Wx Watch: Storms Aloft

To oversimplify — a necessity when discussing meteorology — weather comes in two basic varieties. One is the kind of systems and events that, once detected, have more easily predictable movements. The other are those that, by their nature, constantly throw us curves. Good examples of relatively easy-to-predict phenomena include cold fronts, extratropical storms, and even the strength and direction of winds aloft. For example, once the formation of a cold front is observed it's a comparatively simple matter to determine its intensity, then ascertain its speed and direction of movement. This gives forecasters the luxury of predicting the time of frontal passage at airports along the front's path. The faster a front's or storm's movement, the more accurate the prediction.

Slow-moving weather drives forecasters nuts. A storm or front that's been ambling here and there at a snail's pace doesn't have a reliable track, so predicting future movements is problematic. In this category are stationary fronts, slow-moving hurricanes, storms that form or re-form along the United States' Mid-Atlantic and Northeast coasts, and cut off lows aloft. Making any predictions about these weather events is much, much trickier. Here, weather predictions hinge less on the character of the events themselves, and more on the conditions adjoining the weather in question.

Cutoff lows aloft — also called closed lows aloft — make good illustrations of the challenges that forecasters and pilots must face when dealing with a slow-moving system.

Cutoff lows form within low pressure troughs high in the atmosphere. There are about five or six of these troughs that undulate around the Arctic Circle. These troughs show up very well on 500 millibar constant pressure charts (which depict the pressure patterns at approximately 18,000 feet msl), and they can dip as low as 30 degrees north latitude. While these troughs are at high altitude, they have a tremendous effect on the weather below, and on the flow of the winds aloft — including the jet stream.

Cutoff lows occur when pressure within a trough aloft deepens, and becomes isolated within the trough's large-amplitude pressure pattern. The large-amplitude pressure pattern, or wave, looks like a big "U" on constant pressure charts. The cutoff low appears as a circle (or a series of concentric circles) within the trough's "U." It's a mini-hurricane of its own, covering several states at a time.

As with any low pressure system, winds blow counterclockwise around the cutoff low. The tighter the spacing between the height contours defining a cutoff low's pressure gradient, the stronger the winds.

Cutoff lows develop vertically as well as horizontally. A vertically "deep" cutoff low can be a mean animal, because low pressure and strong winds can exist from the surface to the top of the troposphere. To visualize a deep cutoff low, imagine a big hole in the sky — a sort of mega-tornado. You can see the vertical profile of this low pressure funnel by looking at constant pressure charts for various pressure levels. A deep cutoff low will show up as a vertical stack of circular height contours. In the illustration accompanying this article, you can see how cutoff low pressure can reach from the surface on up, its center unwavering in position.

At the surface, expect to find a surface low, with a cold front extending to the south or southwest. Typically, stationary or occluded fronts — surefire markers of slow-moving systems — will form in the quadrants to the north of the surface low.

The structure of a cutoff low is one thing. It's the cutoff low's weather that'll ruin a pilot's day. Moisture drawn into the low's circulation rides up its east side on southerly winds. Clouds thicken as the winds pick up, and to the west a cold front will probably be brewing. Even though cutoff lows are mainly creatures of autumn and winter, thunderstorms are likely — especially in the southern tier of states.

In deep lows, 50-knot-plus winds and a strong counterclockwise motion can drag that moisture all the way up to the Plains states, where colder temperatures can produce icing conditions. Many times, the force of the circulation will drag clouds and precipitation all the way around a cutoff low. This has come to be known as "wrap-around" precipitation. In these cases, ice and rain can occur near troughs and occlusions to the west of a surface low.

This is not the usual state of affairs. Ordinarily, the colder, drier air behind a low and its associated cold front will prevent any moisture from affecting areas to the west and southwest of a surface low, thanks to the presence of post-cold frontal conditions. However, cutoff lows produce anything but "usual" weather.

A recent trip to Wichita, Kansas, and Lincoln, Nebraska, gave a good illustration of how cutoff lows can do the unusual. On the westbound leg from Frederick, Maryland, there were tailwinds of 20 to 30 knots. Right away, you know something's not right with an easterly wind aloft.

We were flying along the northerly portions of a cutoff low, and it was the low's counterclockwise flow that gave us our push. Though the National Weather Service had yet to fully mention it, this meant that the low aloft would have to be somewhere to the south and west, in, say, Oklahoma.

After two days in Wichita, it was time for the trip to Lincoln, and a look at some weather charts, AM Weather, and The Weather Channel. By this time, the NWS had identified a cutoff low aloft over Nebraska, and placed a surface low over eastern Iowa. A cold front was trailing to the south of the surface low, and widespread light precipitation was reported over portions of Iowa, Illinois, and Missouri.

Even so, the weather in Wichita involved just a few cirrus clouds, unlimited visibility, and light winds. The forecast for our arrival at Lincoln was equally benign: 3,000 scattered, visibility greater than six miles, no precipitation, and light winds.

But it was not to be. Wrap-around precipitation from the cutoff low had reached northern Kansas and eastern Nebraska. Halfway enroute, we were enveloped in cloud. Temperatures at our cruising altitude of 7,000 feet dropped to freezing, we began to take on rime ice, and light turbulence developed.

A descent to 5,000 feet took care of the ice, but the turbulence stayed with us to Lincoln. To top it off, Lincoln's ceiling dropped to 1,200 feet, its visibility went to 2.5 miles, and surface winds were out of the northwest at 20 knots, gusting to 30. This was a far cry from the forecast, but a great example of some of the strange goings-on that a cutoff low can bring.

We left that mess behind after a brief layover. Eastbound, the tops were at 8,000 to 9,000 feet, and the altimeter setting plunged to 29.62 near the cold front.

Pilots in the Midwest had to deal with that cutoff low for the next four days as it slowly, slowly dragged itself east, leaving widespread IFR and MVFR conditions in its wake. Kansas City got 1.8 inches of rain out of the deal, Omaha had 1.7 inches, and Nashville endured 2.35 inches.

Most of this rainfall accumulation came about due to the cutoff low's characteristically slow movement. It's in their nature to sit and spin, with gloomy overcasts and persistent rain.

Cutoff lows aloft may bring strange winds and unforecast weather, but it's important to remember that they're not rarities. In September 1994 alone, six of them passed over the continental United States. Eight per month is the average during the fall and winter months.

Here's something else to remember: The computer models that predict atmospheric patterns have good track records when it comes to the formation of cutoff lows, but they're not so hot at predicting when they'll break up and move downstream. Maybe that's why cutoff lows still carry a bit of mystery. A studious briefer or meteorologist can give you up to 48 hours' advance notice of a cutoff low's shenanigans. But he probably can't tell you how long you'll be IFR once the gloom sets in.

Largely ignored in textbooks, the cutoff low is a major player in fall and winter weather. Keep your eyes and ears open for the warnings — tailwinds west, headwinds east, fast drops in altimeter settings, precipitation and ice where yesterday's forecast said it wouldn't be — and you'll be as ready as you can for the adverse weather that follows.