Above Normal Significant Fire Potential forecast for November

November 3, 2019 – The 4-month Predictive Service outlook calls for warmer and drier than normal conditions in the North Ops region through February 2020. Very dry conditions remain in place across the region as November begins.

There are hints of a large-scale pattern change to more low pressure in the eastern north Pacific beginning in the middle of November, and that could lead to the arrival of the fall-winter rainy season, but confidence remains low due to model inconsistency.

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In the meantime, with dry air in place and high pressure west of the U.S. West Coast, there remains a potential for more N-NE/Offshore wind events and high significant fire potential, especially from the western Cascade-Sierra slopes to coastal regions from Lake County south. These are the driest areas, and they are where the N-NE/Offshore winds mainly occur, leaving them vulnerable to large fire development until the fall-winter rainy season begins. Any fires in these situations may exhibit rapid growth and very active fire behavior.

In November most areas below 5000 ft from the 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, with the emphasis on the first half of the month. All other areas have Normal Significant Fire Potential in November, and the entire region is in the Normal category from December through February.

Past weather discussion

October was very dry in the North Ops region. Early in the month weak frontal systems brought light precipitation to the northern mountains. Other areas received little to no precipitation during October. Frontal systems moved through the region once or twice per week, but outside of the early ones the fronts were dry. Dry N-NE/Offshore winds brought RH and fuel moisture values down to critical levels following each frontal passage.

Moderate to strong N-NE/Offshore wind events occurred more often in October than in any single month in the past 30 years, and the event of 26-27 October was as strong or slightly stronger than the October 2017 “Wine Country” event that produced dozens of fires and over 40 fatalities.

The prevailing dry windy weather in October almost completely erased the benefits of above normal rainfall in northern and eastern areas in Aug-Sept. The frequent cold fronts also led to cooler than normal temperatures in the region in October. 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.

Fuels and drought

The annual grass crop is well above normal for a fourth consecutive year, and is fully cured. Live fuel moisture values are near late fire season levels, which in a typical year is at the driest levels prior to the arrival of fall rains. Dead fuel moisture values increased to well above normal in September due to wetter than normal weather at upper elevations, but the dry and breezy October weather has dropped dead fuel moisture values below normal.

The region-wide 100-hr fuel moisture chart indicates values near record levels as November begins. The area with the driest longer-term conditions extend from the western Cascade- Sierra slopes to coastal areas from Lake County south, including the Bay Area and Sacramento Valley.

Another way to illustrate the dry conditions is the Evaporative Demand Drought Index (EDDI), which shows increasingly dry conditions toward the southern and western portions of the region.

The U.S. Drought Monitor is showing increasing dry and drought conditions in southeast and east-central CA, with abnormally dry conditions surrounding Lake Tahoe

November 2019 – February 2020 highlights

  • Warmer and drier than average.
  • Tropical E’rn Pacific ENSO-neutral, but steadily warmer than ave.
  • Fine fuel crop cured and heavier than normal.
  • Very dry air, fuels, and soils as November begins. Offshore wind season underway. Some indication of wetting precipitation mid Nov.
  • Above Normal Significant Fire Potential in early November from the lower western Cascade-Sierra slopes to coastal areas from Lake County-south.

Normal Significant Fire Potential otherwise. Normal large fire occurrence from November through May is defined as less than one large fire per PSA per month.