The wind whipped Valley Fire devastated the communities of Middletown and Hidden Valley Lake on Saturday September 12th. Now that much of the smoke has cleared, officials have had a time to tally the loses: some 1000+ homes have been destroyed, and sadly, three lives were lost to the inferno making this one of California's most destructive wildfires. Contributing to the massive losses was the fire's rapid rate of spread, which was was quite remarkable, consuming 40,000 acres in the first 12 hours. The rapid advance begs the questions what conditions lead to this tragedy. Specifically, what caused the the strong winds that drove the fire and contributed to long distance spotting that hampered containment efforts? The answer is, in part, somewhat surprising: Tropical Moisture.
In this post I'll make the case that tropical moisture, streaming north from the remnants of Hurricane Linda, may have played a significant and unexpected role in generating the strong winds that propelled the Valley fire through Middletown and Hidden Lake Valley. To set the stage, lets start by looking at the mid-level (500 hPa) relative humidity and geopotential height fields leading up to the day of the fire (Fig. 1).
Two days before the fire, on 11 Sept., Hurricane Linda is apparent as a series of closed contours west of Baja California and as a extensive plume of high relative humidity air overspreading southern California (gray shading). Over the next two days Linda's circulation decayed, but the moisture plume advanced north along the California coast, overspreading the Bay Area during the day on the 12th of September.
This surge of tropical air was mostly confined to the mid-levels of the atmosphere, overriding much drier air near the surface. For example, the radiosonde launched from Oakland International airport at 00 UTC on 12 September (Soild lines, Fig. 2) shows the high humidity air at 500 hPa, but an extremely dry layer of air near 850 hPa.
What is notable, however, is that by Saturday afternoon (dashed lines, Fig. 2) the dry layer had significantly moistened and cooled. So too, the winds in this layer, which ultimately affected the fire, increased dramatically, reaching sustained speeds of 45 kts (~52 mph) out of the northwest.
The cooling at 700 hPa, and the commensurate increase in winds, was caused by rain falling from clouds associated with Linda in the mid-levels and evaporating into the dry air near the surface. In fact this process was captured in compelling detail by the roof top LiDAR and webcams at SJSU (Fig. 3). The LiDAR data shows the arrival of mid-level clouds, the bases of which progressively lowered through the afternoon. Then around 20:00 UTC virga is observed, and also captured on by the webcams (inset photo). The evaporative cooling in this band of precipitation aloft causes a strong downdraft, which descended to the top of the marine layer and caused a series of gravity waves (undulations in the shading near the surface). These waves showed up in local wx observations as some peculiar fluctuations.
But the story doesn't end there. The evaporative cooling was far more extensive in a layer offshore, west of the Bay Area. Figure 4, for example, shows RUC model analyses from the morning and afternoon, including precipitation (shaded colors), 850 hPa geopotential heights (black contours), and 850 hPa winds (wind barbs). From these figures it is clear that a "meso-high" forms just offshore due to the evaporative cooling in the precipitating region (marked as an H). Since cold air is denser than warm air, the evaporative cooling generates higher pressure, a phenomenon that is well known with severe thunderstorms, but not often seen here in Central California.
The net result is a significant increase in the pressure gradient over the North Bay, and since winds are driven by pressure gradients, we see a corresponding increase in the NW flow. This burst of NW winds is particularly evident in the radar wind profiler data at Bodega Bay (Lower Panel Fig. 4), where at 00 UTC 45 Kt winds are observed just above the surface.
These same winds were manifest over the area of the Valley Fire. Figure 5, for example, shows the time series of wind and temperature from the Kelseyville Remote Automated Weather Station (RAWS). There is a significant increase in NW winds in the afternoon of the 12th, with peak sustained winds of 25 mph and gusts to 35 mph. 35 mph winds are no laughing matter, especially when it comes to a fire moving through late season cured fine fuels.
But perhaps the biggest effect of the wind was to cause a tilted plume structure, which contributes to long distance spotting (Fig. 6). The strong winds cause what would ordinarily be an upright convective column to lean in the downwind direction, carrying burning embers far in advance of the flaming front of the fire. The resulting spot fires then drive a nonlinear fire propagation that can be extremely difficult to contain, and extremely dangerous. The fire is no longer simply moving from point a to point b, but jumping in leaps and bounds across the landscape, with individual fires merging from all directions.
In summary, while many factors likely contributed to the Valley Fire's rapid advance there is clear evidence that tropical moisture contributed to enhanced NW winds that propelled the fire. Specifically:
(1) Topical moisture moved north, overlaying much drier air near the surface.
(2) Evaporative cooling due to rain falling from aloft into the dry air caused a meso-scale region of high pressure.
(3) The "meso-high" increased the pressure gradient over Lake County, generating strong NW winds
(4) These winds contributed to the rapid rate of spread and long range spotting.
This case would be an interesting modeling exercise. For example, to further isolate the role of tropical moisture, we might run two simulations: one with and one without precipitation effects. Do the strong winds still develop if we turn the precipitation off? How much slower would the fire spread be in the absence of the enhanced NW winds?