The winter precipitation for the Central Coast for the previous fire season (2020) was normal to slightly drier than normal, due to several significant atmospheric river events. We started the rainfall season in better shape than most areas in California and throughout the west. But this winter (2020-2021) has been one of the driest winters on record for California and the Southwest. This alone is concerning for the upcoming season, as fire resources are likely to be stretched thin.
While our region had significant rainfall end of January with a strong atmospheric river, it has received minimal precipitation since then other than the mid-March storm. Our region is still classified as abnormally dry (1,2). The dead fuel moisture indices, have dropped substantially since the end of January (2,3). These indices are used to determine fire potential and predicted rate of spread, and in particular the potential for extreme fires.
In particular, the 1000-hour and 100-hour, fuel categories are now well below normal and at record levels for this time of year. Dead fuels respond solely to current environmental conditions and are critical in determining fire potential. In particular, 1,000+ hour fuels do not burn easily, but if they do burn, they will generate extreme heat often causing dangerous fire behavior conditions (3).
Chews Ridge rainfall (4) for the water year starting October 1, 2020 is currently 11.62” Since January 1st, precipitation has been 9.14”, with much of falling during the atmospheric river event in late January. We are currently at 50-66% of the normal seasonal rainfall amounts over the Chews Ridge area of responsibility (5).
Unfortunately, there are no significant storms forecast for our area in the near future. The outlook through June 2021 is for below normal rainfall, with near normal temperatures (5).
This is the best map showing current drought conditions for California. It is frequently updated.
(1) US Drought Monitor https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?CA
(2) National Interagency Fire Center, Geographical Area Coordination Center https://gacc.nifc.gov/oscc/predictive/outlooks/myfiles/assessment.pdf
(3) US Forest Service Wildland Fire Assessment System https://www.wfas.net/index.php/dead-fuel-moisture-moisture–drought-38
(4) Chews Ridge Weather station (CNRFC) https://www.wrh.noaa.gov/mesowest/timeseries.php?sid=CHWC1&num=168&banner=NONE
(5) California Nevada River Forecast Center https://www.cnrfc.noaa.gov/
More Background Information
The fuel moisture index is a tool that is widely used to understand the fire potential for locations across the country. Fuel moisture is a measure of the amount of water in a fuel (vegetation) available to a fire, and is expressed as a percent of the dry weight of that specific fuel.
For example, if a fuel were totally dry, the fuel moisture content would be zero percent. Fuel moisture is dependent upon both environmental conditions (such as weather, local topography, and length of day) and vegetation characteristics.
When fuel moisture content is high, fires do not ignite readily, or at all, because heat energy has to be used to evaporate and drive water from the plant before it can burn.
When the fuel moisture content is low, fires start easily and will spread rapidly – all of the heat energy goes directly into the burning flame itself. When the fuel moisture content is less than 30 percent, that fuel is essentially considered to be dead. Dead fuels respond solely to current environmental conditions and are critical in determining fire potential.
The dead fuel moisture threshold (10–hour, 100–hour, or 1,000–hour), called a time lag, is based upon how long it would take for 2/3 of the dead fuel to respond to atmospheric moisture.
Small fuels (less than 1/4 inch in diameter), such as grass, leaves, and mulch respond more quickly to changes in the atmospheric moisture content, and take 10 hours to adjust to moist/dry conditions.
Larger fuels lose or gain moisture less rapidly through time. Fuels that are 3 inches to 8 inches in diameter, such as dead fallen trees and brush piles can take up to 1,000 hours to adjust to moist conditions, and are represented by the 1,000–hour dead fuel moisture index. 1,000+ hour fuels do not burn easily, but if they do burn, they will generate extreme heat often causing dangerous fire behavior conditions.
Learn more about dead fuel moisture here.