Media stories have blamed the catastrophic fires on many things: a dry environment after the typical summer drought, unusual warmth the past several months, excessive rainfall producing lots of flammable grass, strong winds, global warming, and the lack of vegetative maintenance (clearly of the power right-of-ways) by the local utility (PG&E).
But none of the stories I have read get at what I believe is the real truth behind this unprecedented, severe, and explosively developing wildfire event:
A unique mountain-wave windstorm produced the strongest winds in the historical record at some locations. An event produced by the unlucky development of just the right flow regime that interacted with regional mountains to produce extreme winds beyond contemporary experience.
In short, this blog will make the case that the extreme nature of the wildfire was the result of a very unusual weather event, one that our weather models had the ability to forecast and warn about, if only their output were applied more effectively. The blog also suggests that better use of state-of-the-art weather prediction offers the hope of preventing a similar tragedy.
The Unique Wind Event
Although there have been a lot of media reports about windy conditions, few have described the extreme, often unprecedented, nature of the winds on Sunday night and Monday morning (October 8/9th). Some have even mocked PG&Es claims of hurricane-force winds, suggesting wind speeds of 30-40 mph.
Let's clarify a few things. There was a wide range of winds that night, with the strongest winds on ridge tops and on the upper lee slopes of terrain. Some winds was startling.
For example, at 10:30 PM on 9 Oct 2017 the wind gusted to 96 mph on a 3400 foot peak NE of Geyersville, about 20 miles NNW of downtown Santa Rosa. They reported sustained 74 knots (85 mph). Those are hurricane force winds (sustained of 64 knots or more).
At the Santa Rosa RAWS station (U.S Forest Service and Bureau of Land Management) at 576 ft elevation, the wind accelerated rapidly Sunday night to 68 mph (see below).
A few miles to the NNW and a bit higher (2000 ft), winds at the Hawkeye site accelerated abruptly to 79 mph.
Unprecedented
What is really amazing about the winds at these sites, was that they were unprecedented: the strongest winds on record, with records going back to 1991 (Santa Rosa) or 1993 (Hawkeye). And we are not talking about winds during the fall, but winds any time during the year. Even during the stormy winter season when powerful storms can cross the region.
At low-levels, the situation was more mixed. For example, at Napa Valley Airport (36 ft), the sustained winds at 11:15 PM October 9 (37 knots) were the strongest observed (looking back to 2001) at that location from July 1- November 30, while at the Santa Rosa Airport (KSTS) the sustained winds only reached 28 mph, with 40 mph gusts.
So why were the winds so strong and unprecedented at higher levels in the hills? These winds were key for causing the wildfires to explode and to so quickly move into populated regions. And the winds undoubtedly damaged power transmission lines and thus helped start electrical fires, which may, in fact, have initiated the big wildfire runs. And why were the lower level winds less severe? What can explain such differences?
Mountain Wave/Downslope Winds
When strong flow interacts with terrain, the air can be greatly accelerated. The schematics below shows you some situations, when air accelerated over and downstream of mountain crests.
That night (Sunday evening), strong to moderate easterly/easterly flow was approaching the terrain north of San Francisco, something shown by the 6hr forecast of height (like pressure) and winds at 850 hPa (about 5000 ft)--a forecast valid at 11 PM Sunday night (see below). The strong winds and their orientation was the result of cooler air and high pressure moving into the Northwest during the previous day.
A group at the Desert Research Institute runs a forecast model (WRF) at very high resolution (2-km grid spacing). Here is their 6-h forecast for sustained surface winds at 11 AM Sunday.
OMG...there it is. You can see the banded structure of strong winds over and immediately downstream of major terrain features, with lower winds near sea level. I inserted a terrain map below...you can see how the wind maxima were oriented the same way as the ridge lines.
Other major modeling systems also predicted the strong mountain-wave winds. For example, here are the max wind gusts predicted by the NOAA/NWS High Resolution Rapid Refresh Model for 2 AM Monday. Same banded structure, with gusts near Santa Rosa of 50-55 knot (58-63 mph)
Professor Bob Fovell of the University of Albany completed another high resolution simulation initialized at 5 PM on Thursday. Here are is a vertical cross section though the Tubbs fire that affected Santa Rosa. You can see the acceleration of winds (sustained) on the slopes.
The creation of such downslope mountain-wave type windstorms is very sensitive to the characteristics of the air moving towards the mountains. You not only need strong approaching flow, but the proper vertical structure of temperature and winds. Clearly such conditions don't happen often--otherwise similarly strong winds would have occurred before. There is no reason to expect that such extreme wind conditions were made more probable by global warming.
So I think we can outline what happened Sunday/Monday of last week.
The vegetation was very dry after little rain over the summer (quite normal). The ground vegetation was drier than normal because the summer had been usually warm (by 1-4 F as shown by the NOAA Western Region Climate Center map for the last 90 days.
On Sunday afternoon, winds approaching the mountains of northern CA increased, and the vertical structure of an inversion over cooler air was established. A strong mountain wave/downslope wind event was initiated, bringing winds of 60-90 mph to the crests and upper lee slopes of the regional terrain. Such winds helped initiate the fires (possibly due to interaction with power lines) and then caused the resulting fires to explode. The fires, driven by the strong, gusty winds, pushed very rapidly into populated areas.
The good news in all this? Our models seemed to be able to simulate this event, providing some warning of the imminent wind acceleration.
What could be done with such information? Much better warnings of a potential blow up? Shutting off the power to threatened communities? There are lots of possibilities. But one thing is for sure: we can not let this happen again. And the first step is to really understand what happened, without assuming we know the answer beforehand.
Too many people are suggesting the wildfire event is all about climate change, when it may prove to reflect a severe weather event unrelated to global warming. Similarly, some of the same folks claimed that the great rainfall with Hurricane Harvey was all about global warming, when a stalled storm was probably more to blame. Only by knowing the true cause of disasters and acting on that information can we protect people in the future.
from Cliff Mass Weather and Climate Blog http://ift.tt/2xKIPMO
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