Why did the Beach Gulch Fire Explore 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.