Warm temperatures limit the winds, but create very dry conditions. Offshore winds arrive early this week, causing further problems with fire growth.
SACRAMENTO, Calif .– The Caldor Blaze swept through the mountains just southwest of the Tahoe Basin, masking much of the popular tourist area in toxic smoke.
Warm winds with gusts of up to 35 mph were forecast for Saturday, raising fears that the winds could spread the embers of forest fires from the tops of parched trees and start new fires.
Cal Fire Fire Captain Stephen Horner said Saturday would be “a very pivotal day.”
Weather outlook from meteorologist Carly Gomez
Triple-digit heat will bring its own concern for the growth of fires. Strong atmospheric winds are not expected to play a role in the leading edge of the fire.
Warm temperatures continue to make it dry with very minimal moisture recoveries overnight. This means that the air is so dry that the margin for getting any kind of moisture in the air is much greater. We observe daytime humidity up to around 15-20%, while nighttime humidity has not returned to the typical 50-75%, instead we saw humidity around 25-30% during the night. At 3:30 a.m. Saturday, the humidity was about 26% near the Caldor fire.
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At 9:45 a.m. on Saturday, the humidity dropped to 20%
Humidity over the weekend should be in the teens until Sunday.
Summers in northern California don’t produce much dew on grass in the morning, but dew point temperatures aren’t too far away. During triple-digit heat, the dew point temperatures are so far apart that vegetation has no chance of burning even slower. Tinder is so dry that it will rise in an instant and might spread embers in a new area nearby. These lead to point fires at close range.
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With these types of temperatures projected over the weekend, the strong winds are limited and we are more likely to see these point fires at a shorter range. The problem now lies in point fires that land in difficult terrain.
The Caldor fire is approaching steep canyons and areas where ski resorts thrive. These areas make passage incredibly difficult for firefighters. It also makes it nearly impossible for bulldozers to help with containment lines.
Upstream fires also burn much faster, so that will be the main driving force behind the blaze this weekend. Imagine holding a matchstick upright. It will burn at a normal rate. But as soon as you turn the match down, the flame burns faster, gripping the highest points of the match. This is similar to reflecting on uphill terrain. It is not necessarily completely blown away, but there is more vegetation and wood to cling to; allowing it to burn faster.
Smoke will continue to spread over northern California on Saturday, bringing dangerous to unhealthy air quality in most cases from the Sierra to the Sacramento Valley.
No improvement in air quality is expected before offshore winds push smoke east on Sunday evening. This will bring better dispersion of pollutants, but will also begin to fan the flames to the northeast for Caldor’s fire.
SEE MORE: As more evacuation orders are issued, the Caldor fire draws closer to South Lake Tahoe as firefighters fight the flames.