Most of us remember from ground school that air circulates clockwise around a high and counter clockwise around a low pressure center. It’s useful, I guess, to know this to help understand what is happening overall with the weather. But most of the time I just look at the streamlines on the winds aloft forecast map, note that the arrows are blowing directly from my destination to the departure airport, and groan about the yellow or orange color that indicates the headwind is really strong.
The counter clockwise circulation around low pressure gets lots of attention every hurricane season when satellite and radar images show the wind racing around an intense low pressure center. I don’t pretend to understand all of the ingredients necessary to cause a tropical storm to form, but we all hear repeatedly that it is the heat of warm ocean water that fuels the storm. And the low pressure of a cyclone is at the surface so a falling barometer is a very positive indication that nasty weather is on the way.
But there is another low pressure system that is not all that common but makes the flying weather over a huge chunk of the country just plain miserable for days and that is the closed low aloft.
Actually, closed low aloft is now a little used meteorological term. The more common name for these nasty phenomenon is now cutoff low. By either name this is a low pressure system that forms in the upper atmosphere and the circulation is closed, meaning the wind makes a complete counter clockwise circle as it does in a tropical storm or hurricane.
For a closed low to form it must be cut off from the jet stream. Forecasting the formation of a cutoff low is notoriously difficult. Because the low is not being propelled by the jet stream it is almost impossible to predict reliably if it will move, where it will move, or when it will move. And forecasters don’t have much success in predicting when the cutoff low will break apart.
A cutoff low can bring days of strong winds at the surface and aloft, and drenching rains over the same spot. Convective activity pops up all over the place under the low, and the strong winds that shear with altitude and small changes in distance creates huge areas of moderate or greater turbulence. Flying into a cutoff low is simply something none of us wants to do.
As luck would have it a cutoff low formed over the southern states a couple days before I planned to fly from home base in western Michigan to Savannah. The low brought severe storms to eastern Texas, Louisiana, Arkansas and Alabama, and serious flooding in Georgia. I hoped the low would break up or move away before my trip. It didn’t.
The northern edge of the low pressure circulation was very clear on the weather radar mosaic. I could fly along the northern edge almost to the east coast and then make a right turn for Savannah and miss most of the worst weather. But the southeast wind aloft was blowing 50 and more knots. A trip that normally takes 3+45 would take 5+30 so a fuel stop was required. I picked Greensboro as the most southeasterly spot before the weather became really terrible.
By the time I slogged into GSO it was raining, the surface temperature was barely 50 degrees, and it was howling out of the east. Then I had to turn back to a southwesterly course for Savannah and fly through several bands of rain, continuous turbulence, and 50 knots of headwind component even though I stayed at 4,000 feet.
About 30 miles north of Savannah I flew out of the clouds into clear air, the temperature jumped up 10 degrees C, it was in the mid 70s on the ground and the wind was less than 5 knots. The people at the FBO told me “you should have seen how bad it was here this morning.” I think I did see that just a few miles north.
That’s the way it is with a cutoff low. Terrible inside, but not far away the weather is fine. Of course, I got to fly back home the next day and the low was still there but had moved just enough that yesterday’s headwind became mostly a crosswind so I had little help. A closed low aloft is surely Mother Nature’s punishment for pilot’s past sins.