Extreme Fire Weather Event underway as over 100,000 acres burn across Southern CA

A few months ago, it was Wine Country, this week it is Southern California. Powerful Santa Ana winds…with gusts up to 80 mph… are creating conditions favorable for explosive fire development and extreme fire behavior. Multiple fires have already broken out across Southern California leading to the destruction of (likely) hundreds of structures in what is usually the early part of the dry season in the region.

The largest fire…the Thomas Fire…has now burned an incredible 96,000 acres since early Tuesday morning! An area equivalent to 150 square mi/241 square km. Over 116,000 acres total have been burned in total across Los Angeles and Ventura Counties, with over 200,000 people ordered to evacuate.

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Wildfire near 405 Freeway. (Fox News)
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A wind-driven brush fire was burning out-of-control in the Santa Paula area on Dec. 4, 2017. (KTLA)

Very powerful Santa Ana winds…easterly winds blowing down the Santa Ana Mountains of Southern California are driving these highly destructive fires. A strong ridge of high pressure sitting over western North America…locked in place by a highly amplified atmospheric wave pattern…is causing enhanced surface pressures over the Interior West. This elevated, dense, high pressure air is sinking rapidly down the mountainous terrain towards lower pressure out at sea causing the powerful downsloping and gap winds…the Santa Anas…which through Friday morning may produce gusts near 80 mph fanning fires. And because the air flow undergoes “adiabatic heating” as it sinks to lower elevations and compresses under higher atmospheric pressure near sea level, the temperature of the air both warms and dries *significantly* drying fuels as the high wind speeds fan the flames over those fuels.

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Southern California terrain.
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4 am PST December 7th atmospheric sounding over San Diego, CA showing *extremely* dry atmosphere over the area produced by strong high pressure system and very dry, warm winds blowing off terrain in near surface environment.

The blocking upper-air pattern favoring these unusually warm, dry conditions out west (with unusually cool conditions in the Eastern US) will continue into next week unfortunately. While fire weather conditions will vary (mild improvement as far as winds and humidity this weekend), no significant rainfall is expected during the period for Southern California…in what should be their wet season.

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Warm colors display strong upper-level ridging of the jet stream over the Western US and Canada with well above normal temperatures and helping to produce the extreme fire weather conditions in Southern California.
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The Global Forecast System (US) Model forecasts continued blocking pattern over the Eastern Pacific with strong ridging over the Western US (with some fluctuations). This pattern suggests low precipitation and continued problems with fire weather conditions into next week.
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GFS Model shows no measurable precipitation over most of the Western US during the next 10 days. Only Western Canada and Western Washington.

Current research suggests that California’s increasing shift toward more drought conditions (which can fuel and extend wildfire seasons) is being caused by the reduction in sea ice in the Arctic (a symptom of human-induced climate change). As a side note, the Arctic is experiencing extremely abnormal temperatures this November and early December, with temperatures up to 30-50 degrees F above normal in some locations and top 2 or 3 record low sea ice extent for this time of year over the Arctic Ocean (record low in the Chukchi Sea and Bering Sea).

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Author: Meteorologist Nick Humphrey

Meteorologist and geoscientist in Lincoln, NE. Seattle, WA native. Love weather, storm chasing/photography and planetary science.

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