Winds Aloft Forcast when planning for a cross country flight
Most of us remember the days of our private pilot training when we calculated the effect of winds aloft on a flight plan for a cross-country. We would get the winds aloft forecast and interpolate those forecast for our altitude and flight path. Then with our trusty E6B, we would find the headwind/tailwind component and the wind correction angle based on the crosswind component. I say, “remember” because rarely do I find licensed pilots going through this exercise after their private pilot check ride, especially for flights in the mountains. There are several reasons for this departure from our training. First, many pilots today are using computer programs to generate flight plans and this usually includes a winds aloft calculation. Second, for mountain pilots, many feel that the winds aloft forecast for their mountain cross-country flights are either nonexistent or seem to be inaccurate, especially when interpolated over great distances. For example, when planning a flight out of Eagle (KEGE) or Aspen (KASE) the nearest wind stations are 80 to 120 miles away from the departure point.
The seeming inaccuracy of the forecast is usually only noticeable below 18,000 feet. Of course, this is where most piston engine airplanes are operating. So how can we utilize the winds aloft forecast for mountain flights below Class A airspace? There is actually a great deal of information to be gained from these forecast, even when flying out of KEGE, KASE or the other surrounding mountain airports.
The two closest wind stations to the central Colorado Rocky Mountains are at Denver (DEN) to the east and Grand Junction (GJT) to the west. Both of these stations exclude the forecast for 3,000 and 6,000 feet. (Hopefully you know why!) The lowest two forecast levels are at 9,000 and 12,000 feet. As a rule of thumb (this is based on experience, not scientifically documented facts) the wind velocity over the Vail, Avon, Eagle, Aspen, Steamboat Springs and surrounding mountain areas will often be 30% higher than the interpolated forecast at 12,000. The reason for this is the same reason your airplane will fly. Remember Bernoulli? Just as the air flowing over your wings accelerates because of the airfoil, the mountains tend to have the same effect on the winds aloft. The mountains actually create a venturi effect as the winds pass by the obstructions. The mountain valleys also tend to redirect the winds. This means your computer generated flight plan will not be accurate over the mountains at altitudes at and below 14,500 feet, neither in velocity nor direction. (The highest peak in Colorado is approximately 14,433 feet, Mt. Elbert, near Leadville (KLXV).) So, if you will do the velocity interpolation between DEN and GJT at 12,000 feet and add about 30%, you’ll have a rough idea of the velocity to expect overall. Keep in mind that as you approach mountain passes, this number may still be too low. You might even add another 30% to the estimates as these winds are accelerated through the mountain passes. Sometimes the acceleration is much more.
Another piece of useful information from the winds aloft forecast is to get an idea of how the ride will be over the mountains. As the wind velocities increase, so does the turbulence. When the forecast is for velocities at or exceeding 20 knots at the altitude of your flight, you probably won’t have a completely smooth ride. Of course, like everywhere else, thermal activity also adds to turbulence over the mountains. When the winds are forecast at 30 knots, you might expect occasional moderate turbulence. At 40 knots you will generally see continuous moderate with occasional severe. Sometimes we see forecast of 60+ knot winds at 12,000 feet. These are good days to take a helmet! These would not be the days to take your spouse or friends out for their first mountain flight.
When flying over the plains, we generally don’t concern ourselves with the winds aloft forecast at altitudes well above our intended cruise. In the mountains, this is a mistake. There is useful information to be gained by reviewing the forecast well into the flight levels. For example, if the winds at 18,000 or 24,000 feet are forecast to be much greater than the winds at 12,000 feet, you can expect that the rule of thumb of 30% from above is probably too low. The mountain wave effect of the winds in the flight levels will tend to dip down into the lower altitudes and not only increase velocities but also degrade the ride.
Another thing to consider from the forecast is to compare the wind direction of the DEN and GJT stations. When these are forecast to be more than 10 to 20 degrees different at the same altitude, this is another indication that you might expect some turbulence. For example if the winds aloft are forecast to be from 270 at GJT and 340 at DEN, somewhere along a line between GJT and DEN there is usually a dramatic shift in wind direction. (Usually east of KEGE and KASE) Not gradual swings as you might expect from divergent forecast over the plains. This dramatic shift will usually produce moderate turbulence and possibly wind shear.
If you want to get a good idea of how winds aloft are affected as they pass over the mountains, here is a tip. Pack a picnic, get a comfortable chair or a blanket and set yourself down next to one of our beautiful mountain streams or rivers. Observe how the water flows over and around the boulders and rocks. The winds do the exact same thing over the mountains! If you are a proficient mountain pilot, you could pack the picnic, grab your chair and your fly rod and venture off to the Gunnison or Delores Rivers!