A real meteorologist will tell you that when a cold front catches up with a warm front and an occlusion occurs, it is a signal that the low pressure storm system is weakening.
I suppose that is scientifically correct but if you ever happen to fly through an occlusion, the word “weakening” will have a hollow ring.
Often the folks who draw surface weather charts don’t even include occlusions and stick with the blue cold fronts and red warm fronts. Take a look at the weather radar picture that accompanies this. That sharp wrap-around is a sign of an occlusion yet I looked at all the surface charts I could find that day and only one showed the purple (red and blue mixed up, get it?) symbol of an occluded front.
The meanness in an occlusion is the result of crazy mixed-up wind shear turbulence that develops when a cold front forces itself on a warm front. There is not usually convective activity but the atmosphere is disturbed enough to make some pilots feel like they have blundered into a thunderstorm.
The flying clue is that while there are generous airspeed excursions as the airplane moves through the disturbance, there are no strong up and downdrafts.
I was dealing with an occlusion one day on a westbound trip. It was not on the weather chart that I saw before takeoff and the area forecast mentioned only a strong cold front.
I knew what was coming, though, for as I flew along at the lowest available IFR altitude I could see churning in the clouds that were overhead. That is one sign that an occlusion is happening. As the ceiling lowered and I got ever closer to the cloud bases, the air became turbulent. Then when I was in the clouds, the wind shear turbulence was more than generous.
I have told this story before so will just do a brief summary here.
I was originally headed for Dayton, Ohio, to get fuel. There was a long string or airplanes lined up for the ILS there, though, and I was growing weary of the bumps. So I retreated to Ohio State’s airport in Columbus where the weather was better and where I landed on a really windy runway.
The wind at Columbus was out of the southeast. A short distance to the west at Dayton the wind was equally strong out of the southwest. If I had any doubt about an occlusion, that settled the matter. The surface position of the occlusion was between Columbus and Dayton.
We usually think about northwest surface winds after a cold front but after an occluded front passes the wind will initially be out of the southwest. It will later shift to the west and then to the northwest as the center of low pressure moves away.
I waited a while at Columbus, to let the atmosphere calm itself, which it will often do at a given location after an occlusion passes by. Then I had a relatively smooth trip on to Wichita, my final destination for the day.
If I had known in advance what was going on and had planned ahead, I would have taken a more southerly route, to stay farther away from the strong surface low pressure system that was wrapping things up that morning. I have done that on other long trips with good results.
Occlusions don’t happen too frequently. I guess I might have had to deal with a dozen or so in 57 years of flying. But when one does present itself, you can get a better ride if you know what is going on and make a plan to avoid the worst of it.
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