Strong Diablo/Santa Ana Wind Initiate Major Fires over Californina

It has happened again: strong, dry offshore-directed winds have initiated large wildfires over California.

Fires that have covered much of California with smoke, destroyed thousands of homes, and tragically killed nine people as of this writing.

Importantly, the winds were again highly predictable, poor warning were given to the communities, a failing electrical system may have contributed to one of the fires, and many of the people affected were living in the wildland-urban interface.

This blog will examine the meteorology of the event.

The Current Fires

There are two major fires burning right now.  First, is the Camp Fire in the foothills of the Sierra Nevada east of Chico, CA, which is burning towards the southwest and has destroyed the town of Paradise.  Nine people have died and approximately 7000 structures have been lost.  The town of Paradise is essentially gone. (see map below--acquired here).

The second major fire complex is in southern California affecting the area from Thousand Oaks to Malibu (the Hill and Woolsley Fires).  The fire has burnt to the ocean and is now moving towards Malibu.

Satellite Pictures

The satellite imagery is stunning.  Both fires started early on November 8th as offshore-directed winds surged.   The NASA MODIS satellite imagery around noon PDT shows a plume smoke heading from the Camp Fire towards the southwest, while the fires near Thousand Oaks, were just starting.

The change by noon on November 9th is amazing.  Huge smoke plumes that not only covered the entire Bay Area and adjacent locations of central/norther California, but extended many hundreds of miles offshore. The southern CA smoke plume was dense, but much smaller in size.

The air quality implications of the smoke are very serious.  The air around the Bay area was far worse than Beijing, reaching into the very unhealthy levels.  The EPA AirNow plot of particulate levels (PM2.5/PM1.0)  for Saturday morning shows that this serious health threat continues.  Unhealthy conditions are found over the densely populated Bay area and the values at some locations to the north are at very unhealthy levels.    The health implications of these smoke concentrations are very serious, leading to increased levels of asthma and respiratory problems.

The Origin of the Fires

The fires that began on November 8th were associated with strong offshore-directed winds, known as Diablo winds over central and northern CA and Santa Ana winds over southern CA.  The cause of these winds are no mystery:  an increase of high pressure over the intermountain West (e.g., Nevada) that leads to a strong offshore-directed pressure differences (pressure gradients), which in turn accelerate the winds.  The winds can be locally increased substantially by effects of of mountains (downslope wind acceleration associated with mountain wave dynamics).

To illustrate, here is the official National Weather Service chart for 7AM November 8 (Thursday). The solid lines are isobars, lines of constant pressure.  You will notice high pressure over eastern Washington/Oregon that is pushing into Nevada, and a trough of low pressure over coastal California.  Between them is a large pressure difference that accelerated the winds.

This pattern is most frequent in the fall.  High pressure over the eastern Pacific has retreated, while high pressure builds over the interior as the continent cools and the flow aloft increases.    The key "set up" is having a large ridge of high pressure aloft and a trough pushing southward into the SW US.  Exactly what we had Thursday morning (see map).

Winds gusted to 60-75 mph in "favored" locations.   The max gusts for the 24-h ending 9 AM over the region showed that near Chico there were winds gusting 72 mph in the foothills above Paradise and nearly as strong in the mountains west of Lake Tahoe (see below, click to expand).    The observational system is limited, but it was suggesting the limited extend of the strong windsto  near the crests and upper western slopes of the Sierra.

Minimum relative humidity during the same period dropped to under 10% along the Sierra foothills:

Southern California observation sites showed 50-65 mph gusts at a number of locations for the same 24-h period.

And the relative humidities were very, very low, with many stations around southern CA less than 10%:

Relative humidity of descending air is usually very low, as air warms as it descents (warm air can hold more water vapor than cold air).  Plus, the source region of the air (the intermountain west) is quite dry.

The occurrence of strong winds and low relative humdities were skillfully forecast during the previous days.  The Desert Research Institute CANSAC effort runs the WRF model at high resolution twice a day.  Here are the sustained winds predicted by their 2-km model domain for the run initialized at 4 AM on Wednesday and valid at  AM Thursday (see below).  Forecast of sustained wind around 50 mph  along the Sierra Nevada western slopes.  Gusts would be 30-50% higher typically.  Excellent warning

For southern CA, the same model showed winds accelerating at the same time (see below) to 50 mph around east and north of LA.

And then going nutty strong by 1 AM Friday.

Other weather forecast systems showed similar forecasts.  The bottom line:  the strong winds were expected and well forecast.  

These strong winds and associated low humidity were hitting a landscape dry after a summer of little rain and warm temperatures, something typical of California, with its Mediterranean climate (dry summer, wetter winters).   The landscape was modestly drier than normal around the Camp Fire, with precipitation over the past six months below normal by 3-6 inches.   Conditions around southern CA were near normal.

So the situation was highly favorable for fire.  Dry conditions at the surface, low relative humidites, and strong winds that could encourage fire growth and push the fire rapidly away from its initiation point.

And then there is fire initiation.  There is some initial evidence that the Camp Fire near Chico was the result of a failure of a high-tension power line that started the fire.    PG&E, even with very winds forecast, decided not to de-energize their line--probably a big mistake.  Just like the Wine Country fires of October 2017.  And just as disturbing, local officials did not use the Amber Alert system to warn people of the exploding Camp Fire.

The initiators of the southern California fires is still not known.

The latest surface smoke forecasts over California from the NOAA HRRR smoke system is not optimistic for today:   a LOT of smoke over the Bay area and LA (see forecast for 6 PM this evening)

Bottom Line:   Our ability to forecast these wind events has become very, very  good.   We knew how dry the conditions would be.  The threat was clear.  It is unfortunate that powerlines appears to start the Camp Fire and that the lines were not de-energized before fire was initiated.  It is doubly unfortunate that we do not seem to be able to warn people effectively when such fast-moving fires are occurring.    And the threat of so many people living at the urban-wildland interface is made clear again.