Diablo Winds Trigger Massive Wildfires in the North San Francisco Bay Region

As it happened, the strong upper trough digging out west that drove a strong cold front southward across the Great Basin east of the Sierra Nevada (see previous post) brought strong and bone-dry ‘Diablo Winds’ to the north San Francisco Bay Region, especially Napa and Sonoma Counties.  The strong surge of dry, cool air crossing the Great Basin pushed westward across the mountains of California.  By taking this trajectory, the rushing air becomes even drier and warmer, as it is warmed by compression in its rushing descent toward the coast.   These winds typically become very strong along ridges and hill tops, then become even stronger as the rushing air funnels down coastal canyons, then fans out below the canyons across the coastal plains as it flows toward the sea.

Diablo Winds are the same type of offshore, very dry and locally strong and gusty wind that is known as the ‘Santa Ana Wind’ in southern California – and indeed there was a Santa Ana Wind event as well.


The Diablo Wind event is underway.  Here, at 4 PM Pacific Time  Sunday, we see about the lowest humidity possible on the planet we call Earth:  At Santa Rosa, a temperature of 88 degrees with a dew point of 2 (yes, I said 2 above zero is the dew point), yields a relative humidity of, ahem, four (4) percent.  Wow.  A dew point of 18 up to the north at Ukiah is no slouch, either.  This is inline with the dew points in northern Nevada, but the humidity is much higher in the Great Basin because the temperature is much cooler.  Bishop, east of the Sierra, is competitive with a temp of 87 and dew point of 6. 


The surface pressure analysis at 09/00 GMT (5 PM Pacific Daylight Time, 7 PM Central Time, October 8th)  Surface cold front extends from lows over Colorado to eastern California with a surface trough extending on northwestward near the California and Oregon Coast.  Cold, dry air is pushing southward across Nevada, kicking up dust.  Some of this then pushes across the mountains of California, rushing toward the coastal trough, warming and drying further by compression processes as it descends in elevation toward the coast. (Just like a bicycle pump warms when you pump your bike tires due to increased air pressure.)  Note the many isobar lines, indicating a very strong pressure gradient, or push of air, causing the strong winds.


And here is the concurrent 500 MB upper trough (about 18,000 feet ASL) that drove the surface event.




At 09/00 GMT (5 PM Sunday, October 8th):  A full gale in Bishop (in east-central California) with north winds gusting to 45 knots (50 MPH), which continued for several hours.  Just an hour previous (1st chart above) winds were calm there.  Blowing dust is causing the automated report of ‘haze’ and a visibility of 2 miles.  Bishop is a bit over 4,000 feet in elevation.  Air will warm roughly 5.5 degrees for every 1,000 feet of descent due to compressional heating caused by higher air pressure at lower elevations.  This converts 72-degree air at 4,000 feet, in theory, to 94 degrees.  Since no moisture is added to the already dry air in its compressional warming, the already dry airmass dries further.  This hour, with the temp at Santa Rosa down 5 degrees and the dew point up 3 degrees (to 5 above zero) over the previous hour, the humidity is an insignificant one percent greater – now 5 percent.  It is very difficult to achieve such low humidity in populated regions of the planet.  Death Valley, Antartica, the Sahara and the Atacama desert probably achieve such low humidities at times, I imagine.  (I smell a small research project..)


Lets look at the weather event unfold with the automated observations at the Santa Rosa Airport:

Sunday at 12:53 PM PDT: A warm but calm day this hour (81 degrees):

KSTS 081953Z VRB03KT 10SM CLR 27/02 A2981 RMK AO2 SLP086


27.2°C (81°F)


2.2°C (36°F) [RH = 20%]

Pressure (altimeter):

29.81 inches Hg (1009.6 mb)

[Sea-level pressure: 1008.6 mb]


variable direction winds at 3 MPH (3 knots; 1.6 m/s)


10 or more miles (16+ km)


An hour later at 1:53 PM, the wind starts, slowly at first.  However, even with only very light northeast winds, the temperature has jumped 6 degrees in an hour!  The humidity, already low, has dropped from 20 percent to 9 percent!  Clearly, a change in airmass has occurred – from dry and warm, to very dry and warmer:

KSTS 082053Z 07007KT 10SM CLR 31/M06 A2978 RMK AO2 SLP077


30.6°C (87°F)


-5.6°C (22°F) [RH = 9%]

Pressure (altimeter):

29.78 inches Hg (1008.5 mb)

[Sea-level pressure: 1007.7 mb]


from the ENE (70 degrees) at 8 MPH (7 knots; 3.6 m/s)


10 or more miles (16+ km)


Another hour goes by and the wind has increased, gusting to about 25 MPH.  The temperature has risen another degree to 88 and the dew point has dropped further to 4 percent (a dew point of 2 above)!  The song by Neil Diamond “Cracklin Rosie” comes to mind.  At this low humidity level, any slight spark from just some static electricity can start an inferno instantly in the bone dry but overgrown vegetative conditions in the area following 6 months of virtually rain free conditions.

KSTS 082153Z 03016G22KT 10SM CLR 31/M17 A2976 RMK AO2 SLP072 


31.1°C (88°F)


-16.7°C (2°F) [RH = 4%]

Pressure (altimeter):

29.76 inches Hg (1007.9 mb)

[Sea-level pressure: 1007.2 mb]


from the NNE (30 degrees) at 18 MPH (16 knots; 8.3 m/s)

gusting to 25 MPH (22 knots; 11.4 m/s)


10 or more miles (16+ km)

At 09/00Z (5 PM PDT):  Temperature has dropped a bit due to the setting sun, but doesn’t matter, the wind is there and the humidity is almost Mars like.

KSTS 082353Z 05015G26KT 10SM CLR 28/M15 A2976 RMK AO2 PK WND 03026/2349 SLP071


28.3°C (83°F)


-15.0°C (5°F) [RH = 5%]

Pressure (altimeter):

29.76 inches Hg (1007.9 mb)

[Sea-level pressure: 1007.1 mb]


from the NE (50 degrees) at 17 MPH (15 knots; 7.8 m/s)

gusting to 30 MPH 


At 11 PM PDT, it is still blowing.  Peak wind in the last hour 35 KT (nearly 40 MPH).  Even at this late hour with a cooler temperature, the humidity is a bone dry 11 percent.  The visibility is dropping a bit, indicating smoke from some distant fires.

KSTS 090553Z AUTO 01013G25KT 9SM CLR 23/M09 A2982 RMK AO2 PK WND 03035/0506 SLP092 


22.8°C (73°F)


-9.4°C (15°F) [RH = 11%]

Pressure (altimeter):

29.82 inches Hg (1009.9 mb)

[Sea-level pressure: 1009.2 mb]


from the N (10 degrees) at 15 MPH (13 knots; 6.8 m/s)

gusting to 29 MPH (25 knots; 13.0 m/s)


9 miles (14 km)

At 2 AM PDT, an ‘overcast’ is registering at 4,700 ft, which of course is smoke, as it is far too dry for clouds with a surface humidity of 11 percent.   The fires are getting well underway at this point.

KSTS 090853Z AUTO 01017G31KT 8SM OVC047 22/M11 A2982 RMK AO2 PK WND 01031/0847 SLP090 


21.7°C (71°F)


-10.6°C (13°F) [RH = 11%]

Pressure (altimeter):

29.82 inches Hg (1009.9 mb)

[Sea-level pressure: 1009.0 mb]


from the N (10 degrees) at 20 MPH (17 knots; 8.8 m/s)

gusting to 36 MPH (31 knots; 16.1 m/s)


8 miles (13 km)


4700 feet AGL


overcast cloud deck at 4700 feet AGL

Since the fires started after dark Sunday night, there was no visible indication by satellite until Monday.. so below are a few Monday images of the smoke plumes.


Monday , October 9th, 9:46 AM PDT:  The sun angle is high enough to see the smoke plumes, still streaming out to sea north of San Francisco.  A huge area of smoke from the overnight fires extends well off the coast already.  A ‘southerly surge’ of coastal stratus pushes northward along the Big Sur Coastline south of Monterey Bay.  These are usually very shallow, so any cooling and high humidity is limited to areas near the immediate coast.  Of course, the southerly surge is well south of the fire zone.


At 1:45 PM:  The off-shore wind event is about exhausted and the smoke sits in place under nearly calm conditions.  Another smoke plume is seen over the southern Sierra Nevada just south of Sequoia National Park.  Another plume is over the northern Sierra in Yuba County.  The southerly surge has bumped up a bit of fog against the coastline around Santa Cruz, which seems to be bouncing back out into the Monterey Bay again in a wave pattern.  (Marine layers do all sorts of silly things like this.)


At 4:45 PM, a weak onshore flow has commenced with the smoke plume now starting to push inland.  This is beginning to bring somewhat cooler and more humid air inland, but it is far too little too late, as the fires, already well established in the try foliage, await the next surge of dry, warm air.  Inland, the Sierra Nevada smoke plumes are flaring up in the heat of the day.  The southerly surge has morphed into a few fog swaths off the the coast near Monterey.  A finger of fog extends northward under the dense smoke plume.

An excerpt from the forecast discussion issued Monday evening from the NWS Forecast office, SF Bay Area, makes for some interesting reading:

San Francisco Bay Area Forecast Discussion

946 PM PDT Mon Oct 9 2017

.DISCUSSION…as of 9:46 PM PDT Monday…For the last 25 years or so the Oakland hills fire has been the seminal fire event that was seared into Bay Area residents psyche. For the current generation of North Bay residents todays firestorm will leave an indelible scar and for years to come we will all recall the Columbus Day firestorm. The wind event that roared through the North Bay last night is long behind us as ignitions occurred on the tinder dry fuels, fire literally exploded and raced along the landscape. Fuel analysis just ahead of the winds showed they were at all time record dry levels preceding the front. Recall we had nearly 5 years of extreme drought followed by record rains last winter that produced a bumper crop of grasses and fine fuels on top of drought and disease stressed heavier fuels. The Hawkeye  raws station in northern Sonoma peaked to 79 mph during the event with the Santa Rosa raws at only 599 feet reporting an incredible gust to 68 mph that may have been fire induced as the temperature spiked to 91 degrees at 4:30 am. The NWS has meteorologists in route to support the incoming incident management teams.

In the fire business, the common refrain is fire goes where wind blows and unfortunately there will be another dry cold frontal passage later Tuesday night into Wednesday morning. To be crystal clear the magnitude of this front will be MUCH weaker than the Sunday night system. However, if there is still a lot of open fire line at this time tomorrow night it will be of concern once again.

Good writing and information from the staff at the NWS forecast office for the SF Bay Region.








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