Weather Forecasts on Forest Service Radios

Columbia Dispatch broadcasts the weather forecasts for the Forest Service on the Radio’s, at two times a day, about 10 am and I think 4 or 4:30. If you’ve been scanning your radio, at these times, you’ve heard the weather forecast. It gets repeated over different repeaters, so you hear the forecast two or three times (and with varying levels of signal quality, too).

The information is really good weather forecast, but there are terms they use that are not in our standard weather forecasts you get on the 6 o’clock news. They also refer to zones, and I never could figure out if the forecast was for where I was or somewhere else.

Here’s the cheat sheet on the info. The map is included for details, but essentially, almost all the PCT on Mt Hood area (south of the Columbia) is in Zone 607, with East side of Mt Hood-Barlow Pass-Frog Lake boarding into Zone 639 and Pinheads and Olallie Lake bordering into Zone 610. For the most part, any weather for Zone 607 would be applicable, given the geography and weather patterns. The PCT on the North side of the Columbia is in Zone 660. (There is a map for all of Oregon, if you are interested, I can send you that one, too.)

A few terms they use in the weather:

• LAL – Lightening Activity Level – see below • Haines Index – see below

Lightning Activity Level: The chart listed below will be used to forecast Lightning Activity Level (LAL):

• LAL = 1 No Thunderstorms • LAL = 2 Isolated Thunderstorms • LAL = 3 Isolated Thunderstorms (Increased Confidence/Threat) • LAL = 4 Scattered Thunderstorms • LAL = 5 Numerous Thunderstorms • LAL = 6 Scattered (But Exclusively Dry) Thunderstorms

Haines (1988) developed the Lower Atmosphere Stability Index, or Haines Index, for fire weather use. It is used to indicate the potential for wildfire growth by measuring the stability and dryness of the air over a fire. It is calculated by combining the stability and moisture content of the lower atmosphere into a number that correlates well with large fire growth. The stability term is determined by the temperature difference between two atmospheric layers; the moisture term is determined by the temperature and dew point difference. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior.

The Haines Index can range between 2 and 6. The drier and more unstable the lower atmosphere is, the higher the index.

• 2 : Very Low Potential -- (Moist Stable Lower Atmosphere) • 3 : Very Low Potential • 4 : Low Potential • 5 : Moderate Potential • 6 : High Potential ------(Dry Unstable Lower Atmosphere)

Portland Fire Weather Zones