Weather forecasters expect the wildland fire danger in some areas of California to be extreme Tuesday night and into Thursday. Strong north to northeast winds accompanied by low humidity has triggered Red Flag Warnings putting land managers on edge. PG&E is so worried it is proactively turning off the electrical power to 600,000 addresses in nearly 30 northern, central, coastal, and Bay Area counties.
We consulted a fairly new fire danger forecasting tool to see how it analyzed what firefighters will be faced with for the next few days. The Hot-Dry-Windy Index (HDW) is described as being very simple and only considers the atmospheric factors of heat, moisture, and wind. To be more precise, it is a multiplication of the maximum wind speed and maximum vapor pressure deficit (VPD) in the lowest 50 or so millibars in the atmosphere.
The products, displayed as charts, show the index for the preceding 10 days and the forecast for the next 7 days. For the current and following days you will see results of the Global Ensemble Forecast System (GEFS), which is a weather forecast model made up of 21 separate forecasts, one control (in red) and twenty perturbations. The reasoning for showing 21 different forecasts is to quantify the amount of uncertainty in a forecast by generating an ensemble of multiple forecasts, each minutely different, or perturbed, from the original observations.The HDW only only uses weather information – fuels and topography are not considered by HDW at all. If the fuels are wet or have a high live or dead moisture content it will not be reflected in the data.
At the top of the page is the HDW forecast for an area about 15 miles northwest of Santa Rosa in the Bay Area of California. It predicts the Index will be above the 95th percentile on October 8, above the 90th percentile on October 9, and back up to the 95th percentile on October 10.
Below are HDW predictions for other areas in California.