April 13, 2021

Heat Wave!

 Find your sunglasses.  Stock up on sunscreen.  And get your shorts and tee shirts out.  You will need them.   A Northwest spring heatwave is about to begin.  

The start to spring has been chilly and damp, but that will be a distant memory by this week.   Consider the latest ensemble forecast (running the model many times, each a bit different) for the NOAA/NWS GFS forecast system (see below). Steadily rising temperatures from the low 60s today to the mid-70s on Saturday and Sunday.

The ensemble members are all very similar, which means we should have confidence in the prediction.  

I know your next question.  What about the highly skillful European Center forecasts?  Here they are (below).  Highs of 74 and 73 on Saturday and Sunday, followed by a cool down next week.

What do we owe this turn to torrid conditions?  

A very high amplitude upper-level ridge of high pressure over the eastern Pacific.   The forecast of upper-level (500 hPa, around 18,000 ft) conditions at 5 PM Wednesday, shows the upper-level ridge extended into BC, with troughs (the L's) on both sides.  This is known as an OMEGA block, because it looks like the Greek letter omega.  Very persistent.

And by Friday afternoon the ridge of high pressure amplifies right over us.  Such high-pressure areas are associated with sinking air (therefore cloud-free) and warming conditions aloft.  At the same time, there is easterly or offshore flow at low levels, which isolates us from the cool Pacific Ocean.

To really warm you up, let me show you the predicted air temperatures just above the surface (2-meters).

The forecast for Wednesday at 5 PM, shows temperatures rising into the mid-60s on both sides of the Cascades, with moderate northerly and northeasterly winds

By Friday afternoon at 4 PM the story is very different, with temperatures surging into the 70s over portions of western Washington.  Note that western Washington is warmer than eastern Washington because of sinking and compressional warming over the western slopes of the Cascades.

Temperatures are even warmer on Saturday (note, this a lower-resolution forecast) and the winds are weaker as well, so it will seem warmer still.

It will be marvelous.



The agenda for the Northwest Weather Workshop is up....please check it out here.
The meeting will be on a half day (morning of May 1) and will be online.  Anyone interested can attend and we recommend you register if you want to be on our mailing list.

April 11, 2021

Frosty Morning and an Extraordinary Land Breeze

Much of the state experienced a hard freeze last night, a combination of unusually cold air and clear skies, which allowed the earth to radiate heat to space.  Plus, generally light winds that lessened the mixing of warmer air down to the surface.

As a result of these ideally chilly conditions, a number of daily minimum temperature records were broken on both sides of the state.  

A map of the minimum temperatures last night (Saturday night/ Sunday morning) shows teens over NE Washington (and a 9F over northern Idaho), with low 30s and 20s across western Washington.  

Note the cold temperatures along the Washington coast....we will get back to that in a second!  

Proximity to water has a major impact on temperatures, something illustrated by a close look at Seattle and vicinity low temperatures last night (see below).  Temperatures only dropped into the mid-30s near Puget Sound or Lake Washington, but dropped to the mid-20s over the eastern suburbs of Seattle.  Classic in these situations.

But what really caught my eye this morning was not the frosty temperatures around the region, but the formation of one of the best land-breeze situations I have seen in years.

But first some land breeze 101.

Below is a schematic of sea breezes and land breezes.   A sea breeze occurs when the land becomes warmer than the water in a coastal region during the day, with low-level air moving inland.   Sea breezes cool the coastal zone.   Land breezes are the opposite, occurring when the land cools below the temperature of the nearby water.  In that case, air moves from land to water.

Western Washington and Oregon rarely get land breezes.  

Why you ask?  

Because our water bodies  (like the eastern Pacific and Puget Sound) are relatively cool (around 50F all year round) and the land often doesn't cool below the water temperature, particularly during the warm half of the year.

This morning we had a land breeze of note, one you will be telling your grandchildren about.  The winds at 7 AM showed the offshore-directed, easterly (from the east) flow along the coast (see below)

And the visible satellite at this time along the Washington coast was amazing (below).  You see the clear zone along the coast and the line of clouds offshore?  

Blame the land breeze.  The land breeze extends offshore and is converging with contrasting (northerly) winds offshore, resulting in low-level convergence (air coming together at low levels) that produces upward motion and thus clouds.   

You have really good eyes?  Can you see the low clouds in the Chehalis River valley with a small tongue of river stratus jetting out into Hoquiam bay?

And if you want a little help seeing what I am talking about...check out the next image.  The breeze winds are shown by the blue arrows, the Chehalis cloud jet by the orange arrow, and the offshore clouds at the edge of the land breeze indicated by the red arrow.

To show you how far weather model technology has come, let me show you the forecast for 8 AM from the high-resolution (1.3 km grid spacing) WRF model run by my group at the UW.  

The first image shows the wind speed near the surge by wind bards, with wind speed indicated by colors (white, then blue, then green, then orange).   You can see the jet of winds coming out of the Chehalis River and the expansion of the land breeze offshore, where it converges with northerly winds.

A forecast of the winds and temperatures (color shading) at the same time is found below, with white and dark green being the coldest temperatures. You can see the cold air from the land pushing over the ocean, warming as it moves westward.

Impressively, the forecast model cloud forecast, predicts the low clouds in the Chehalis Valley and even the line of clouds paralleling the coast offshore.

Enjoy the sunny week ahead.

April 09, 2021

New Podcast. Frost to heat this week, plus an in-depth look at the effects of the weakening La Nina.

 A strong front is approaching the Washington coast as I write this, with well-below normal temperatures behind it.    The revenge of La Nina.

Rain will spread across the region later today with strong winds buffeting western Washington and the Cascades.

Forecast of sea level pressure and low-level temperatures at 8 PM tonight (Friday).  Blue is cold air and the blue line indicates the frontal location

The air behind the front will be unusually cool for this time of the year, dropping snow levels below 1000 ft.  6-12 inches of snow in the Cascades on Friday night and Saturday morning. Expect frost on Sunday morning.

But then everything shifts, with a veritable heatwave later this week.

My podcast (see below) will describe the details of this stunning change in our weather, plus a more in-depth examination of the effects of the collapsing La Nina in the tropical Pacific.

Here is my podcast:

Click the play button to listen or use your favorite streaming service (see below)

You can stream my podcast from your favorite services:


The Northwest Weather Workshop, the annual gathering to talk about Northwest weather, climate, and major meteorological events, will take place on May 1, 2021. This year we will have a special session on the meteorology of the September 2020 regional wildfires. The meeting will be online. More information, the agenda, and registration information is found here: https://atmos.uw.edu/pnww/

April 07, 2021

La Nina is Collapsing

This winter has been dominated by La Nina. a period of below-normal sea surface temperatures in the tropical Pacific.   The implications of La Nina have been profound for the West Coast, with above-normal snowpack in the Northwest and dry conditions over the southwest U.S.

Typical conditions during a La Nina Winter

But there is major news on the La Nina front.   La Nina is now collapsing and should be history in a few months.

To show this, let's start by looking at the sea surface temperature anomaly (difference from normal) for the central tropical Pacific,  an area called the Nino 3.4 region (see map).

As shown below, the sea surface temperatures in this area have been colder than normal (blue colors), hitting a maximum of  1.8C below normal in late October.  Anytime the temperature is .5C or more colder than normal, we consider it a La Nina period.   

But look closely:  the cold temperature anomaly has weakened considerably, down to about .6C.   La Nina has radically weakened.  

In fact, if it gets much lower (less than .5C below normal) then we would call it a neutral or normal period.

Another way to see the situation is to look at the sea surface temperatures under the waves--in the upper few hundred meters of the ocean.  This is shown by east-west vertical cross-sections along the equator over the upper 300 meters of the Pacific for 7 February through late March.  Orange/red signify above-normal ocean temperatures and blue below normal.

In early February there was substantial cold water in the upper portions of the Pacific, with warm water limited to the western portion of the ocean.  But by the end of March, almost all the cold water was gone and warm water has surged eastward below the surface about three-quarters of the way across the Pacific.  As this warm water mixes to the surface, La Nina will rapidly become history.

Nearly all of the latest atmosphere-ocean model forecast ( see below) are projecting that the central Pacific sea surface temperatures in the Nino 3.4 area will warm, resulting in a transition to Neutral or La Nada conditions.   I should note that such forecasts are not reliable in spring, a phenomenon known as the "spring forecast barrier."    But by July we should have a very good idea of what next winter will bring.

These rapid changes out of La Nina conditions are good news for California, which has experienced two dry winters in a row, since neutral conditions should bring increased chances of moist atmospheric river conditions to the Golden State.    And such neutral years are often associated with the most active weather in the Northwest, something appreciated by meteorologists.

April 05, 2021

Is Outside Air COVID Safe? Are Masks Needed Outdoors?

When I visit parks in Seattle and the region, bike along the Burke Gilman Trail, cross-country ski at WA snowparks, or go hiking in the Cascades, I note that many people are wearing masks outdoors to avoid COVID-19.  A lot of people.

Some folks are clearly afraid and fearful of COVID exposure outdoors.  For example, I often see individuals making a point to avoid unmasked bikers or joggers on the Burke-Gilman trail...moving 5-10 feet off the trail and sometimes turning away. Occasionally it gets tense, as when one local scientist (not me) was angrily called a "Republican" when he was walking in a park without a mask.  

The fear of outdoor spread even hit the management of Seattle Parks last year, when several parks and their parking lots were closed because of COVID fears.

Photo by Nate Loper Through a Creative Commons License

It is time to bring science to this issue and to relieve the worries of many who wish to enjoy outdoor recreation.

In this blog, I will tell you about measurements of carbon dioxide (CO2) I took around Seattle this weekend, measurements that suggest little threat of COVID transmission outdoors. I will also review some of the latest scientific literature, which again does not indicate a significant threat of COVID transmission in the outside air.

The message of this information is obvious: wearing masks outdoors does not appear to be necessary.

My Field Experiment Around Seattle

    In my experiment, I used a sensitive CO2 sensor that makes use of proven measurement technology (see below).  Human breathing puts out a lot of CO2, roughly 4 percent by volume or roughly 40,000 parts per million (ppm).  The background level of CO2 in the free atmosphere is currently approximately 415 ppm.... a concentration much, much less than coming out of our mouths.

Thus, CO2 can act as a tracer of the air coming out of our respiratory systems, something mosquitoes know all about since they use the plume of CO2 we exhale to find us.  By measuring CO2 levels, it is possible to determine the relative concentration of air coming from humans and the degree of dilution by the surrounding air.

There have been a number of peer-reviewed scientific papers that have shown that CO2 concentrations are a potent tool for evaluating the potential for disease spread by viruses and other agents, with higher concentrations of CO2 associated with more transmission of disease (some examples of relevant scientific papers found here and here).  

Most of these papers use CO2 to measure the potential for transmission in indoor spaces, so why not take it one step further and evaluate potential disease transmission in the outside environment? That is what I did around Seattle on Saturday, a very nice day (high around 63F) with lots of people in the parks and outdoor locations.

My CO2 unit reported about 400-405 ppm away from roads and people, and this value represents the background level of carbon dioxide.

The Burke-Gilman Trail

My first stop was the crowded Burke Gilman trail, with folks walking, running, and whizzing by continuously.  There was no increase in CO2 concentrations compared to the background at any time (around 400).  Clearly, the diffusion/dispersion/dilution of CO2 by outside environmental air was very large.  And the same would be true of any COVID virus breathed out by anyone on the trail.

Magnuson Park

I then traveled to Magnuson Park, one of the most popular parks in Seattle.  First, I walked about a half-mile, back and forth, on a crowded path.  No increase in CO2 values above background (still around 400 ppm).  

Then I decided to really push things and walked around crowded picnic shelters, some with as many as 15-20 peoples without masks. (I suspect they thought I was being a bit odd as I circled around the groups, but this was for science!)   I could find no enhancement of CO2 above the background levels....natural movement of air diffused their CO2 emissions completely.    I did find a weak signal downstream of a raging barbecue fire...but even that was muted (about a 10 ppm increase 40 feet downwind).

So the bottom line so far:  natural ventilation and mixing in the outside air was effective in diffusing the CO2 emitted by people in an outside park environment, even when there was a high density of people.  And if CO2 enhancement by all the people was undetectable, the same would be true of COVID-19 virus emission by any infected individuals.  Parks appear to be entirely safe locations and masks are not necessary.

University Village Shopping Center:  Outside and Inside

Next, I went to the University Village shopping center.  There is a nice little urban park adjacent to the Apple Store.  It was packed with people enjoying the nice weather.  If there was going to be a place where human CO2 would be evident, this would be it, particularly with nearby structures lessening the wind flow.

I walked all around the park, even close to groups of people.  Only background levels of CO2 were observed.  No hint of people's CO2.  Atmospheric mixing was still large, and thus there was little threat of catching COVID-19 from the general environment.  And I found similar results near lines of individuals waiting to enter some U. Village stores.

Supermarkets and Indoor Restaurants

As a little side test, I then went into a busy local supermarket.  The CO2 concentration zoomed up to 830 ppm.  A warning, perhaps, about food stores.  

Then I went into a restaurant, one that was well ventilated (I could feel the breeze) and with reasonable spacing between tables.   The CO2 level was a very modest 520.  Turns out the restaurant had a CO2 meter, which read 530.   Good confirmation of my calibration and a positive note about the restaurant providing a safe environment for its patrons.

A Visit to A Gym

Then I stopped in a local gym, a major one located in northeast Seattle.  Lots of ventilation and the CO2 level was a very modest 465.   A safe place for one's workout and less than the value in my home (which is about 530).   People are required to wear masks in the gym, but quite honestly they probably aren't needed.

Reality Check

I shared my results with some local air quality experts and a well-known epidemiologist.  They thought my results were reasonable and not surprising.   They acknowledged that COVID transmission is far less problematic outdoors.  

One suggested that the risk outside was not zero:  imagine if you had a conversation with a highly infectious individual for an extended time at close proximity (within a foot or two), with little wind and the person was upstream of you.  Perhaps.  But you can imagine how unlikely this situation would be.  And easily avoided.

The epidemiologist, although generally agreeing about the safety of outdoor air, suggested the potential for a very short (transient) exposure to the long-distance breath of a COVID-infected person.

He noted the example of smelling cigar or pot smoke at a distance--might one get exposed to COVID the same way?  I suspect this is highly unlikely. First, large virus-containing droplets tend to fall out close to the infected (that is why there is a six-foot "rule" for separation).  And smaller particles tend to get diluted with distance. But just as important is that current best science suggests that extended or prolonged exposure to a COVID-infected individual is necessary, with the Centers for Disease Control indicating 30 minutes or longer (see here).  That is simply not going to happen outside if you and the source are moving around.

This never made any sense and prevent people from enjoying a healthful environment

Finally, what does the current scientific literature indicate about outdoor transmission?  The scientific literature is very thin on this topic, but here are some examples:

  • Qian et al., 2020:   Examined 1245 confirmed cases in 120 cities in China and identified only a single outbreak in an outdoor environment, which involved two cases. 
  • Nishiura et al., 2020:  Transmission of COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment (95% confidence interval). (they could not exclude the possibility that the few "outdoor" transmission might have occurred by undocumented indoor transmission)
  • Bulfone et al., 2021:  A review of all the literature on outdoor transmission.  They found only five studies, two of which are above.  They noted the poor quality and inconsistent approaches of most of the studies so far.

Quite honestly, one can only be disappointed in the quality of the "studies"  on outdoor transmission, which is surprising at this point in time.  Importantly, there is no compelling published research that demonstrates significant outdoor transmission.  

The Key Message

My observations of CO2 concentrations suggest that the diffusion, dilution, and dispersion of human exhalations are very large outdoors, making the threat of outdoor COVID-19 transmission very, very low.  This is consistent with the (limited) scientific literature on the topic.

Outside air is very safe, and it gets even safer during the summer since ultraviolet radiation rapidly kills airborne virus.  The logical conclusion from these results is that wearing a mask outdoors is probably unnecessary. Closing outdoor parks makes no sense at all.

If any of you see some weaknesses in the above logic, let me know.  That is how science works.
Public Radio Station KNKX and Cancel Culture
:  What Happens When a Public Radio Station Rejects Viewpoint Diversity and Goes Political.

April 02, 2021

Why are their contrails in the sky? And the weekend forecast. All in my latest podcast

 The Northwest sky this morning is full of contrails.  

You can see them in the visible satellite image at 9:46 AM this morning over southern Washington and northern Oregon (they are the thin lines)

Why do contrails exist and why so many today?  Can contrails change the climate?    And what about the claims of chemtrails?

All of these questions are answered in my podcast today (see links below or on the left panel of this blog).

And for my Patreon supporters, I will have a special in-person Zoom session tomorrow morning (Saturday, April 3) to answer your questions and talk about a special topic.

Here is my podcast:

Click the play button to listen or use your favorite streaming service (see below)

You can stream my podcast from your favorite services:


The Northwest Weather Workshop, the annual gathering to talk about Northwest weather, climate, and major meteorological events, will take place on May 1, 2021. This year we will have a special session on the meteorology of the September 2020 regional wildfires. The meeting will be online. More information, the agenda, and registration information is found here: https://atmos.uw.edu/pnww/

Heat Wave!

 Find your sunglasses.  Stock up on sunscreen.  And get your shorts and tee shirts out.  You will need them.    A Northwest spring heatwave ...