August 5, 2019 – The 4-month Predictive Service outlook calls for warmer and drier than normal conditions in the North Ops region through November.
The high elevation snow pack has led to fairly high soil and fuel moisture values at elevations above 6000 ft, and these areas will likely remain at the lower end of normal, fire-wise, for the remainder of the season.
Lower elevations (below 3000′) will see increasing grass/brush fire occurrence in August, and the potential of fires there growing to large sizes is Above Normal. Middle elevations (3000-6000 ft) will join the lower elevations in Above Normal Significant Fire Potential in August due to hot and dry weather combining with drier fuels.
The semi-persistent SW-W flow pattern that has limited lightning occurrence so far this fire season will continue to be more prevalent than high pressure ridging, and this will continue to keep lightning occurrence below average in August. Due to the moist conditions, any lightning strikes at upper elevations will not be as likely to produce large fires, compared to 2018 when the snow pack was below normal.
In August most areas below 6000 ft have Above Normal Significant Fire Potential, and this will continue in September.
In October areas east of the Cascade-Sierra crest drop back to Normal while most areas west of the crest below 6000 ft remain Above Normal.
Even with below normal rainfall in October and November, enough is expected to drop all areas back to Normal Significant Fire Potential in November.
Normal large fire occurrence in August is defined as 3-6 large fires in the NW Mtns, NE CA, and Northern Sierra PSAs and 1-3 large fires in all other areas. In September normal is defined as 2-3 large fires in the Mid Coast, NW Mtns, and Sacramento Valley PSAs and 1-2 large fires elsewhere. In October normal is defined as 1 or more large fires in the Mid Coast, Sacramento Valley, and Northern Sierra PSAs and less than 1 large fire in all other PSAs. And, in November all PSAs average less than one large fire.
Fuels and Drought
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The annual fuel loading of the grass crop is well above normal for a fourth consecutive year, and is mostly cured at elevations below 6000 ft inland from the coast. A crop of late spring weeds mixed in with the dead grasses will cure out in August. Live fuel moisture had been above normal in most areas and at most elevations until recently, but hot and dry weather in late July has pushed live fuel moisture values below 100% in some locations, and live fuels are expected to be much more vulnerable to fire in August as hot and dry weather continues. Dead fuel moisture has been tracking near the “normal” curve recently, but normal values themselves are fairly extreme in August, and more hot and dry weather will push fuels indices closer to extremes in upcoming weeks.
Other fuels issues that will contribute to above normal significant fire potential this fire season include the following: A rare heavy snow event in the northern Sacramento Valley has led to lots of dead and down trees and limbs, and an increase in tree mortality in the past year, as shown in the Tree Mortality Task Force density map.
August-November 2019 Highlights
- Warmer and drier than average through November.
- Weak El Niño now expected to end this summer, ENSO-neutral expected as fall begins.
- Above average fine fuel crop, mostly cured below 6000 ft in August.
- Live fuel moisture values dropping to near normal at lower elevations in August.
- Below normal amount of summer lightning due to frequent SW-W flow.
- Offshore wind season typically begins early to middle September.
- Above Normal Significant Fire Potential most areas below 6000 ft in August and September and west of the Cascade-Sierra crest in October. Normal Significant Fire Potential all areas in November.