winter, persistent high pressure has frequently developed over the western U.S.

High pressure that has often brought dry conditions and afternoon sun.  But such high pressure also results in extensive fog, particularly in valleys and basins.

Fog is a serious threat with such high-pressure conditions, often resulting in multi-vehicle collisions, injuries, and loss of life.

For example, yesterday there was a 49-vehicle chain collision in the Central Valley of California between Bakersfield and Fresno as a result of dense fog.

Below is a visible satellite image from yesterday morning, which clearly shows the fog. 

 Such fog in the Central Valley is known as Tule Fog (named after the Tule grass that is extensive over the region).

Fog accident events often progress in a similar way.   

A collection of vehicles is driving too fast for conditions.     The first vehicle in the "pack" enters the fog and hits the brakes suddenly, setting off a chain reaction behind that car that can involve dozens of vehicles.

Washington State is home to many of such fog related accidents associated with high pressure.

Low-level fog is frequently present during high-pressure periods during winter in eastern Washington, with a long list of vehicle pile-ups during the "fog season" from November to February (see satellite image example below on January 17)

For example, a 38-vehicle pile occurred on I-90 near Kittitas on December 7, 2022 (see below).  I could list a dozen other such chain accidents associated with fog over the Columbia Valley.

The danger of such fog events are inceased during cold weather, when freezing fog makes roads slippery and dangerous (according to the 2022 event).

As mentioned earlier, these west coast events are often associated with high pressures aloft, as shown by the upper level (500 hpa, about 18,000 ft) upper level map for yesterday morning.  In meteorological parlance, we call this a ridging situation.

High pressure aloft reduces upper-level clouds, allowing the surface to radiate heat to space, and allowing the surface to cool to saturation (producing fog).   The high pressure aloft is also associated with sinking air that warms by compression aloft.  

Warming aloft and cooling near the surfae produces stable, inversion conditions, which are favorable for fog.

Finally,  the Northwest is one of the foggiest locations in the U.S., so being cognizant of the threat of fog while driving is important.