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.

Flash Heatwave and Wildfire Threat over Western Washington

 Tomorrow is going to be fascinating. 

 There is going to be a sudden, intense heatwave over western Washington, with temperatures jumping into the mid to upper 80s at many lowland locations.

And then much cooler on Wednesday.

This west-side heat will be associated with powerful easterly (from the east) winds over the Cascade crest and western slopes, which will provide a fire threat.

Consider the temperatures forecast by the UW high-resolution ensemble (many forecasts) prediction system for Seattle.  Tomorrow afternoon, around 4 PM (red arrow), most of the forecasts are for the upper 80s!

Let me show you some forecast maps of surface temperatures.

Tomorrow morning at 5 AM, the region is relatively cool, with the warmest temperatures on the lower western slopes of the Cascades.

By 11 AM, things are really starting to warm on the western slopes of the Cascades, Olympics, and coastal mountains, with some locations in the 80s!

But 5 PM is another...and very warm... story.  Western Washington will be MUCH warmer than eastern Washington, with much of the west in the mid to upper 80s.  Cooler near the water.

Why so warm WEST of the Cascades?  

Because strong easterly (from the east winds) will be descending the western slopes of the Cascades and coastal mountains, and descending air warms intensely by compression.

Take a look at the surface winds at 11 AM tomorrow, with stronger winds shown by yellow and orange colors. The western slopes will be gusty!

Warmer air can hold more water vapor than cooler air, so relative humidity falls rapidly as the air descends.  At 5 PM tomorrow, relative humidity will be below 20% in large swaths of western Washington and Oregon.

Strong winds and dry air can dry out surface fuels and stoke windfires.  In western Washington and Oregon, virtually ALL the big fires are associated with strong winds from the east.

An important tool for wildfire prediction is a parameter called HOT-DRY-WINDY (HDW), which combines wind and dryness.  

The forecast of HDW for tomorrow at 11 AM, with red and orange colors being high values.  Major areas of concern are north of Portland,  southeast of Seattle, and over the northern Oregon coast.

The National Weather Service has a red-flag warning out, which indicates the potential for wildfire on the west side of the Cascades.

The saving grace for this situation is that heat and dangerous values of hot-dry-wind will not be in place for long, and we had some rain this week.  

Still, folks need to be careful.

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