LINKING INTENSE WESTERN WILDFIRES WITH WEATHER PATTERNS AND CONDITIONS

Fire Management Today, Summer 2007 by Prevedel, David A

The history and lore of the Federal land management agencies is embossed with the names and legends of large wildfires. The Big Burn (Montana 1910), Tillamook (Oregon 1933), Sleeping Child (Montana 1961), Sundance (Montana 1966), and Yellowstone (Wyoming 1988) are among these historic events.

For many years, the occurrence of these large western wildfires has been associated with conjecture regarding fuels, terrain, weather, and ignition types. Historically, we know that most western vegetation communities developed under the influence of wildfire. But today, with changing land use, fire prevention, and urban growth, these burns can now pose significant risks to human life and property.

Within the last two decades, fires have burned with uncharacteristically high severity. Our observations show that many large wildfire occurrences seem to follow specific-often predictable-weather patterns. During times of extreme fire behavior, this predictability is significant to firefighter and public safety, as well as to the allocation of personnel and equipment.

We studied and reviewed large wildfires from 2000 through 2004 in the Western United States and attempted to correlate weather patterns and events that were present during extreme fire behavior. Images were obtained from the visual spectrum of the Advanced Very High Resolution Radiometer (AVHRR) Tyros satellite series that is operated by the National Oceanic and Atmospheric Administration (NOAA).

During our study period, the satellite receiving station was operated by the Forest Service's Intermountain Region in Ogden, UT From 1994 to 2004, the AVHRR receiver was used here to map wildfires on a daily basis (Prevedel 1994).

Each afternoon during our 4-year study period AVHRR satellites were monitored, including NOAA-17, NOAA-16, NOAA-14, and NOAA-12. All of these satellites are in polar orbits at an altitude of approximately 517 miles (833 km). Pixel resolution with these satellites is approximately 1,000 meters (3,281 feet).

During the last 3 years of our study, Web-based NOAA Geostationary Operational Environmental Satellites ("GOES") water vapor images were also used as an ancillary source to the polar-orbiting satellites. (See .)

Study Observations

While mapping wildfires, we noticed that some of the major fire events occurred in cloud-free "zones" on the west and northwest sides of large high pressure systems that were typically centered over the Great Basin and "Four Corners" area.

When these high pressure systems build, a clear weather surface thermal trough will frequently form ahead-or on the upper left side-of the high pressure. When the ridge breaks down and shifts east, this thermal trough intensifies and also shifts east. The ridge breakdown and eastward shift is often associated with a weak westward flow of very dry air that brings upper-level cooling while allowing the lower levels to remain hot. This phenomenon increases fire intensity (see fig. 1).

These westward flows can be observed on the GOES water vapor images as a "tongue" of very dry air that results in low relative humidity at the ground surface (see fig. 2).

Much more active fire behavior occurs with the thermal trough overhead. Fires located outside the trough to the north and west might be active, but this activity is more diminished. In addition, smoke from these fires appears "lazy" and unconsolidated-in contrast to the consolidated, intense, and linear smoke from fires associated with the trough.

The summer high pressure systems over the Great Basin and Four Corners areas are generally dominated by clouds and even summer thunderstorms. These observations of high pressures systems are critical because they could move or shift several hundred miles in a single day.

We also found that with the NOAA AVHRR satellites, the high pressure systems and accompanying surface thermal troughs could be observed and monitored several times daily.

In meteorological terms, this frequently observed fire weather pattern is known as the "breakdown of the upper level ridge" as described by Chris Maier of the National Weather Service in Salt Lake City, UT.

What To Watch For

Typical ground weather conditions over "fire blowups" associated with the upper level breakdown include:

1. Very clear weather. These clear weather surface troughs appear as voids in the atmosphere when viewed on the satellite image.

2. Strong winds that develop from the southwest or west within the late afternoon, generally from 1500 to 1800 hours.

3. Rapidly falling relative humidity.

4. Smoke plumes and fire progression that travel in a northeastern or eastern direction.

5. A large high pressure system (often associated with clouds) immediately to the south or east.

Ancillary meteorological data and patterns that trigger five "watch outs" for fire blow-up conditions:

1. A short wave in the isobars (preferably weak) indicating a drop in barometric pressure is approaching-followed by the breakdown, or eastward shift, of the upper ridge.

2. Often, a subtropical jet stream.