A Strong Cyclone is Developing Off the Northwest Coast

It is not as powerful as the "bomb cyclone" of last November, but it is impressive for this time of the year.   

Our first significant Pacific cyclone of the season.

The sea level pressure forecast for 5 PM today (Monday) shows the low center clearly, with a respectable central pressure of 980 hPa 

And you see the clouds starting to swirl around the low in the infrared satellite imagery.

The latest UW WRF forecasts are in....so let me show you what should happen.

Tomorrow (Tuesday) morning at 11 AM, the deepening low will be west of Vancouver Island, with an intense pressure difference to the south of the low...which means strong winds.    A strong front, with substantial pressure differences and winds, will be along our coast.

This low will not be in a hurry to leave us and is nearly stationary at 11 PM Tuesday.

Winds will get quite gusty late tomorrow afternoon and during the evening.  According to Seattle WindWatch, winds will gust to about 30 mph over Seattle later on Tuesday.

And the plot of predicted wind gusts at 8 PM Tuesday shows very strong winds (40-50 kt) over Northwest Washington and along the coast.   This early in the fall, you can expect some branches down and scattered power outages.

Finally, this offshore low should produce heavy rain over the Olympics and southwest BC, with loads of rain offshore.   The fire season west of the Cascade crest will be over.  And the windfires east of the Cascades will rapidly fade.

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A Below-Normal Wildfire Year

 Being at the end of September, with cooling temperatures and substantial precipitation forecast, it is possible to make a definitive statement about the Washington wildfire season:  it is about to end and 2025 will come in below normal.  

During the next five days, there will be substantial precipitation across the region (see predicted totals through Wednesday afternoon below), and temperatures west of the Cascades will not get out of the 60s.  Much cooler east of the Cascades as well, with no thunderstorms expected.

This meteorology will close out the fire season, something supported by the guidance from the Forest Service Northwest Interagency Coordination Center, which predicts a dramatic drop in fuel flammability over the eastern slopes of the Cascades (see example below, where gray shading shows the normal values and the blue line shows the predicted values).

The fact that there are a few fires burning now should not make one conclude this is an above-normal year.   Wildfires are and have been a fixture of the Northwest environment.

But consider actual data.  

The figure below shows annual burned acres for Washington State since 2014, with the total for this year so far shown.  We are now below any other year, and with rain coming, one can expect only minor increases from the current total.  

There is little chance the 2025 total will reach the 10-year average (467,274 acres).

Similar results are found on the WA DNR website, which shows burned area for a subset of the above (the lower-elevation DNR lands).  This year has been below normal.

The fact that we had a modest wildfire year, even with a drier than normal summer, shows that the controls of wildfire are more complex than the trends in one or two meteorological parameters.  For example, wind is as important as temperature and moisture.

In a future blog, I will discuss this important point in more depth, noting we have experienced a wildfire DEFICIT over the region compared to historical norms.  

The Northwest is a place where wildfire is a normal part of the environment...and has been for thousands of years.

The Most Dangerous Day for Washington Wildfires for the Remainder of the Year

When it comes to rapid wildfire spread and growth,  strong winds are essential.

Virtually all major periods of rapid wildfire growth are associated with gusty winds.

It was clear that today would be a risky period on the eastern slopes of the Cascades.

Why?  Because we had some moderate fires going on the eastern slopes and strong westerly winds were forecast to push over and down the Cascades.

The Labor Mountain Fire Today

Before today, three lightning-initiated fires had been burning on the eastern Cascade slopes (see map):  the Wildcat fire east of Mt. Rainier, the Labor Mountain fire south of Leavenworth, and the Lower Sugerloaf Fire to the northeast of Leavenworth.

The visible satellite imagery this morning, around 9 A.M., clearly showed smoke from the fires, with the Labor Mountain fire being the most active (see below).

The satellite image of the heat emission from the fires confirmed that this fire was modestly active at 8 AM (the red dot shows the fire).

But when today's front approached from the west, cooler air flooded into western Washington, causing the pressure to rise.  This created a large pressure difference across the Cascades, forcing rapidly increasing westerly winds (winds from the west) down the eastern slopes of the mountains.

The UW WRF model predicted this change.  Consider the predicted winds near the surface over the central Cascades (below).  At  8 AM, the westerly winds were predicted to be modest, with the strongest near Ellensburg.

Huge strengthening by 2 PM

And the fire temperature observed from satellite around 1 PM clearly showed the fires increasing rapidly in response.

Surface observations documented the winds accelerating.  For example, the nearby Dry Creek USDA RAWS site showed the winds surging to around 40 mph.

A map of the maximum gusts today through 6 PM shows several locations with gusts over 30 mph east of the Cascade crest.

The great danger of the wind-driven Labor Mountain fire resulted in evacuation notices for those west of the fire (see below).

As of 8 PM, winds have started to decline, but fire-maintained gusts will continue for much of the night.

Good news.  In a few days, the fire danger for the whole region will recede as a series of wet Pacific fronts move in.

Valley Smoke Plumes

Information about my Atmospheric Sciences 101 class below
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The visible satellite imagery this morning was fascinating, with smoke in mountain valleys jetting out into the lowlands.

There are two fires on the eastern side of the North Cascades (Sugarloaf and Labor Mountain fires...see map below)

 

With sea-level pressure higher on the eastern side of the Cascades than over western Washington, the smoke is being pushed westward through the Cascade Passes and then jetting out into western Washington.

You can see this in the visible satellite image this morning around 9 AM, with smoke pouring westward in Stevens Pass and pushing out into Snohomish County north of Seattle.

The Seattle PanoCam this morning, looking north around sunrise, dramatically showed off the smoke plume.  Kind of beautiful.

This plume created a localized region of poor air quality that pushed into western Washington, north and east of Seattle.  Another smoke plume moved westward through the near-sea-level Columbia Gorge (red and purple are the worst air quality).

Look at the air quality at Gold Bar, west of Stevens Pass. You can see the sudden increase in smoky air this morning after midnight.  Not a good time to go on a hike in the pass! 

Even more impressive, the vertical laser profiler at Marysville showed the dense smoke moving aloft last evening, being densest between 500 and 1800 meters (1600-6000 ft).  Yuk.

Turning to the Olympic Peninsula, some smoke from the Bear Gulch Fire moved southward before taking a hard turn to the west (see visible satellite image below)

Some of the smoke was evident far offshore around noon today:

The good news is that by Thursday, the winds will reverse (be out of the west) and the smoke will be pushed back into eastern Washington.

Furthermore, by next Monday, rain and stormy conditions will move in...and that should essentially end the wildfire season.  

As a teaser, below is the predicted precipitation total for the next ten days.

There will be no Western Washington fires after this.  

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Starting tomorrow, I will be teaching Atmospheric Sciences 101, which you can take either in person or online.  

The description of the class is here: 

https://atmos.uw.edu/~cliff/ATMS101Y2025.html

For those over 60, you can take the class as an Access Student at a very low cost.  Here is information about the ACCESS program:

https://registrar.washington.edu/register/access/

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Get outside and enjoy the weather with Play Tennis Seattle. Find nearby hitting partners, coaches, classes, and join the Seattle tennis community. 

https://www.joinplaytennis.com/seattle

 

How Unusual Was This Summer?

 Today is the last day of summer, so it is a good time to evaluate what has happened over the past three months.

Let's start with the average daily temperature.  Below is the deviation of daily temperatures from normal for the western US.

Cooler than normal over much of California and southern Nevada, but warmer than normal over Washington and western Oregon--by around 2-4°F.

For precipitation, much of California and eastern Oregon were wetter than normal, while most of Washington was drier than normal by a few inches.

Let's take a closer look at how unusual this summer was by looking at a few local observing locations.

Starting with the June 21- September 20 observations at Olympia, WA., from the late 1940s to this summer,  we note a slot increase by around 3°F over the period, with this year being warmer than normal but not record-breaking.

Interestingly, the high temperatures this summer were very typical, with only a slight long-term trend in heatwave temperatures.

This is something the media has not been noting....the warmest temperatures are not rising much.  The big warm-up is in the lowest temperatures each day.

 

For precipitation (below), we see a slow drying trend in a region-- a region that does not get much rain over the summer.  This summer was drier than normal, but not nearly the driest.

Now let's turn to Wenatchee in the Columbia Basin of eastern Washington.  Clear warning trend, and this summer was the warmest on record.  

Precipitation at Wenatchee?   Modest drying over the years.

As shown above, general warmth and dryness this summer was not universal over the western US, and mainly over Washington State and western Oregon.

Why, you ask? 

 Because a persistent upper-level pattern developed, with higher pressure/heights over southwest Canada (red colors below) and lower heights/pressure over California.  High pressure is associated with sinking air, producing warmer and drier than normal conditions.

Why was this pattern so persistent?   I cannot tell you for sure.

Finally, how good was the seasonal forecast made by the National Weather Service before the summer started?  As shown below, not that great....got the warmth over the NW but missed the cooling over the southwest.

Temperatures will warm up during the next few days, but nothing serious over western WA--mid to upper 70s at most.  

Enjoy it while you can.  In one month, we will be entering the murk of Northwest winter.

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Prelude to Autumn: A Serious Front will Arrive Tomorrow Night

A serious "autumn" front will reach the Northwest coast tomorrow night, bringing regional rain and a reduction of local wildfires.

A front that arrives one day before the official start of autumn on Monday morning.

The visible satellite image this afternoon shows the front offshore (indicated by the red arrow).

This front has our name on it

A 5 AM Sunday morning, rain from the front will be moving through western Washington.

And by 11 AM Sunday, the frontal rain will reach eastern Washington, with residual showers over the western slopes of the Cascades.

The forecast total through 5 PM Sunday is healthy for September, with up to an inch in the mountains.  This rain, combined with the 60s through most of next week, will reduce wildfire activity substantially.

Talking about temperatures....western Washington will cool into the upper 60s on Sunday (see forecast below), warm into the 70s for a few days, followed by a sustained decline, with highs remaining in the 60s.  There will be a feeling of autumn in the air.

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Why did the Beach Gulch Fire Explode Yesterday?

 The Bear Gulch Fire in the southeast Olympic Mountains really revved up yesterday morning and afternoon.

Four days ago, it was smoldering, with only minimal smoke (see below, arrow points to the fire).

Yesterday,  during the late morning and afternoon, it really started to increase:

And today, the plume of smoke was impressive, stretching hundreds of miles into Canada.

Just after sunset from Seattle last night, looking towards the Olympics, one could clearly see the smoke plume from the fire.

The fire increased dramatically yesterday, but why?

In fact, my last blog, made before the blow-up, gave the reason:  the sudden development of strong, dry easterly winds over the region.

I have spent a great deal of time studying western Washington and Oregon wildfires, and a major finding was that virtually all the big, westside blow-ups are associated with strong, dry easterly (from the east winds).

In fact, I just wrote a paper on the subject that was just published (here).

Yesterday morning, there was a surge of such easterly flow as transient high pressure built into eastern Washington, and a thermal low moved northward up the coast. High pressure to the east and lower pressure to the west drive easterly flow since air tends to go from high to low pressure near terrain.

You can see the development of the easterly flow in the lower atmosphere by looking at the weather observations from planes landing and taking off from SeaTac (see plot of winds and temperatures below).  Time is on the x-axis in UTC and increases to the left.  (16 /12) is yesterday at 5 AM. The Y-axis is height in pressure.  850 is about 5000 ft.  Wind barbs are shown (blue) as well as temperature (red lines, °C).  

The easterly winds really increased (to about 30 knots) yesterday morning.

Easterly winds descending the Cascades were warmed by compression and were very dry.

Warm air.  Dry air.  Strong winds.   All very favorable for fire.

Fortunately, the winds weakened yesterday and reversed, so the fire should decline substantially now.  No hot weather is predicted over the next week, and rain will come in late Saturday.

We are going to make the transition to fall weather, and there is probably no going back.

PS:  one more fun fact about yesterday.  Several stations away from the mountains experienced an unusually large diurnal range:  the difference between high and low temperatures in one day.  

 47 to 88 F at Olympia: 41F difference!  Several other locations were like that.  Why do you think that mountains work against such large daily temperature differences?  Leave your answers in the comments.

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