October 2, 2019 – The 4-month Predictive Service outlook calls for warmer and drier than normal conditions in the North Ops region through January 2020. The outlook also calls for slightly more than the usual amount of high pressure ridging in the eastern Pacific, which would lead to more than the typical amount of N-NE/Offshore wind events.
The recent wet weather in the north and east has led to high soil and fuel moisture values there, and these areas are not likely to see any ignitions grow rapidly or to large sizes until the 2020 fire season.
Areas from the lower western Cascade- Sierra slopes to coastal regions from Lake County south remain dry and are vulnerable to large fire occurrence, especially during periods of dry N-NE/Offshore wind events. Any fires in these situations may exhibit rapid growth and very active fire behavior.
The forecast into the middle of October calls for dry conditions in these areas with 2-3 N-NE/Offshore wind events.
In October most areas below 5000 ft from the lower western Cascade-Sierra slopes, throughout the Sacramento Valley and foothills, and in coastal regions from Lake County south through the Bay Area have Above Normal Significant Fire Potential.
All other areas have Normal Significant Fire Potential in October, and the entire region is in the Normal category from November through January. However, if very little to no rain falls in late October we could see Above Normal potential extend into November in the same areas that have Above Normal Potential in October.
Normal large fire occurrence in October 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. From November through May all PSAs average less than one large fire per month.
Fuels and Drought
The annual grass crop is well above normal for a fourth consecutive year, and is cured below 5000 ft. Live fuel moisture values are at or slightly above normal throughout the region, but they are below the critical 100% value at middle and lower elevations. Higher elevations have seen soil and fuel moisture values increase in recent weeks and any ignitions above 5000 ft are not likely to see active fire behavior or rapid spread rates.
The region-wide dead fuel moisture value increased from near the “normal” curve to well above normal in September due to wetter than normal weather at upper elevations, but it is important to note that lower elevation grassy/brush-dominated areas remain much drier.
Periods of warm and dry weather are expected to keep areas from the lower western Cascade-Sierra slopes to coastal areas from Lake County south fairly dry at least into the middle of October. Another way to illustrate the difference between wetter high elevation areas and drier lower elevation areas to the southwest is the Evaporative Demand Drought Index (EDDI), which shows wet conditions in the north and east and normal to slight drought conditions farther to the southwest.
Also, an increase in dead and down trees and limbs in the northern Sacramento Valley caused by a heavy snow event in February contributes to a higher threat of large fires.
Past Weather Discussion
On September 4-5 a monsoon thunderstorm event in the North Ops region produced over 1,600 lightning strikes, igniting over 180 fires, a handful of which grew to large sizes. There were five other lightning events throughout the month, and these were all caused by cool Pacific frontal passages, resulting in fewer ignitions than the early September event.
For the entire month nearly 2800, or 70% of normal lightning strikes were recorded in the North Ops region. All of these events brought rainfall to the North Ops region, mainly to northern and eastern areas where rainfall was above normal for the month. This added to above normal rainfall in the same areas in August.
Areas from the Sacramento Valley to coastal areas from Lake County south were drier than normal in September, and for the summer. Temperatures followed a similar trend, with northern and eastern areas cooler than normal. Coastal areas were the warmest, and this coincides with above normal sea surface temperatures (SSTs) off the U.S. west coast. The tropical eastern Pacific is in the ENSO-neutral phase, but SSTs there are expected to remain slightly warmer than normal into the winter.
October 2019-January 2020 Highlights
- Warmer and drier than average.
- Tropical E’rn Pacific ENSO-neutral, but steadily warmer than average.
- Fine fuel crop cured and heavier than normal.
- Live fuel moisture values below critical levels at lower and middle elevations, but slightly higher than seasonal averages.
- Offshore wind season underway.
- Above Normal Significant Fire Potential in October from the lower western Cascade-Sierra slopes to coastal areas from Lake County south. Normal Significant Fire Potential otherwise.
- October’s Above Normal areas could see those conditions continue into November if fall rains are delayed.