Wind, low fuel moisture, dry air all factors in power shutoff

Forecasts of abnormally windy weather conditions last week triggered the first widespread implementation of PG&E’s Public Safety Power Shutoff (PSPS) program.

The goal of the program is to minimize the likelihood of wildfire disasters like those that occurred over the last several years.

For this PSPS event, PG&E expected that nearly 800,000 customers in 34 counties would be affected by planned power outages. This included residents of many western and central Contra Costa County cities who received advance notice that transmission and distribution lines would be de-energized once severe weather began to materialize.

Several variables besides strong winds factor into potential fire weather events. Low fuel moisture, a measure of the water content of vegetation available to a fire, it also important.

California’s Mediterranean style climate features dry summer weather. Since no precipitation was recorded in the Bay Area prior to this PSPS event, low fuel moisture conditions existed.

Wildfire potential is also enhanced by low relative humidity conditions. The strong offshore winds expected to develop during this PSPS event were very dry, since they had no over-water trajectory.

Given the dry fuel and air conditions that existed prior to the predicted strong wind event, the main forecast issues were the strength, timing and location of the strongest winds.

Looking for answers

Meteorologists and fire weather professionals rely on computer forecast models to answer these questions and advise utility and governmental emergency response personnel. The key factor was the strength of a high-pressure ridge that developed to the north and east of our state.

Strong winds are a function of surface pressure gradients, which meteorologists measure in millibars (mb). This is the metric equivalent to inches of mercury that many people use.

In the case of fire weather winds that develop over Northern California and the Bay Area, the pressure gradient between Winnemucca, Nev., (WMC) and San Francisco (SFO) is a common yardstick. These sites are chosen because they parallel the fire weather wind direction and have long periods of recorded weather data.

As of this writing, the highest forecast pressure gradient was 18 mb. Based on historical data, an event of this strength would rank in the top 0.5 percent of all recorded WMC-SFO pressure gradients. Thus, the strong wind forecast combined with low fuel moisture and dry air triggered the PSPS event.

The accompanying chart displays one computer model available to meteorologists 60 hours before the expected peak winds. Other models run were very similar. Solid lines indicate surface pressure contours, in 4 mb increments. Winds were forecast to blow from a high-pressure center over Idaho to a low-pressure trough off the coast. The tight packing of pressure contours over California denotes the area of strongest winds. Low air moisture areas are shown in red.

Based on the latest forecast models, the critical ingredients of low fuel moisture, dry air and very strong offshore winds that are conducive to wildfire conditions seemed to be in place.

Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to
clayton_909@yahoo.com