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Wildfires continue to burn up and Watch Russian Mom (2016) Korean moviedown the state of California, with the most damaging blazes located in the picturesque Napa and Sonoma Valley region, about 60 miles north of San Francisco. This area has seen at least 3,500 homes and businesses destroyed, including entire wineries, since the fires exploded in size amid a fierce windstorm on Sunday night and Monday. The number of fatalities continues to climb, with the figure standing at 21 on Wednesday afternoon.

On Wednesday, forecasters hoisted dreaded red flag warnings for critical fire weather conditions across Napa and Sonoma Counties, particularly in higher elevations, where winds could gust as high as 55 miles per hour by Wednesday evening. The wind gusts, coming from the north, will follow the passage of a weak front moving across the San Francisco Bay Area.

SEE ALSO: How California's firestorm spread so mind-bogglingly fast: From 'Diablo' winds to climate trends

These winds will once again be blowing offshore, which means they'll be devoid of Pacific moisture, and will produce extremely challenging conditions for firefighters struggling to contain the blazes.

Diablo winds, which are known as Santa Ana winds in Southern California, occur when air is forced to compress as it moves around an inland area of high pressure. Winds gain speed as the air rushes down hillsides and accelerates through canyons. In the process, air temperatures increase, and humidity drops, both of which enhance the likelihood that wildfires will spread.

The National Weather Service warned on Wednesday morning that "very high fire danger" will develop in the North Bay mountains and East Bay hills starting Wednesday evening, and lasting through Thursday. More strong winds are also expected this weekend.

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The fire weather conditions are not expected to be quite as severe as they were when the fires first exploded in size early in the week. According to the NWS:

It is important to put this upcoming wind event in perspective with the one we just had that has lead to the devastating fires over the North Bay. For Sunday night we did see local gusts of 60 to 80 mph and even in North Bay Valley locations winds were gusting over 35 mph. In this case winds for higher elevation spots (especially over 1,500 feet) winds will locally gustin the 45 to 55 mph range while valley areas should see gusts peaking in the 20 to 30 mph range. However the values over the North Bay Mountains and East Bay Hill may be high enough to topple some additional trees and power lines.

The strong winds anticipated this weekend are forecast to be slightly more intense than the offshore wind event of Wednesday night. This means that firefighters will have only relatively narrow windows of beneficial weather conditions in which to gain the upper hand of these fires, which are far from under control.

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As of early Wednesday, the Tubbs Fire, which destroyed large parts of Santa Rosa, California, had charred 28,000 acres, and was 0 percent contained. Nearly 700 personnel have been deployed to combat the Tubbs Fire alone, according to CalFire.

Flames from a wildfire approach a pair of horses in a field, in Napa, Calif. on Oct. 9, 2017. Credit: AP/REX/Shutterstock

The Atlas Fire, in Napa County, had grown to 26,000 acres, and was 3 percent contained. In all, well over a dozen fires are burning across the state.

Firefighters have called in their biggest possible assets to assist with the fight, as a Boeing 747 "SuperTanker" began making fire retardant drops north of San Francisco on Tuesday.

How the fires spread so fast

A combination of factors converged to create one of the worst firestorm's in the state's modern history, beginning late Sunday and continuing off and on throughout the week.

First, the offshore winds, known as "Diablo winds," were extremely strong on Sunday night in particular, gusting as high as 79 miles per hour in higher elevations in Sonoma. This meant that any fires that started would spread extremely quickly, and be nearly impossible to control.

The fires encountered an ample supply of dry vegetation, primarily seasonal grasses and plants known to firefighters as fine fuels. Months of drier and milder than average conditions followed the wettest winter on record in this area, leading to a surge in the amount of highly combustible fuels.

This aerial photo provided by the California Highway Patrol shows some of hundreds of homes destroyed in a wind-driven wildfire that swept through Santa Rosa, Calif., early Oct. 9, 2017. Credit: AP/REX/Shutterstock

The summer of 2017 was the hottest on record across the state, and numerous record-shattering heat waves occurred that further dried out vegetation. In September, San Francisco set its all-time high temperature record, when the thermometer hit 106 degrees Fahrenheit.

The heat, particularly the relentless heat waves that struck the Golden State in September, played a crucial role in setting the stage for the devastating fires. 

"[The] record heat in early September capped an already extremely warm and dry summer to date, which acted to dry out vegetation even more than would typically be the case this time of year," said Daniel Swain, an atmospheric scientist at UCLA.

"Finally, the longer-term context of record multi-year drought just a year or so ago meant that some of the region's forests remain residually stressed. The net effect: 'fuel moistures' (the amount of water in vegetation) were at or near record-low values when these wind-driven fires ignited, which almost certainly contributed to the extreme rate of spread and the likelihood of spot fire ignition during this firestorm," Swain said.  

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