Has Global Warming Changed the Nature of Northwest Heatwaves?

 On July 9th, the temperature surged to 98F at Seattle-Tacoma Airport.  This was a record for the day and 22F above normal.

But that is not what surprised me.....SeaTac had gotten that warm before.

It is was HOW it got that warm.

I have completed a lot of research on Northwest heatwaves and published roughly ten articles on this subject in the peer-reviewed literature.   Historically, to get that warm in Puget Sound you needed easterly (from the east) winds over the Cascades, winds that descended the western slopes of the Cascades. (see schematic).

Easterly winds are good for Puget Sound lowland warmth in many ways.   First, they remove the influence of the cool Pacific Ocean, which is only around 50F.   Second, the air comes from the generally warmer interior of the continent.   And finally, air sinking down the western sides of the Cascades warms by compression.

Strangely, the situation on July 9th did not fit the traditional heatwave situation.  

Why? 

 Because there was little evidence of easterly flow.   Seattle Tacoma Airport got very warm without the powerful assistance of atmospheric compression.

I was not a little bit shocked by this realization.  Could I prove this was more than a fluke?

So I decided to do a little research...which I am going to write up this year.  I found all the times when the temperatures at SeaTac reached 95-99F.    (Remember that the highest temperature on record there was 108F in 2021).

Then I examined the regional weather patterns associated with the first five events at these temperatures and the last five events achieving the same (again 95-99).  Here are the two collections of dates.

The first group occurred before global warming was significant.

The second group after human impacts would have been expected.

For each of these collections of dates, I found the average sea-level pressure patterns and 5000 ft winds (actually 850 hPa pressure winds).   

The results were stunning.   

Let me show you the upper level (again around 5000 ft winds) before and after global warming for such events.  For the earlier events (below global warming, GW), major heatwaves required easterly winds (look at the little arrows).  The shading gives the wind speed.  After GW, the warming assist from strong easterly winds was not necessary.

Why not necessary?  The atmosphere starts out warmer for a start, with summer temperatures now 2-3F warmer than 50 years ago.  And there are other potential factors associated with global warming as well, such as drier soils and alternations of weather patterns.  Furthermore, SeaTac Airport temperatures have undoubtedly been warmed by several degrees due to the construction of the third runway, airport development, and increased urbanization in the area.

Then I asked another question:  What about the really extreme heatwaves, like the 2021 event where SeaTac got to 108F or the 2009 event where 103F was reached?

Do they require easterly flow?  

I checked.....and the answer is yes.  To get uber heat, an assist from downslope compressional warming is required.

Anyway, this is a fascinating topic:  how are the weather patterns required for extreme heat changing?   One I plan to examine in great detail.

Record Breaking Precipitation on the Driest Day of the Year

 As noted in an earlier blog, the end of July and first week of August is typically the driest period of the year.  The time for outdoor weddings.

And for Seattle, today is historically the driest day of the year.

Going against climatology, a relatively wet weather system moved in today and the wetness is not over, as shown by the NWS radar imagery around 8 PM Monday (see below).  At this time, some healthy rainbands were moving in from the northwest.

The air feels "sticky" with high dewpoints (into the lower 60s), resulting from rain falling into relatively warm air.

The precipitation totals so far today have been impressive (see below).  Bellingham has received 0.78 inches (a record for this day) and some of the foothills to east of Bellingham have gotten over 1 inch.  Lot of rain in southern BC and our coast got about 3/4 inch.  Central Puget Sound received roughly .25 inches with lots of variability.  

So far Seattle Tacoma Airport has only received .13 inches, less than the previous record of .17 inches.  But the day is not over yet!  Decent rain in the Cascades and mountains of NE Washington.

More rain is predicted for the next day (see forecasts for the 24 h ending 5 PM Tuesday) and the  influence of this rain and much cooler than normal temperatures will be a substantial reduction in wildfire threat for the next week over much of the region.   As shown in the forecast map below, northwest Washington and the western slopes of of the regional terrain get most of the additional precipitation.

Things should dry out after tomorrow (Tuesday) and temperatures will move back to normal and then a bit above-normal levels (highs in the low 80s in much of western WA).  Today was coolest day in a long time for the region, with only a handful of stations getting to 90F in southeast Washington.

The precipitation and cool conditions have substantially lessened fire danger  (see official estimates below, green is good).  Western Washington is in good shape for the next week, with drier conditions returns later in the week over the eastern side of the Cascades).

My next blog will describe how this year has been of quite a few grass fire events but relatively few forest fires.

The Wettest Driest Day in Seattle History

 July 29th is climatologically the driest day of the year in Seattle.

On this July 29th (Monday), an unusually wet system is predicted to move into the central and western portions of Western Washington.

The result?  The wettest day on record for the climatological driest day of the year at Seattle Tacoma Airport.    

You will tell your grandchildren about this day.

Consider the historically record-breaking wet days at SeaTac on July 29th (see below).

The wettest day was 0.17 inches in 1993.

The NOAA/NWS ensemble system is going for about 0.25 inches on Monday, although there is some variability in the forecast.

The  UW ensemble high-resolution prediction system is predicting very wet conditions for Monday and early Tuesday over western Washington, with the total precipitation through 5 PM Tuesday exceeding an inch in the mountains and about 0.5 inch around SeaTac. 

Which smashes the record!

The highly skillful European Center total precipitation accumulation prediction for the same period is similar (below).

Several daily precipitation records will fall that day.  

What about temperature?   

As you can expect, it will be much cooler than normal, with highs only reaching the mid-60s over the western lowlands.   Below is the predicted temperature anomaly from normal from normal at 5 PM Monday.for the European Center surface air temperature forecast.

Wow.   Most of the area will be at least 10F cooler than normal and some locations 15D cooler and more.  Eastern WA will enjoy some cool relief.

My professional recommendation for Monday.   Find a sweater and an umbrella.  

Rain is Coming to the Northwest During the Driest Time of the Year

 Every meteorologist knows that climatology only provides typical or average conditions and that natural atmospheric variability can provide some major excursions from normal conditions.

That will certainly be true next week.  Climatologically, the last week of July is the driest of the year in western Washington (see plot at SeaTac below).  Best time of the year to plan a barbecue or an outdoor wedding.

But this year, the last week of July will bring rain...and even significant rain in some locations.

Let me show you.

The weekend will be generally dry, with a wet system moving in Sunday night/Monday morning.

The 24-hour totals ending 5 AM Monday show lots of rain along the coast, with moisture extending into NW Washington.

The next 24-h totals ending 5 AM Tuesday show western Oregon and Washington getting thoroughly dampened with as much as a half inch of rain.  A lot for this time of the year.  Even eastern WA gets a piece of it.

And light rain continues into Tuesday.

How certain are we of this rain?  

Take a look at an ensemble of many predictions for rain accumulation in Seattle for the next week using the National Weather Service's GFS model. Each gray line is a different forecast with the blue line being the high-resolution prediction.  Considerable variation (and thus uncertainty), but nearly all the forecasts predict rain.

And don't forget the temperature.  The highly accurate Weather.com website has the high on Monday not even getting into the 70s (see below).

This cool/wet period should have minimal thunderstorm activity and will help reduce subsequent wildfire activity.

Northwest wildfires are most frequently grass fires. Climate change is not an important contributor to grass fires.

When many folks think about Northwest wildfires they visualize a burning forest.

But in reality, most Northwest wildfires are grass or range fires, with trees playing a minor role.

As we will see, grass fires have little to do with climate change, but a LOT to do with flammable, human-spread invasive grasses and human ignition.   And such grass fires can subsequently contribute to wildfires in adjacent forests.

Consider a visible satellite image from early this afternoon (from a NASA MODIS satellite).  You can clearly see the smoke from some fires over eastern Washington and Oregon.  This satellite also can sense the heat from the fires (orange dots).   

Look closely and you will notice that few fires are over elevated terrain with dense forests.  Most are associated with extensive grasses and range vegetation (including small shrubs).

For reference, here are the current fires from the Northwest Fire Coordination Center.

One of the most active Washington fires is the Black Canyon fire northwest of Yakima, which has spread over 4500 acres in grass (see photo below).

The biggest fire in the region is the Lone Rock Fire of northeast Oregon, which has now spread over 131,000 acres.  Grass and shrubs.   Most of the fires in our region...and most of the smoke production..is from grass and range vegetation.

In fact, the USDA Fuelcast website diagnoses how much grass/range fuel is available (see latest determination below).  Bountiful, above-average values are in place over eastern Oregon and Washington (dark green colors) thanks to a moist spring and leftover dead grass from last year.

It is important to note that large dangerous levels of light fuels such as grasses are NOT due to drought, but rather above-normal precipitation during growth periods.

Summer heat and drought have little impact on grass fire potential since our normally dry summers are virtually always able to cure and dry out grasses and the like.

Consider the Columbia NWR RAWS site near Othello (a picture of the nearby terrain is shown).

The ten hour fuel moisture percentage (the moisture level of light vegetation) for this site for the past several months is shown below.   Under roughly 12% is dry enough for fire.  Clearly, such fuels have been dry enough to burn for months. During every summer such fuels are ready to burn.  With or without global warming.

But something HAS changed that makes grass/range fires more likely:  the replacement of native (less flammable) bunch grasses with invasive, flammable grasses such as cheatgrass (also known as grassoline).

Here is a map of where cheatgrass has moved in.  Large areas of our region.

In addition, these human-spread flammable grasses are more likely to be ignited these days, with large increases in population as well as fire-inducing human activities (from fireworks and off-road vehicles to gunnery practice to deficient power infrastructure, to name only a few.)

Strong winds play a critical role in spreading grass fires, something of particular note on the windy, lower eastern slopes of the Cascades.

Too many media outlets blame grass and range fires on climate change without factual bases or supportive information.   Grass/range fires also play a large role in inducing forest fires, something I will discuss in a future blog.

Will this be the driest July on record for the region?

 Several folks have asked me whether we will break the record for the driest July in history.  

Several locations, including SeaTac Airport, have had no precipitation this month, and to put it bluntly, little is predicted for many locations.  

First, how much has fallen so far?   For western Washington, some locations have received a bit, such as the Cascades (from some thunderstorms last week) and along the coast.

Eastern Washington has had less, most of it from a few showers from hit-and-miss thunderstorms last week.

Tomorrow, there could be some very light precipitation across the western portion of the Northwest due to a weak upper-level trough of low pressure moving up the coast.  The latest forecasts suggest that air will be sufficiently unstable for a few convective showers, particularly over the Cascades and the coast, but nothing you would write home about.

To illustrate, here is the latest UW WRF model simulation of clouds at 2 PM tomorrow.  

Looks impressive, right?  You can tell it is convective by the mottled appearance of the simulated clouds.  Lots of mid-level cumulus.

But the rainfall is very modest....here is the forecast total through 5 AM Monday.  A few convective showers in southern Oregon, light precipitation along the coast, but very little in the interior.

The NOAA/NWS HRRR model is similar.

The UW extended accumulated precipitation from this morning through the end of the month (below) predicts considerable rain over coastal BC, but little over much of the Northwest, except for the far southwest that gets a piece of  the monsoonal moisture streaming up from Arizona, Nevada, and Utah.

The bottom line is that SeaTac and other sites around here could easily end up with no July rainfall.

Now the shocker:  this would not be any kind of record.  Several previous Julys have done the same.

To show this, here is the July rainfall at SeaTac going back to around 1950.  Six previous years have had a trace or zero.  There does seem to be a trend towards drier Julys, something that is consistent with global warming simulations.

At least you don't have to worry about carrying an umbrella.  And with dropping temperatures, the weather should be nearly perfect for outdoor recreation or a hearty barbecue.

The Upcoming Cooling over the Northwest

We have been in a persistent pattern of above-normal temperatures without major heatwaves for the past week.   

But after a short warming today and tomorrow, a cool-down to normal or even below-normal temperatures is in store for the region.   A week without significant thunderstorm activity, but with coastal drizzle.

 To understand what will happen, consider the forecast upper-level maps for the next week.  

But not just any maps.  Let me show you the average of many forecasts...an ensemble of predictions.  

Why?  Because an average of a diverse collection of forecasts generally has more skill than any individual forecast.   These maps show the upper-level heights (solid lines) at 500 hPa pressure, about half of sea-level pressure.  Alternatively, you can think of them as representing pressure at 18,000 ft.    The shading shows the difference from normal (blue is below normal--troughs, reds indicate above normal or ridges).   

There is an offshore trough and a ridge centered over Alberta.  This pattern results in modestly above-normal temperatures for the NW.

In contrast, the pattern tomorrow is warmer for us, with the ridge strengthening and moving a bit westward.  Expect mid to upper 80s in the western WA lowlands tomorrow.  Around 105-110F in the Columbia Basin.  Hot.

By Wednesday, the trough will move into coastal BC and a substantial cooling will be evident.

And this cool trough continues to move in through Friday.  Not a warm pattern for us.

The latest NOAA National Blend of Models (statistical combination of many forecast models) indicates a substantial cooling from Saturday through mid-week, with highs only getting into the mid-70s on some days.  Mid-50s at night, which means good sleeping!

In eastern Washington, the decline will be dramatic, from roughly 110F to lower 90s in Pasco.

 

Precipitation?

Coastal drizzle and light rain, but little inland (see the European Center forecast for total precipitation through next Saturday morning).  No thunderstorms, which means no more lightning-induced wildfire starts.  This is climatologically the driest time of the year, so nothing unusual.

To put it a different way: expect temperate, moderate, and delightful weather this week after a warm weekend.   The string of 80s at SeaTac will be ended.  And the below-normal wildfire situation over Washington will continue.

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.

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.

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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.