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Significant fire potential is projected to be above normal up to 6000 feet during July and all elevations during August except for near coastal areas, including the North Coast and Bay Area-Marine Predictive Services Area (PSAs). All areas and elevations are forecast to have above normal potential during September, with areas west of the lower Cascade and Sierra Crests to the Pacific Ocean retaining above normal potential in October.
The weather pattern during June was diverse, with a mix of warm and dry upper ridging intermixed with unusually cool and wet upper trough passages. The most widespread cool and wet period occurred June 3-5. June is typically a drier month, with the four observed cool and wet periods resulting in above normal precipitation for the northern quarter of California. However, to the south in central California, near to below normal precipitation was observed. The pattern of observed temperatures was similar, with cooler than normal observations across the far north and east, while near to above normal observations occurred elsewhere. The snowpack started out well below normal at the beginning of the month and was largely gone by the end of the month, with snow now relegated to elevations above 8000 feet.
Dead fuel moisture fluctuated but was generally near to above the seasonal averages, although moisture rapidly lowered the last week of the month due to an extended period of unusually warm and dry conditions. Energy release component (ERC) values exceeded the 90th percentile value across the Bay Area and Sacramento Valley PSAs at the end of the month. Herbaceous fuels continued to cure during June, with a cured or mostly cured fuels found below 3000 feet and partly to mostly cured fuels between 3000 to 5000 feet. Herbaceous green-up was most prolific between 6000 to 7000 feet at the end of the month. Live shrub and tree canopy moisture levels continued to experience a mix of moistening and curing, which was species dependent. Chamise, which is dominant across the greater Bay Area and in the foothills of the Sacramento Valley, continued to cure and likely contributed to fire spread as the month progressed. Sage, which is a primary species found across the far north and east, had peaked and was largely curing during the month but was less flammable. Manzanita generally moistened with mixed flammability based on aspect and elevation.
There were a few gusty wind and lower relative humidity periods, including two northerly wind events on June 13-14 and June 21, as well as a notable dry westerly event June 27-28 that created elevated fire danger. Cloud-to-ground lightning accompanied several of the Pacific storm systems and the first monsoonal moisture period of the season occurred June 22-23, with just under 1000 cloud-to-ground lightning strikes across northern and eastern portions of North Ops. Daily wildfire ignitions fluctuated but averaged between 15 to 20 per day. Several large fires were reported across the Greater Bay Area and Sacramento Valley-Foothills PSAs and were mainly in a grass-hardwood mix.
The July through October weather outlook shows near to above normal temperatures and near to below normal precipitation. The weather pattern during July is expected to be a mix of warm upper ridges interspersed with cooler upper trough periods. A few monsoonal moisture surges are also expected, but the main moisture surges are likely to stay south and east of the geographic area placing northern California on the periphery of these surges. Monsoonal moisture and associated lightning will likely be less common during August as Pacific troughs and onshore flow are expected to dominate. The North American Monsoon is likely to end in September, with a pattern that could lead to more intense heat and drier wind events. Confidence decreases during October, although La Niña should be present, with some drier than normal conditions to start the month. The most likely month for lightning ignitions appears to be July for northern California. Onshore or westerly winds are expected to be more dominant during July and August, providing timely and impactful marine influence near the Coast, but dry breezes farther inland.
Drought conditions are expected to intensify some during the next few months, but likely not as dramatic compared to the previous two years due to less potential for extended hot and dry periods. Regardless, dead fuel moistures will spend ample time at unusually dry and critically dry levels, especially away from the coast. Live fuels will continue to become more flammable during the summer, with the transition most noticeable during July across the low and mid elevations when several species are likely to cure. The herbaceous fuel loading, based on measurements and field assessments, are near to above normal, with the above normal loading favoring the Far East Side PSA. Other fuel wildcards that impact significant fire potential include large areas of blow-down with cured leaves and needles due to intense storms last December across portions of the Tahoe, Eldorado, and Six Rivers National Forests, as well as a high likelihood that tree mortality has returned in greater numbers due to the extended drought.
Historically in July and September, PSAs average one to three large fires and two to five large fires during August. The exceptions include less than one large fire in the Bay Area PSAs during this period, with the Far Eastside PSA generally observing less than one large fire during September. During October, all PSAs average one large fire or less.