Weather Phenomenon

One of the critical fire weather patterns in the West is the breakdown of the leading edge (upper ridge) of the summer subtropical high pressure system typically centered over the Four-corners Region. When this high pressure ridge builds over the west, a surface thermal trough will frequently form ahead of the high pressure. When the ridge breaks down and shifts east, this thermal trough intensifies and also shifts east. The ridge break down and eastward shift is often associated with a weak westward flow bringing upper level cooling while allowing lower levels to remain hot. This increases fire intensity.

Much more active fire behavior occurs with the thermal trough overhead. Fires outside the trough to the north and west may be active, but activity is lesser and smoke from these fires appears “lazy” and unconsolidated, as opposed to consolidated, intense, and linear with smoke from fires associated with the trough. 1/

Look for the following as an indication of this weather pattern:

A clear air trough on the northwest edge of the subtropical high pressure system. This high pressure will have a clock-wise rotation and will contain clouds of subtropical moisture. Puffy cumulous clouds will often develop around the edge of the trough. The clear air trough will often be 40 to 60 miles across at the narrowest point and 100 to 200 miles in length. The clear air trough will often distort and form a “dimple” in the high pressure system.
Moderate to high Haines Index (5-6).
A “tongue” of very dry air wrapping around the leading edge of the high pressure ridge and flowing into the clear weather trough (refer to water vapor image). This results in low relative humilities at the surface. The water vapor image will locate dark areas as potential regions of the moderate to high Haines Index.

1/ Adapted from Chris Maier, National Weather Service, Salt Lake City, Utah

Current Water Vapor image:
(Dark bands indicate lower amounts of atmospheric water vapor)

Water Vapor for the Western United States

NOAA Satellites and Information Site. This site shows the current water vapor loop using the GOES satellite platform. This site is updated every half hour and shows the last six hours.

High Density Winds for the Western United States

NOAA Geostationary Satellite Server. This site shows the current water vapor image with an overlay of the high density winds. This site is updated every three hours and shows the last 48 hours.