Northern California Preparedness Level 2  

National Preparedness Level 1  

National Preparedness Level Updated November 4 at 2 p.m. MDT (on a scale from 1 to 5)

November 22, 2019

Five large fires have burned 17,041 acres in California, Colorado, Kansas, Oregon, and Virginia. Eleven new large fires were reported this week.

National Weather: Weak off shore flow will continue across California on Friday and will keep humidity levels low. The flow will intensify early Sunday morning as pressure gradients align to create a weak Sundowner wind. Santa Ana and Mono Winds are possible in California on Monday and Tuesday as a strong northerly flow develops in the wake of a strengthening system that will cross the Continental Divide by Tuesday afternoon and move into the central Great Plains, where it will begin producing significant snowfall. As the system begins to pull off to the northeast toward the Great Lakes on Wednesday morning, a cold and windy northerly flow will develop over the Plains. Also on Wednesday, another strong, wet, and cold system will begin to move on shore into Oregon and California. Widespread heavy snow will be possible in the mountains. Some valley locations could see some snowfall as well. The Sierra and the southern Cascades could benefit greatly from this system. The system will weaken as it moves into the Great Basin on Thursday. However, the deep troughy nature of the system will allow for it to pull down unseasonably cold temperatures from Canada into areas west of the Continental Divide for Thanksgiving.

National Interagency Fire Center statistics show that in 2002-2006, an average of 12,000 (16%) of the wildland fires were started by lightning per year. These fires burned an average of 5.2 million acres per year.

   Two-thirds of lightning fires occur June-August. lightning fires peak in the late afternoon and early evening. Three-fifths (61%) of all fires started by lightning occurred between 2:00 and 10:00 p.m.

   55% of lightning fires occur outdoors, and 41% occur in structures. Deaths and injuries occur mostly in structures (89% and 86%, respectively).

   Because most lightning fires occur outdoors, the most prominent form of material ignited is "growing living form," which includes trees, brush, and grass. Materials found on residential structures that are commonly ignited include roofs, sidewalls, and framing. Electrical wiring is another material often ignited, as the electrical current in lightning is drawn to electrical wires.

   Civilians suffer more injuries than fatalities in lightning fires each year. Most casualties result from lightning structure fires rather than outside or other types of lightning fires. 89% of lightning fire civilian fatalities and 86% of injuries occur in structure fires.

Lightning fires are started by strikes to ground that have a component called a continuing current. All positive discharges have a continuing current, and about 20% of negative discharges have one. Ignition depends on the duration of the current and the kind of fuel the lightning hits. Ignition in fuels with long and medium length needle cast, such as Ponderosa pine and Lodgepole pine, depend on the fuel moisture. Ignitions in short- needled species, such as Douglas fir depend far more on the depth of the duff layer than on the moisture. Spread of the fire after ignition usually depends on fuel moisture in all cases.

The ignition efficiency on a 1 km pixel is given on a per discharge basis. That is, if the efficiency is high, then about 9 discharges will result in one ignition; if the efficiency is extreme, about 5 or fewer discharges will result in an ignition. The ratio of positive and negative discharges is built into the calculation. (Latham and Schlieter 1989) document the algorithm.

The fuel type and depth are conversions of the 1 km resolution current cover type (Hardy and others 1999) for this specific calculation. The moisture input is the 100-hr dead fuel moisture.

August 2002 - The lightning ignition efficiency algorithm has been corrected due to discovery of an error. The resulting maps reflect higher lightning efficiency than previously.

Haines (1988) developed the Lower Atmosphere Stability Index, or Haines Index, for fire weather use. It is used to indicate the potential for wildfire growth by measuring the stability and dryness of the air over a fire. It is calculated by combining the stability and moisture content of the lower atmosphere into a number that correlates well with large fire growth. The stability term is determined by the temperature difference between two atmospheric layers; the moisture term is determined by the temperature and dew point difference. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior.

Haines Index is computed from the morning (12Z) soundings from RAOB stations across North America.

The Haines Index can range between 2 and 6. The drier and more unstable the lower atmosphere is, the higher the index.

• 2 : Very Low Potential -- (Moist Stable Lower Atmosphere)
• 3 : Very Low Potential
• 4 : Low Potential
• 5 : Moderate Potential
• 6 : High Potential ------ (Dry Unstable Lower Atmosphere)

Each day during the fire season, national maps of selected fire weather and fire danger components of the National Fire Danger Rating System are produced by the Wildland Fire Assessment System (WFAS-MAPS), located at the USDA Forest Service Rocky Mountain Research Station in Missoula, Montana.

  Keetch and Byram (1968) designed a drought index specifically for fire potential assessment. It is a number representing the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper soil layers. It is a continuous index, relating to the flammability of organic material in the ground.

  The KBDI attempts to measure the amount of precipitation necessary to return the soil to full field capacity.

It is a closed system ranging from 0 to 800 units and represents a moisture regime from 0 to 8 inches of water through the soil layer. At 8 inches of water, the KBDI assumes saturation. Zero is the point of no moisture deficiency and 800 is the maximum drought that is possible. At any point along the scale, the index number indicates the amount of net rainfall that is required to reduce the index to zero, or saturation.

  The inputs for KBDI are weather station latitude, mean annual precipitation, maximum dry bulb temperature, and the last 24 hours of rainfall. Reduction in drought occurs only when rainfall exceeds 0.20 inch (called net rainfall). The computational steps involve reducing the drought index by the net rain amount and increasing the drought index by a drought factor.

• KBDI = 0 - 200: Soil moisture and large class fuel moistures are high and do not contribute much to fire intensity. Typical of spring dormant season following winter precipitation.
• KBDI = 200 - 400: Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity.
• KBDI = 400 - 600: Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively.
• KBDI = 600 - 800: Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.