Fire Weather Forecasting for Forest Products Industry in Northern and Central California: Revisit Fire Spread and Lower Atmosphere Stability Indices

Since 2008, Fox Weather, LLC has been using our MtnRT® system to provide fire weather forecast maps and text reports to support the Forest Products Industry in California (Forestry).
The products consist of:

a) Spot Fire Weather Forecasts (time interval 1 hour out 7.5 days (180 hours) text reports for over 100 stations in California.
b) Precipitation Forecasts of rain and snow across terrain in map and text formats.
c) Fire Spread Index Forecasts (FSI) out 7 twenty four hour periods. (Graphical forecast map series at 6 hour intervals).
d) 6 Day Lower Atmosphere Stability Index (LAsi), formerly our version of the Haines index, generated from our WRF 12km and 6km forecast domains, specific to central and northern California.

A reference for our Fire Spread Index product will be provided. We maintain an archive of our Fire Spread Index forecasts. A current day Fire Spread Index record has been used in our Fire Weather reconstructions for historical fire seasons in Sonoma, Santa Clara , and Santa Cruz Counties in California. References for these will also be provided.

We renamed our Haines Index product to LAsi in order to stress the analytical nature of the product, i.e. stability of the lower atmosphere mainly below 700 mb with respect to growth of plume fires. The LASI conceptually includes more and varied scenarios and applications, not just a few standard ones.

Examples and description will be provided for the products listed above in c, and d.
We will show some typical maps to illustrate how Fox Weather LLC uses the LAsi Index as an analysis
tool along with a map or grid of terrain elevations. We will show maps from selected late winter, and
spring periods for illustration, as well as from summer.

1. Characterizing the transition between dry profile in the lower atmosphere and neighboring
marine layer moisture with the marine inversion. It appears that most of spatial variation in
LAsi across terrain, are in fact terrain based, with predicted values of LAsi varying as a marine
layer starts or tries to traverse the terrain barrier, with terrain elevations above or below the
main marine inversion.

a. We will show an example of analysis with LAsi, in which neighboring terrain features
are above and below the inversion.

i. Case in which a marine layer is starting to develop into an area of previously dry
unstable airmass (high LAsi with a trend towards lower LAsi).

2. Some Analysis Questions We Encounter

a. What does our analysis tell us about a high LAsi gradient across terrain? What is
physically occurring in this situation? Is it based on warmer surface temperatures aloft
immediately above the marine layer, or above a cold valley? This can be a significant
indicator of potential fire behavior as the fire propagates into the area of differing local
airmass stability over short vertical and horizontal distances.

3. We have found the LAsi index coupled with wind speed or Fire Spread Index to be a valuable
tool for anticipating Red Flag-type conditions up to a week in advance, using a WRF 12km basic
model input.

A. The most extreme conditions of vertical instability and dryness of fuels appear to relate well with
funnel cloud or tornado circulations occurring in active fire areas e.g., Carr Fire in Whiskeytown,
California in July 2018.
B. Our use of the LAsi index is more analytical than categorical (quick) answer in an operational setting.
Operationally, we mostly focus on the coupling of high LAsi Index with high Fire Spread Index,
because that quickly provides the best analytical information for our discussions.