Tuesday, October 10, 2017

The Northern California Fires: Driven by the Diablo Winds That Were Predicted Days Before

A large area north of San Francisco was devastated Sunday night/Monday morning by explosive wildfires that have killed at least 13 individuals, destroyed over 1500 structures, and burned over a hundred thousand acres. Over one-hundred people are missing.

Picture courtesy of KRDO.

This sudden catastrophic event eclipses the damage of the highly publicized Hurricane Nate, during which no person is known to have lost their life.

As we shall see, the northern California wildfires were produced by the rapid development of strong winds, which gusted to 50-70 mph in places.   Importantly, the winds were highly predictable, being forecast by current operational weather models days in advance.

Here are the max winds (in knots)  north of San Francisco during the event (provided by UW grad student Conor McNicholas). Some locations had winds over 55 knots (orange and red) and lots of places reached 30-45 knots (green colors)

High-resolution NASA MODIS satellite imagery show the explosive development of the fires.  Around noon on Sunday, October 8th, California is clear, with little evidence of smoke.

 One day later, massive smoke plumes are moving westward from a series of fires north of California.

The key element of this event was the rapid development of very strong offshore (northeasterly) winds, with gusts to 40-70 mph, and rapidly declining humidity during Sunday evening and the early morning hours of Monday. 

To illustrate, here are the observations at Napa Valley Airport (KAPC) from 1154 PDT (1854 UTC) Sunday through 2:24 AM PDT on Monday.  Temperatures early in the day were in the 70s F, with southerly winds and moderate dew point (upper 40s). But subsequently the winds switch to northeasterly, the dew point dropped into the teens (very dry) and the winds gusted to 35-40 knots (40-46 mph).


Why did dry winds pick up so rapidly?  Ironically, it was due high pressure, associated with cold air, passing to the north and east of northern California.

Looking at National Weather Service large-scale pressure analyses, one can view the changes.  The lines are isobars of constant sea level pressure.  On Saturday at 5 AM, there was high pressure over the Pacific and a cold front was moving into the Northwest.  Everything was fine in California.


By 5 AM on Sunday, the front had reached northern CA and high pressure was pushing eastward over Oregon, Idaho, and northern Nevada.  A trough of lower pressure was beginning to develop over coastal CA.


By 2 AM Monday morning, the world had changed, a very strong pressure difference had formed over northern CA, as high pressure pushed east and southward to the east of the Sierra Nevada and a trough of lower pressure intensified along the coast.  There was a big change of pressure with distance, which meant strong winds.


The UW WRF 12-km forecast of wind gusts and sea level pressure for 2 AM Monday, shows a huge pressure gradient over northern CA, with areas of strong winds. Very strong winds over the eastern Pacific as well.


And the humidity forecast for the same time shows very dry conditions are northern CA (dark brown color).

A higher resolution (4-km grid spacing) model run by the CA CANSAC group at the same time shows powerful northeasterly sustained winds (not gusts) north of San Francisco.

So the set up was the following.   Northern California was at the climatologically driest point of the year, after a summer of little rain (which is normal).  In fact, the latest official drought U.S. monitor graphic did not show particularly unusual dry conditions (see below).

High pressure then build in north and east of northern California, forcing strong  offshore (easterly) flow.  As the flow descended the western slopes of the regional terrain it was compressed and warmed (see schematic).  This warming resulted in reduced relative humidity and pressure falls at the base of the terrain (warm air is less dense than cool air).


In fact, the strong downslope wind over northern California has a name:  the Diablo or Devil's Wind

The pressure falls associated with the Diablo wind helped rev up the horizontal pressure gradients, and thus the surface wind speeds.      So you had antecedent dry conditions, strong winds, warm temperatures and low humidity--all the ingredients needed for explosive fire growth.

And then we have the other issues:  human initiation of fire, mismanaged local forests and grasslands, and folks living too close to fire-prone vegetation.

This event should be considered a severe storm situation, driven by a well-forecast weather phenomenon.   In fact, the forecast models were predicting this event many days before.  To show this, here are forecasts of sea level pressure and temperature made for 2 AM on Monday started 9 hr before (Sunday at 5PM) and 117 hr before (5 AM Wednesday).   VERY similar and both were predicting the strong winds.  The strong winds and dry conditions should have surprised no one.



Could we have warned people better of the upcoming wind event and the potential for fire blow up?  Could we have saved lives if we had done so?

Should the powerlines have been de-energized when the winds exceeded some threshold?  Was poor maintenance of powerlines (e.g., lack of trimming vegetation) a major issue?

I will let others answer these important questions.



10 comments:

Eric Blair said...

After the fires of OR, WA and BC this summer, I have no idea why anyone would not have anticipated what has occurred. Additionally, they've had decades of non - burning of the forests there (as we've already noted previously in the above mentioned areas), so the mass of fuel that was ready to pop was altogether too familiar. Sad.

Unknown said...

I completey agree that this should have been considered a severe weather event. Thank you for documenting it. I hope that forecasts of the potential for such wildfires can be better utilized in the future. It is dismaying that so many people had very little if any warning to escape the firestorms, and tnat this did not need to be the case. - Steve Krueger

John Marshall said...

The implication seems to be that NOAA didn't raise the proper warnings ahead of time, but I don't think that's what you truly intend to say. I know that up here in the PNW, every weather app I have generates ALERTS when NOAA puts out Red Flag Warnings about extreme fire weather conditions.

Did that not happen in fire-sensitive California?

If they did raise the Red Flags days ahead of time, then anyone who looked at any weather forecast in that time should have seen the warnings.

What surprises me in CA (and this seemed to also be happening back when I lived in SOCAL in the 90's) was that when the strong winds of early Fall are forecast, you can almost guarantee fires popping up precisely when they arrived. Given that wind doesn't create fire, it always seemed that there were people (who did watch the weather forecasts) who were starting the fires. Either that or a lot of fires suddenly got away when the first gusts came through. But campfires and other forms of outdoor burning have been prohibited in the whole state since early summer.

Carelessness or intent? When I lived there, the consensus was that it was intent. If so, then the bad guys are paying much closer attention to the forecasts than the media and general public.

Tom Butler said...

If I recall you have criticized the drought monitor for being slow to respond to changing conditions. Is it possible that's the case here? It's really scary to think that something this bad could happen under normal conditions.

Mary and Jim said...
This comment has been removed by the author.
Jan Null said...

per below, NWS SF issued the following Red Flag watch on Thursday and updated it to a Red Flag Warning on Friday:
URGENT - FIRE WEATHER MESSAGE
National Weather Service San Francisco CA
1108 AM PDT Thu Oct 5 2017

...Fire Weather Watch Sunday through late Monday night...

.A classic autumn fire weather pattern is forecast to develop
later this weekend into early next week. Warm temperatures, low
humidity and gusty north winds will coincide with critically dry
fuels. Current indications suggest this will be the strongest
offshore wind event so far this fall.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

URGENT - FIRE WEATHER MESSAGE
National Weather Service San Francisco CA
949 AM PDT Fri Oct 6 2017

...Red Flag Warning Sunday through late Monday night...

.A classic autumn fire weather pattern is forecast to develop
later this weekend into early next week. Warm temperatures, low
humidity and gusty north winds will coincide with critically dry
fuels. Current indications suggest this will be the strongest
offshore wind event so far this fall.

CAZ507-070400-
/O.UPG.KMTR.FW.A.0003.171008T1800Z-171010T1200Z/
/O.NEW.KMTR.FW.W.0006.171008T1800Z-171010T1200Z/
North Bay Mountains-
949 AM PDT Fri Oct 6 2017

...RED FLAG WARNING IN EFFECT FROM 11 AM SUNDAY TO 5 AM PDT
TUESDAY FOR STRONG OFFSHORE WINDS AND LOW HUMIDITY FOR FIRE
WEATHER ZONE 507...

The National Weather Service in San Francisco has issued a Red
Flag Warning, which is in effect from 11 AM Sunday to 5 AM PDT
Tuesday. The Fire Weather Watch is no longer in effect.


* WIND...Northeast winds 15 to 30 mph gusts 40 to 55 mph.

* HUMIDITY...10-20 percent afternoon with night time recovery
under 30 percent.

* HIGHEST THREAT...is located in the Napa County hills as well
as around Mount Saint Helena and the hills of Marin around
Mount Tamalpais.

* IMPACTS...any fires that develop will likely spread rapidly.
Outdoor burning is not recommended.

Lindsey Heuscher said...

Hurricane Nate killed over 30+ people in Central America. Like California, weather can be made worse by terrain and public response to an event.

J B said...

Cliff
You have made some valid points.

"Northern California was at the climatologically driest point of the year, after a summer of little rain (which is normal).  In fact, the latest official drought U.S. monitor graphic did not show particularly unusual dry conditions"

This is correct.
Fuel Moistures from the closes RAWS stations (before the event) where in the normal percentile for this time of year.

"This event should be considered a severe storm situation, driven by a well-forecast weather phenomenon."

This was considered a sever situation. Red Flag Watches were posted up days before the event for most of Northern California including the Bay Area. Red Flag Warnings were also posted up timely for a huge swath of area in Northern California.

Meteorologists producing watches, warnings and forecasts must deal with the reality that inherently all forecasts of meteorological events are by their nature not perfect. With Fire and Wind events you need to add in another input to this equation; FIRE IGNITION.

Unless you know where all the weak power poles are (ones that will fall over in winds and create fires), or where the arsonist is going to strike his match, or who is going to be carless with his stove ashes, or who is going to drag their boat chains along the road, you have no idea, in the VAST response area that was under the Red Flag Warning where disaster will strike. That does not mean you can’t prepare with the timely warnings that “were” put in place.

Fact
*Before the event Fire Crews across the state had their time-off days cancelled and put on special staffing patterns
*Resources were pulled from low potential areas to stage closer to the Red Flag fire zones in anticipation of the event
*Air Resources where held on contract and staged, especial the vary large air tankers (VLAT)
*Air Resources could not fly (as they usually do for initial attack) do to starts that were in the middle of the night
*Ad campaigns where posted from CALFIRE to multiple news agencies before the event of the oncoming danger

California invented the mutual aid mobilization guide for wildfire and are masters of the art (getting more resources where they are needed). The mobilization was a quick response in the chaos.

The Tubbs Fire which burned through Santa Rosa Neighborhoods started up in the urban interface and moved out of it and into the blacktop jungle of a city. Flames took direct aim for the heart of a city, not just the rural outskirts, not just the homes high in the hills or off the grid. The fire jumped a 6-lane highway (101). This was an “exceptional” event. As soon as it left the wildland it turned into a multi-structure (exposure) event.

Everyone who lives in the Urban Interface in California knows how to live with wildfire (State law to mitigate your property from wildfire). It has been a way of life as much as Earthquakes and Floods. But sometimes you get something special, a dragon so wild and dangerous even for firefighters who are accustomed to fighting the beast.

The other fire that I can think of is the Cedar Fire (2003). This made its way down into the blacktop neighborhoods of San Diego. This was the cousin of the North Winds;Santa Ana’s. Oakland Hills Fire was up in the urban interface.

Great explanation on the North Winds (Diablo Winds)

Donald Strong said...


"Minutes to Escape: How One California
Wildfire Damaged So Much So Quickly"
New York Times

https://www.nytimes.com/interactive/2017/10/12/us/california-wildfire-conditions-speed.html?smid=tw-nytimes&smtyp=cur&_r=0

James Grundvig said...

They were wild for forest fires. They were microwave fires created by cell towers and solar flares. And it will happen again, especially with the 5G Network coming online.

https://medium.com/@james.4base/sunspots-and-cell-towers-fueled-the-northern-california-firestorm-6e05c830fc8d