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October 2, 2017 – The Predictive Services weather outlook for October through January calls for warmer and slightly drier than normal conditions through November, followed by wetter than normal and mild conditions during December and January.
October 2017- January 2018 Highlights:
– Drier and warmer than normal weather expected in Oct, then normal.
– Robust cured fine fuel crop and brush to continue to exhibit extreme fire behavior and rapid spread rates well into October, especially in the Mid Coast, Bay Area, and Sacramento Valley and Foothills PSAs.
– Live fuel moisture at normal levels for the end of the dry season, and no longer a hindrance to fire spread below 6500 ft.
– Above Normal Significant Fire Potential: From Mendocino and Shasta Counties south on west side of Cascade-Sierra crest in October, then back to Normal.
– Normal Significant Fire Potential All other areas Oct-Jan.
Monsoon thunderstorm events occurred in the first half of September, starting several new fires. Two weather systems brought wetting precipitation to northern and eastern areas in mid September. Overall, the summer was drier than normal across much of the North Ops region, with smaller areas of above normal precipitation in the eastern and western mountains. Temperatures were above normal this summer.
The wet winter and snow pack promoted above normal live moisture values into early July, but the hot and dry summer weather brought live fuel moisture down to normal levels beginning in August and continuing through September, which is typically the time of the year when fuels are their driest. The exception is the area above 6500 ft., where the long lasting snow pack allowed fuel moisture, in both live and dead fuels, to stay above critical values longer. Drought conditions were mitigated significantly across all of California with the wet winter and have not changed much these past few months, as average rainfall amounts are minimal in the summer. Once the snowpack melted in July the hot dry weather pushed fuels indices to normal values and in some cases above. Fuels indices moderated considerably in September in the northern and eastern areas from the two wet weather systems, as seen in the ERC chart.
North Ops Fuels
The wet weather that occurred in the north and east in September moderated fuels indices in those areas considerably. There was enough precipitation and a long enough duration of cool humid conditions to allow containment of most large fires in NW CA, but the dry and warmer weather in the second half of September has allowed indices to move back to normal levels. Other areas farther south and west were only lightly impacted by the cool humid air mass that moved in. These areas saw indices move back beyond the normal curve toward extreme values once warm and dry weather returned during that second half of the month. The 100- hr Fuel Moisture chart for the Sacramento Valley and Foothills is an example of this. The few existing and the occasional new fires in the north and east are exhibiting more active fire behavior as September comes to an end, but incoming weather systems in early October will tend to have more impact in the north and east due to more rainfall and cooler and more humid air masses, compared to the drier areas farther south and west.
North Ops Outlook
The Predictive Services weather outlook for October through January calls for warmer and slightly drier than normal conditions through November, followed by wetter than normal and mild conditions during December and January.
The impact from the wet September weather systems on northern are eastern areas has been to reduce fire occurrence and moderate fire behavior when fires have occurred. Upcoming weather systems in the first half of October look to have a similar impact, which will mean less time with warm and dry conditions and fewer opportunities for rapid spread and growth of fires, and more opportunities for effective fire fighting efforts during cool humid conditions. Areas farther south and west will not see these impacts until late October or early November.
However, as average rainfall amounts increase even below average rainfall doesn’t rule out the possibility of occasional wet storm systems moving through that could possibly considered a season-ending event in some areas.
The cooler longer nights in October tend to reduce significant wildfire occurrence in northeastern and eastern areas, even without significant precipitation. On the west side of the crest, however, dry wind events are more common in October, offsetting any beneficial effects of the cooler longer nights, especially from Mendocino and Shasta Counties south. This pattern tends to affect areas with grasses and brush more than timber, so large fires in October tend to be shorter duration events that exhibit extreme behavior and spread rates usually for only 1 or 2 burning periods.
In October areas west of the crest from Mendocino and Shasta Counties south have Above Normal Significant Fire Potential. All other areas have Normal Significant Fire Potential in October. From November through January all areas have Normal Significant Fire Potential.
Average number of large fires per Predictive Service Area by month: October: 1 – 1.2 Mid Coast, Sacramento Valley, and Northern Sierra. < 1 elsewhere November through January: < 1 all PSAs