The chance of lightning with little or no rain Sunday and Monday combined with high temperatures and low humidities has led to predictions of elevated fire danger.
Red Flag Warnings are in effect for the northern half of California Sunday and Monday. The highest threat of dry lightning is Sunday afternoon through Monday morning.
Thunderstorms with little or no rain is what started over 500 fires earlier last week. On Monday scattered or isolated dry thunderstorms could hit northern California and portions of Nevada, Utah, Wyoming, and Colorado. The storms are expected to move farther north Monday.
Nick Nauslar, a Fire Meteorologist at the National Interagency Fire Center in Boise wrote about the forecast in a tweet at 11:30 p.m. Saturday, saying, “Hundreds of new fires are likely if this event pans out.”
The Hot Dry Windy Index (HDW) predicts higher than normal fire danger for the area of the Hennessey Fire in the Bay Area Sunday through Tuesday. On Sunday it is above the 90th percentile compared to the average for the date. The HDW is a fairly new tool developed for firefighters to predict weather conditions which can affect the spread of wildfires. It is relatively 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 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.