July 17, 2024

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington.

In fact, there were quite a few thunderstorms yesterday over southwest Oregon (see below, each dot is a lightning stroke) and this activity is moving north as I write this.

So what is going on?  Let's start with the visible satellite image around 7 AM this morning (below).  I have a red arrow pointing to Seattle.    

Wow.  An impressive-looking band of clouds stretches north of the Oregon/Washington border from the ocean to eastern Oregon.  That is a change.

This band even has some lightning in it (see the lightning flashes at 6AM below)

The radar (below) indicates some precipitation with it, although little is reaching the ground.

Why is this cloud/rain band there?   

The key is that the persistent upper-level ridge (high pressure) has moved inland and an upper-level trough is approaching (see upper-level map--500 hPa pressure/roughly 18,000 ft) below).  The heights (think pressure) are shown by the solid lines and the shading shows relative humidity at that level, with white being the highest.

A trough is moving northward along the Oregon coast.  The approach of a trough causes upward motion that increases relative humidity in front of it and releases instability in the atmosphere.  Such instability can cause thunderstorms.

Some amateur weather sites claim that "monsoon moisture" is moving up from the southwest, but that is not operative in this situation.

Here in Seattle, you can see some clouds and incipient instability aloft, as illustrated by a view around 7 AM from the Seattle PanoCam (see below). It's not impressive at that time, but you tell see something is happening aloft.

So what is going to happen this afternoon?

With the trough approaching and the ground warming, instability, convection, and thunderstorm activity will increase, particularly over the Cascades and eastern Washington.

Here is the forecast radar reflectivity (a proxy for precipitation) at 5 PM today from the NOAA HRRR model.   Some significant thunderstorms over the Cascades!

The simulated radar image by the UW WRF model two hours later from the UW WRF model shows significant activity over eastern Washington.

The lowlands of western Washington will probably escape the rain and lightning.

One concern, of course, is lightning-initiated wildfires, something that DNR and others will have to be on the lookout for.
NOTE:  I will do a special online zoom session at 10 AM on Saturday for Patreon supporters.  Topics include the upcoming summer weather, wildfire weather, and more.

July 15, 2024

Fog and Smoke: An Intimate Relationship in the Northwest

 One of the oddities of Northwest meteorology is that westside fog/low clouds and eastside smoke are often directly related.   

Yesterday was a good example.  To illustrate, consider a series of visible satellite images from yesterday and this morning. 

Yesterday morning at 9:45 AM there was lots of low clouds along the Washington coast and just a hint of smoke from the Pioneer Fire on northern Lake Chelan.

But by 7:45 PM yesterday evening, the smoke from the Pioneer fire had exploded, with the plume heading eastward.   Clearly, strong westerly (from the west) winds were present.

This morning, the low clouds had really pushed into western Washington and there was extensive smoke over northeast Washington from the Pioneer Fire.

To demonstrate that the Chelan fire was the only fire act in town, here is the satellite-based fire emission map from yesterday--lots of  orange dots (fire locations) near Chelan indicate the fire.

So what happened?    The west-east pressure difference increased (higher to the west) as cool/dense marine air pushed into western Washington.   This is associated with the upper-level ridge of high pressure moving inland.

This westside push of marine air and increased pressure diffenence across the Cascades resulted a substantial strengthening of the winds above the eastern slopes of the Cascades.   

To illustrate this, here are the winds at Ellensburg (times in PDT).  Big surge of winds there yestergy, with gusts nearly getting to  40 mph.

Take a look at the max winds yesterday across the region (below).  Pretty windy over the eastern slopes and downstream of the Columbia Gorge.  No wonder wind energy is up.

This is turning out to be a very benign fire season so far, with burned acreage far below normal.  Importantly, the latest model runs suggest no heatwaves over the next week.

July 12, 2024

The Weather Regimes of Summer

 Weather patterns tend to get "stuck" for extended periods and we have certainly seen such persistent conditions this summer.    Weather patterns can be self-reinforcing and thus stay in position for many days or weeks.   And oceanic forcing, such as associated with El Nino and La Nina, can cause weather patterns to "lock".

Consider the temperature situation at SeaTac Airport from May 1 to yesterday (below).  The blue bars show the observed highs and lows, brown shading shows the climatological or typical ranges, red shows the record highs and blue the record  lows.

The middle of May through the end of June was associated with below average temperatures, while early July has been warmer than normal.  The cool (and moist) June is a one reason why wildfire acreage in our region has been below normal so far.

Looking at the latest forecast for Seattle for the 10 days, we note that the forecasts are stuck in a very boring pattern, with temperatures remaining in the low to mid 80s.  Average highs for this time of the year are roughly 77-79F.

No major heatwaves are predicted and the nights cool into the lower 60s and upper 50s, so decent sleep is possible without AC.

Why are we stuck in the lower to mid 80s?    To explain, below is the average upper level pattern (at 500 hPa or around 18,000 ft) for the next week. Shading shows the difference from normal.

A ridge is evident (higher heights), but centered inland.  The kind of pattern that makes western Oregon and Washington warmer than normal is not associated with heatwaves warming into the 90s,  such as last week (where the ridge is centered on the coast).

We are now entering the climatologically warmest and driest period of the year.   Enjoy.

July 10, 2024

The Cooling Has Begun

 The differences in temperature between noon today (Wednesday) and yesterday are quite large west of the Cascade crest (see plot below).  Some locations are 15-20F cooler, particularly across southwest Washington and NW Oregon.

The Columbia Basin temperatures were relatively unchanged.

With cooler air moving in across the coastal region, low-level air pressure is increasing there, producing an enhanced difference in pressure across the Cascades.

Such increased pressure differences increase westerly winds (winds from the west), and we can see the wind strengthening over the eastern slopes of the Cascades.  For example, the wind at Ellensburg is now gusting above 35 mph (see plot below).

Such increased winds result in increased wind energy generation (see green line below), which is good...we need it.

The next few days should bring steadily declining temperatures over western Washington.

Let me show you the state-of-the-art UW ensemble of many high-resolution forecasts for Seattle temperatures.  High around 85F today and 80F tomorrow.  Temperatures at night decline to the upper 50s.  Decent sleep beckons. 

The National Weather Service NBM forecast is similar and very boring.  No heat waves. No cold waves.  Just perfect weather rising into the lower 80s.

In contrast, the Tri-Cities will remain around 100F for the same period  (see below).

No rain is predicted on either side of the Cascades during the next week.  

Enjoy the weather.

July 08, 2024

The Current Northwest Heatwave: Facts Versus Hype

 We are now in the middle of a heatwave period in which some locations have broken daily temperature records (records for a specific day)

Let me describe what is happening and why.

Below are the high temperatures yesterday (Sunday, July 7).   70s along the coast and over the marine areas of NW Washington.  Lower 90s in Puget Sound, around 100F in Portland, and low 100s over the Columbia Basin.

Why so warm?   

We start with being near the time of maximum sun strength and length of day.   Temperatures can warm until 5-6 PM this time of the year.

But the real secret is the position of high pressure aloft, positioned today and yesterday in the "sweet spot" for Northwest warmth--over southwestern BC.  The map below shows the upper level (500 hPa) map today, with the shading showing the difference of the values from normal (orange and red are above normal).  Perfect for local warnth.

And yes, there is global warming.  You can give credit for about 2F of the heat to increasing greenhouse gases.   For example, a location that reached 93F yesterday, would have been 91F.   

We still would have had a heatwave without global warming.

So what about the future? 

The Seattle Times is up to its old tricks and stating that a "100 Degree Heat Wave" looms for Puget Sound (see front page clip today) and "among the warmest nights in history."   Scary stuff.

And not true.

Let's look at the surface air temperature predictions from the very high-resolution UW forecast model near the time of max temperatures (5 PM)

At 5 PM, most of Puget Sound country is in the upper 80s and lower 90s, but warmer around Portland, and MUCH warmer (over 100F)  at the lower elevation of the Columbia Basin.

Tomorrow is much of the same story west of the Cascade crest, except a few degrees warmer south and southeast of Puget Sound.  

Sorry, Seattle Times....no century temperatures predicted near Puget Sound.    But much warmer around the Columbia Basin...over 104F in many places.

But what about Wednesday?  Much cooler in the west, and even eastern Washington starts pulling back. 

Why cooling?  Because the upper level ridge weakens and moves eastward and an upper trough of low pressure moves in (see upper level map for Wednesday).  Marine air starts to push into western WA.

But there is a danger in this change.

As cool air and high pressure build into western Washington, it will produce strong winds over the eastern slopes of the Cascades (see wind forecast for late Wednesday).  Reds, grays, and greens indicate stronger winds.

Winds that can rev up and wildfires.  

Finally, what about the Seattle Times claims about us experiencing one of the warmest nights in history?

Just wrong.

Even at crazy warm SeaTac airport the low temperature last night was not even close  to being a record (see below).  The plot shows the highest minimum temperature each year and the red line is last night at SeaTac.  Many years had warmer minima.

And using a far better station for climatological analysis (Olympia), last night's minimum was nothing unusual.  Most years have had warmer minima.

Stay cool....There is no major heatwave predicted for the next week.

July 06, 2024

Unbelievable Change in Temperature in a Very Short Distance

Can you imagine having the temperature change from a chilly 59F to a torrid 101F in only 3 miles?    Or a similar change ascending a modest mountain, with temperature INCREASING with height.

All this happened today in the Bay Area.  Perhaps this should not surprise us considering the craziness of San Francisco!

Consider the high temperatures today around the Bay Area.  59 F on the Pacific side of San Francisco, but 101F at the top of Mount Tamalpais, at around 2500F.   Even higher (105F) at a nearby mountain site.  About 40F change from Stimson Beach on the Pacific to the top of Tamalpais.....3 miles away.


Stunning contrasts.

This situation was characterized by the very cold water of the Pacific and an associated shallow cold air layer near the surface with an extraordinary inversion capping the cold air.

Below is an analysis of the sea surface temperature of California. (sorry, it is in centigrade),  Very cold water is found along the coastal zone of California, the result of upwelling cold water from below.

How cold?  About 9C or about 48F.  Wow...very cold.  This cold water chills the air right above.

We can check out the vertical temperature structure in the area using the ballon-launched radiosonde released at Oakland, CA this morning at 5 AM (see below).  Height is in pressure (850 is about 5000 ft, and the temperatures are in °C).  The right line is temperature, and the left line is dewpoint.

Mama Mia.  Was there an inversion!   A shallow, cool, saturated layer was apparent in the lowest few hundred meters, with a huge inversion overhead in which temperature increased from 14C to 35C (57 to 95F) in about 1500 ft.

What about our area?

Some impressive coastal contrasts exist today, but not in the same league.  Our coastal water is not as cold and the contrasts stretched over more distance.   To illustrate, here are the high temperatures today on the central WA coast.  60s on the coast and about 101F in the inland low terrain.  Still impressive.

Acknowledgment:  This amazing situation was brought to my attention by Dr. Peter Benda.

July 05, 2024

July 4th Smoke, Wildfires, and the White Sky

When you looked at this morning did you notice the hazy whiteness in the sky?   July 4th fireworks are too blame.    And such fireworks have started wildfires, including a major burn near Wenatchee.

Let's start by look at the Seattle Panocam at 6:30 AM this and yesterday mornings (below).  I bet you can see the difference.  At my home, the sky looked white near the horizon.  Why white?  More later.


Yesterday, July 4.

Fireworks smoke has caused significant air quality degradation over much of the region.

The western map at 830 AM (courtesy of the Puget Sound Clean Air Agency), show poor air quality (red and purple colors) over extensive areas downwind of population ceners.

And the US EPA's AirNow graphic also show poor air quality near Wenatchee (see below)

Why Wenatchee?  Because someone ignited a major grass fire west of the town with fireworks (see map below).  That individual has been arrested.

 There is another grass fire, supposedly started by powerlines, southeast of Chelan.    The visible satellite image at 6:30 AM shows the smoke from these fires, plus the Pioneer Fire up Lake Chelan.

Plots of air quality (actually small particle concentrations) at Seattle and Marysville (both shown below) illustrate the rapid increase in smoke last night.  The Marysville numbers were crazy high (about 130).



Finally, why do these particles turn the sky white?

It turns out this is due to something call Mie Scattering.    When light from the sun interacts with smoke particles is it scattered by the particles in many different directions (see below)

Image courtesy of Virtual Labs

Light from the sun includes all wavelengths of visible light (from red to blue/violet).   Light appears white when all wavelengths are represented and if you were above the atmosphere the sun looks pretty white.

Large smoke particles from fireworks scatter all wavelengths of visible light similarly, thus producing a white-looking sky.  This is what is meant by Mie Scattering.  In contrast, small particles in a clean atmosphere (such as the typical gases) scatter shorter wavelengths (like blue) more (called Rayleigh Scattering), which gives the sky a blue cast.    

Heavily polluted cities in China have a lot of big particles, so their skies almost always look white.  Perhaps it is ironic that our fireworks come from China.  They export their white skies to us during our day of celebration of independence.  

July 02, 2024

The Two Most Turbulent Airports in the U.S.

Many folks are flying this summer and most are not fans of in-air turbulence.

So which airports are most prone to turbulence and why?    

And if you are flying to one of these destinations, how can you minimize the risk of a bumpy flight?

All will be revealed in this blog.

Recently, the aircraft turbulence website, Turbli, published its analysis of the most turbulent airports in North America (see results below).  The top two are Portland and Denver.

I can certainly confirm the results for Denver, to which I fly all the time (the National Center of Atmospheric Research, NCAR, is in nearby Boulder).

So why are landings and take-offs so turbulent for these locations?   

It is all about geography and nearby terrain barriers.


As shown in the map below, Portland Airport (PDX) is essentially due west of the Columbia River Gorge.

During winter, cold air and high-pressure build to the east of the Cascades, while low centers approach from the west.   The result is a strong pressure difference across the Cascades that produces strong low-level easterly winds in the Gorge that reach the airport (see model simulation below).

While the winds are strong and from the east at low levels, the winds above are generally westerly (from the west).  This produces a strong vertical wind shear (winds changing rapidly with height.

Strong wind shear is a principal driver of turbulent motions.  Summer conditions are less conducive to Portland airport turbulence.


Denver Airport (KDEN) is northwest of the city and just east of the Front Range of the Rockies (see map).

The Rockies are turbulence generators throughout the year.

During summer, and particularly from June through early September, thunderstorms develop over the Rockies during the late morning and early afternoon and then drift towards the airport.   Thunderstorms produce lots of turbulence.  

Check out his satellite picture for midday on June 23rd...you can see the cumulus clouds bubbling up on the Front Range.   Fly in from Seattle and you will fly through these clouds and will bump around considerably.

If you want to avoid summer turbulence at Denver, FLY IN or OUT EARLY before the thunderstorms rev up.  My rule of thumb:  don't fly in or out of the DIA after 11 AM during the summer.

There are few such thunderstorms during the winter, but there still is plenty of turbulence, this time from strong mountain waves downstream (east) of the Rockies (see schematic below).   Such mountain wave turbulence is encouraged by strong westerly winds approaching the crest of the Rockies for the west, a very frequent situation during the winter!

Such mountain wave turbulence can be extreme.

Happy flying!

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington. In fact, the...