Warm But Not Hot

 During the first two weeks of May, there is often a short warm period that surges to 80°F or more, followed by a serious cool down.

This year will be no different.

Below are the forecast temperatures for Seattle.  Warming to 80F on Monday, followed by cooling into the upper 60s, with no major heat through May 11.

To illustrate this typical temperature pattern, below is a plot of temperatures in Seattle for April 1- May 31, 2024.  Observed temperatures are in blue and record highs in red.  Record lows in blue.

A major warm-up around May 9, followed by cooler temperatures the rest of the month.  Classic.  

You will notice that May brings the end of any frost threat.....good for gardeners to keep in mind.

So why do we often see a spike in temperatures in early May followed by cooling?

First, the sun has become strong in May, as strong as it will be in August.  Solar radiation at noon is summer-like, and days have become longer.  

Below is the solar radiation reaching Seattle since January 2025.  By May 1, we are really cooking.

To get warm temperatures, even in summer, we need a period of offshore flow, since onshore flow off the chilly Pacific will not allow us to get out of the 60s.

To produce the upcoming warming, this weekend will bring offshore-directed (easterly) winds, as illustrated by the winds, temperatures, and heights (pressures) around 5000 ft (850 hPa pressure) on Saturday morning (below).

Why do we typically have fewer really warm days later in the month?  

Because later in May brings extensive low-cloud areas over the eastern Pacific as high-pressure builds offshore.  This high pressure pushes the cloudy/cool air into western Washington. 

We call this "June Gloom," but it typically starts to move in during mid-May.

Here is an example of June Gloom cloudiness from last May.  Enough to make me want to grab a sweater.

Contrail Fest over Eastern Washington

The satellite imagery over eastern Washington this morning looked like someone had gone crazy with a white crayon,  drawing many white lines, most extending roughly east-west.

Here is a closer view.   The white lines are contrails created by aircraft.

Look very closely, and you will see some black lines as well:  the shadows are produced by the contrails stopping the sun's light from reaching the surface.

Contrails are formed by the combustion of jet fuel in aircraft engines.  This combustion produces water vapor that condenses into water droplets, which eventually freeze into crystals in the cold temperatures aloft.   Combustion particles aid the condensation.

Contrails are thicker when the air is already close to saturation.  Thus, contrails are particularly obvious when the air aloft is already moist.   

Look closely at the satellite imagery above, and you will see a thin veil of cirrostratus clouds, indicating air already at saturation.  Adding more water from the jet engines simply makes the clouds thicker.

Or we can look at the temperatures (red lines) and dew points (green lines) from the radiosonde (balloon-born weather observations) at Forks, on the Washington Coast (below).  Near the level at aircraft fly (marked trop on the figure), temperature and dew point are nearly the same, indicating a moist atmosphere near saturation! (the Y-axis is height in terms of pressure; 500 is about 18,000 ft, 250 is about 35,000 ft).

Temperature (°C)

Climate Change and Contrails

Contrails both cool that atmosphere (by reflecting solar radiation to space) and warm it (by emitting infrared radiation down to the ground).   

A number of careful studies have found that the warming is dominant.  

Thus, contrails contribute to global warming.  Much, much less than CO2, but still a contribution.

One way to reduce this warming by contrails is to alter flight paths to avoid cirrostratus clouds or regions where the upper troposphere (where planes fly) is near saturation with water vapor.   Some folks have suggested this be done.

In any case, the effects of contrails are relatively small, much, much smaller than the aggregate effects of global CO2 increases.

A Strong Morning Inversion Undermines Air Quality and Messes Up the Coastal Radar

This morning, a strong low-level temperature inversion caused the air quality to decline over western Washington and created false radar echoes on local weather radar.

Reminder:  an inversion occurs when temperature increases with height, a reversal (inversion) of the normal situation in which temperature decreases with height. 

Let me show what lower atmospheric temperatures looked like this morning.

At 5 AM at Forks, on the northwest Washington coast, temperatures increased by about 10°C (roughly 18°F) in the lower few thousand feet (red line is temperature).  Above the inversion, the temperature declined with height

An inversion was also present at Seattle-Tacoma Airport at 6 AM this morning, with about a 6°F warming between 400 ft and 1800 ft (see below)

Inversions create zones of strong vertical stability, inhibiting the vertical mixing of pollutants, allowing concentrations to increase near the surface.

This was quite apparent this morning as air quality declined to the moderate level (yellow colors) in the graphic from the  EPA AirNow website.

Low-level haze was apparent on the Seattle PanoCam this morning around sunrise.

Strong-level inversions can also mess up local weather radar, acting as an atmospheric lens that bends the radar beam down.  As a result, the radar beam reflects off the surface and is reflected back as a false echo, indicating precipitation where none is occurring.

Such a false echo was clearly apparent on our local coastal radar (Langley Hill, near Hoquium) at 7:21 AM.  It was not raining offshore!

Over land, spring temperature inversions weaken rapidly during the day as the land surface warms,  as illustrated by the temperatures over SeaTac at 4 PM (below).  No more inversion as temperatures at the surface warmed into the lower 70s!

So, take a deep breath...air quality is quite good now over western Washington...and I am going to go for a few-mile run knowing that bad air quality won't be a problem.

Fog In the Wrong Season

This morning, fog was widespread over the lowlands of western Washington, with many of the river valleys in the murk (see visible image below around 7 AM)

Several local weather cams showed the fog.  Olympia had almost no visibility.

And it wasn't much better on the Chehalis River Bridge.

All this foggy action is a bit unusual in late April, which is close to the time of minimum frequency of dense fog for most locations in our region. (see below)

Why is dense fog unusual in April and May?

Because days are much longer and the sun is getting quite strong.  Thus, the surface is getting warmer. But the air is still relatively cold aloft.  

That leads to a rapid decrease in temperature with height, which results in instability and vertical mixing.

Vertical mixing is the enemy of fog, which generally forms when cold, dense air is near the surface, and warmer air is aloft.

Exactly, the situation that was occurring this morning (see the temperature above SeaTac Airport this morning below).

So why the fog this morning?  

A strong high-pressure area aloft developed to our northwest (highs of the 500 hPa pressure surface are shown below...think of pressure around 18,000 ft).  Red indicates the high pressure.

Such an offshore high-pressure aloft causes sinking, which warms the middle atmosphere.  Furthermore, high pressure causes clear skies aloft, which allows the lower layers to cool by emitting infrared radiation.

Warming above and cooling below is perfect for fog formation and the development of a low-level inversion.

California and Oregon Are Getting Our Rain: But That is OK

 There is always a yin and a yang in the weather.

Because of the structure and limited extent of weather systems, one area's precipitation bounty inevitably means less precipitation than normal for an adjacent region.

This is often true on the U.S. West Coast.  When California is wet, we tend to be dry and vice versa.

In April, such a reversal of fortune is occurring, with northern California and Oregon receiving above-normal amounts, and less than normal precipitation over Washington State.

Let's start with the current water year (October 1 to now) precipitation totals (the percentage of normal is shown).

Western Washington and the Cascades are in good shape, which is why our reservoirs are full.  Southern California has been wet.  But Northern California and much of Oregon have been drier than normal.

Climatologically, precipitation tends to shift north in the spring as the moisture-transporting jet stream moves northward, leading to a drying of California, while maintaining substantial precipitation in Washington State and British Columbia.

You can see this effect in the climatological precipitation maps for the region (below).

For January, substantial precipitation (dark blue color) extends from Washington State to Northern California and down the Sierra Nevada. Even Southern California gets a piece of the wet action.

But April is different, with far more precipitation falling over the Northwest than California, with profound drying over the southern portion of the state (see below).  In May, the California lack of precipitation is even more profound

But this year, something different is happening. 

The total precipitation forecast for the next 15 days (below) is for heavier-than-normal precipitation in California, with southern Oregon getting a substantial wet bounty as well.    Washington State will get some welcome precipitation as well, but the real action will be south of us.

The folks in LA and San Diego will be startled by all the rain, but the thorough wetting of California will greatly benefit the huge agriculture industry of the Golden State.  

What is going on?  

This is not climate change, but rather the development of anomalous upper-level lows that are heading into California.  

Below are predicted upper-level (500 hPa pressure, about 18,000 ft) maps, with blue showing troughs of low pressure (which are associated with precipitation).

11 AM Monday....a deep trough over northern California.

5 PM Saturday... another CA trough

Monday, May 4?   Another low-pressure area is heading to California.

Finally, although Washington State has a dry period ahead, we received substantial precipitation last night from a wet system that dropped very heavy precipitation exactly where we needed it:   on the eastern side of the Cascades and over the mountain barrier (see 24 hr below).   

Perfect for supporting agricultural needs over the Yakima Valley region.  

The weather gods are taking care of both California and our region.😊

Will 2026 Be An Above-Normal Wildfire Year in Washington State?

Virtually every spring, some media and activists claim that the upcoming summer will bring above-normal wildfire activity over the Pacific Northwest because of global warming/climate change (see example from last year below).  

The Seattle Times Climate Lab consistently predicts above-normal fire risks

So what does real data actually say?    That will be the topic of this blog.

Let's start with actual wildfire information:  the total wildfire acreage over Washington State's WA DNR (Department of Natural Resources) lands for the last decade (see below).

You will note no upward trend.    Also note that the last few "drought"  years have had below normal wildfire area.   

If we compare the Washington State wildfire area against temperature or precipitation across the state, you will note a poor correspondence (see below).  Clearly,  other elements (e.g.,  fuel availability, wind, lightning, human ignition) are important as well.

So what about this summer?  

An important supporting element for fire is the availability of surface dry fuels (e.g., dry grasses), and there is a website (the U.S. Department of Agriculture's Fuelcast website) that has this information.

The image below shows the latest on the availability of surface fuels such as grasses and flammable vegetation (yellow is below average and green above average).   

Lower than normal over much of eastern Washington but above normal over the Cascades, including its eastern slopes.    

So good news for the lowlands of eastern Washington and the western slopes of the Cascades.  A cause for concern over the upper eastern Cascade slopes...where there was lots of vegetative production by the bountiful precipitation this year. 

The latest European Center seasonal precipitation forecasts are a mixed bag.  For April/May/June, predicts drier than normal conditions over western Washington and the Cascades, fostering a drying of vegetation and surface debris.

On the other hand, it is predicted to be wetter than normal conditions for the critical July to September period when most of our fires occur.

For temperature, the EC prediction is for warmer-than-normal temperatures (see below), which contributes to drying.

But as shown earlier, temperature and precipitation are only part of the story for local wildfires.

Lightning is critical, particularly since it often ignites fire in remote areas.  

Strong winds are essential for the biggest fires, contributing to ignition and rapidly spreading the flames. 

Human ignition is important, and that depends in part on the willingness of utilities to maintain lines and de-energize when advisable.   

Fuel availability is significant, and we have a good idea now of the vulnerable areas (eastern slopes of the Cascades).

To put it concisely,  a simple argument that global warming causes more fires is simplistic and generally wrong, and a more nuanced and data-driven analysis is critical.

The Origin of the Puget Sound Tornado

Around 3 PM on Wednesday, a tornado was spotted over Puget Sound (see picture below).  Technically, this rotating wind feature is known as a waterspout since it developed over water.

Such Puget Sound twisters have occurred before, and in this blog, I will describe their origins.

Monday's waterspout was produced by a thunderstorm associated with cold, unstable air forced to rise by a Puget Sound convergence zone.

As I noted in a previous blog, we had unusually cold air aloft on Monday....a gift from the Arctic Express from Alaska.  The figure below shows the temperature at around 5000 ft at 5 PM on Wednesday, with blue colors indicating below-normal temperatures.

With warmer air near the surface, this created a large change of temperature with height, which produces the potential for great instability in the vertical, leading to convection with towering cumulus clouds and even thunderstorms.

Such thunderstorms are aided by having low-level air convergence, which produces upward motion (see below).  This gives an upward kick to the air to rise.

On Wednesday, we had a very effective source of low-level convergence:  a Puget Sound Convergence Zone, forced by air forced around the Olympic Mountains (see schematic below).   

This combination of low-level air convergnce and and an unstable atmosphere produced a band of cumulonimbus clouds (thunderstorms), which were evident on an infrared satellite image with lightning observations  (see below, red crosses show lightning strikes)

The weather radar image at 3 PM indicated the strong thunderstorm cell associated with the Puget Sound convergence zone (red colors show the heaviest rain).

The Space Needle Cam indicated very heavy precipitation with the thunderstorm cell.  Wow.

But why a tornado?    Where did the rotation come from?  

There is inherent rotation from the converging winds in a convergence zone (see below), and that rotation can be increased by the strong vertical motion in a thunderstorm (see schematic below).   

The increasing spin is analogous to the increasing spin of a skater when they bring their hands in (see below)

Puget Sound is hardly a place where storm chasers gather to view tornadoes.    But there is a long history of weak tornadoes associated with Puget Sound Convergence zones, something I describe in my Northwest Weather book.

For example, a bunch of kids were lifted into the air on June 14, 2001, over West Seattle!  And Bill Gates' childhood home in North Seattle was seriously damaged by a tornado in 1962.

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters.  Will answer questions and talk more about the recent Sound tornado and about drought issues.