Forecasts are excellent tools for being able to pinpoint mountain wave activity. Here's how to use them so you can avoid uncomfortable and dangerous flight conditions on your next mountain crossing.

Area Forecast Discussion

Area forecast discussions are an excellent tool for you to use. They're written by local forecasters, and they give a brief, high-level overview of the forecasted weather in a particular region.

When mountain wave activity is strong enough to be forecasted in these discussions, it is a good indicator that it is not safe for most general aviation aircraft to go flying.

PIREPs

PIREPs, or Pilot Reports, are an excellent indication of mountain wave activity, as these are confirmed reports of mountain waves by other pilots. When you're looking at PIREPS, keep a few things in mind.

First, PIREPs are not forecasts, they are "now-casts" and are really only accurate at the time of the report as weather conditions can change rapidly. Using PIREPs along with winds aloft forecasts can help you determine if the mountain wave is likely to dissipate or strengthen.

Second, pay attention to what type of aircraft is making the report. Mountain waves, and turbulence associated with them, can be experienced at different magnitudes depending on the aircraft you're flying. A large airliner experiencing light mountain wave activity could mean moderate for a smaller general aviation aircraft.

Finally, pay attention to the location where the PIREP was made. A report for mountain wave dozens of miles, or even hundreds of miles downwind of a mountain range suggest that the closer you get to the mountains, the more severe the wave will get.

Winds and Temperatures Aloft Forecast

Taking a close look at the winds aloft forecast can help you determine if there are going to be mountain waves, as well as what type of mountain waves you can expect.

Wind speeds of 20 knots or more and wind directions perpendicular to a ridgeline will aid in mountain wave generation.

While you're looking at the forecast, you first want to determine the average height of the ridgelines you're planning to cross. You also want to take a look at the wind speeds 6,000' above the ridgeline.

If there's significant shear (increase in speed) between the ridgeline and 6,0000' above the ridgeline, it's very likely that you'll have a mountain wave.

So how do you figure out the shear? Simply divide the windspeed 6,000' above the ridgeline by the windspeed at the ridgeline. You'll usually get a number between 0 and 3 (or more with greater shear). A number greater than 1.6 will tend to be a trapped lee wave, which is a mountain wave that can extend laterally for hundreds of miles beyond the ridgeline. anything less than 1.6 will tend to be a vertically propagating wave, a mountain wave that is stationary and localized.

This is a great tool to use in addition to other forecasts to help you pinpoint exact locations of mountain waves so you aren't caught off guard.

If you're interested in learning more about predicting mountain waves, check out our Mountain Weather Course.

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