Wx Watch: Wall-to-Wall IMC

When cold air heads south

September 1, 2003

As all pilots should know, widespread instrument meteorological conditions (IMC) are bad news. What's widespread? I don't know of any hard-and-fast definition, so let's use an area the size of a medium-size state — say, Pennsylvania — as an arbitrary lower limit of coverage. When instrument weather covers this amount of territory the flight-planning process should go into overdrive.

First of all, there's the go/no-go decision. If your departure or destination airport is covered by widespread instrument weather, the choice is simple if you aren't instrument-rated and current: Stay on the ground.

But what if you and your airplane are equipped for instrument flying? There are good reasons why widespread low ceilings and visibilities should make you think twice about launching. A big one relates to fuel consumption. With wall-to-wall IMC you'll need plenty of fuel in reserve in case you miss an approach and have to divert to a suitable, better-weather alternate — which may be a hundred or more miles away. Will you have enough fuel to reach the alternate and shoot an approach, or is the weather at your named alternate so iffy that a retreat to your departure airport sounds good?

It can be nerve-racking to face these sorts of issues while flying on instruments, so we need to be aware of the types of weather setups that create widespread IMC. Now that we're facing the autumn months, the seasonal cool-down becomes a big factor in creating huge areas of lousy flying weather. The sun isn't as high in the sky, the nights are longer, and colder air begins to move south. It all adds up to cooler surface temperatures and more nighttime cooling, and we all know what happens when air is cooled to its dew point — yep, fog can form.

Widespread IMC is nearly always associated with slow-moving weather events like stationary fronts and slow-moving warm fronts. Islands of low pressure aloft — easily detectable on 500-millibar constant pressure charts (they roughly correspond to the 18,000-foot level) — can also cause the weather beneath them to worsen, slow down, and extend great distances as moisture is drawn into the system. These upper lows are called cutoff lows, or closed lows, because the circulation around these high-altitude features is detached from the parent trough. To see them on a chart is to see a bull's-eye of height contours, floating by itself and parked over a region that's been experiencing precipitation and IMC for days.

Pilots planning flights to the East Coast this fall ought to know that cold air damming is another famous cause of widespread IMC. While it doesn't get as much press as, say, hurricanes or destructive cold fronts, cold air damming can be more dangerous to general aviation pilots.

Here's what to watch for:

  • A cold high-pressure system centered over northern New England or the Quebec/New Brunswick provinces of Canada. The high may well produce subfreezing temperatures at the surface.
  • A cutoff low aloft at the 500-millibar level somewhere over Minnesota, Michigan, or Ontario.
  • A surface low-pressure system in the north-central United States and a slow-moving, north-south cold front issuing from the low, pushing warmer, humid Gulf air ahead of it.

What happens is that the clockwise flow around that high-pressure system sends cold air flowing south. It can penetrate as far south as Georgia and — here's a main point — exists as a very shallow layer. Cold air is denser than warm air, so it sinks to the surface as it flows over the Mid-Atlantic and southeastern states, and up against the eastern flanks of the Appalachians.

Now we have this shallow cold layer, running from the surface to 1,000 or 2,000 feet, essentially "dammed in" by the Appalachians, which explains the origin of the term. Problems occur when the cold air makes ambient temperatures drop to the dew point and fog sets in. Fog can be especially dense if the surface has been soaked by rains; this adds extra moisture to the cold layer. It may not be a deep layer of fog, but it can create low IMC (ceilings below 500 feet, visibilities below one-half mile) at the surface.

That slow-moving cold front to the west won't blow the low-level cold air away anytime soon, so the result can be a 1,000-mile stretch of low IMC. Now that's widespread.

On a surface analysis chart, the slim wedge of cold air can show up east of the Appalachians as a warm or stationary front with a pronounced southward bulge in it. This illustrates how the wedge of cold air slides beneath the warm air to the south and forces it back toward the southeastern states' Piedmont regions. Anytime you see this U-shaped front east of the Appalachians, be prepared to encounter some of the lowest weather you'll ever experience.

Similar cold air damming can also occur east of the Cascade Mountains in central Washington State. This happens when the cold-air mass behind the cold front gets blown uphill — against the mountains — by the southerly flows of air ahead of the next low-pressure system to enter the Pacific Northwest. In this scenario, snow-covered terrain at lower elevations can play a role by perpetuating the low-level cold layer over a period of days.

Cold air damming isn't a rare phenomenon. Every autumn, a handful of these situations occur along the East Coast. Some bring terrible consequences, as I described in a previous article (see " Wx Watch: A Bad Day in November," February 1995 Pilot). On November 27, 1994 — the Sunday after Thanksgiving — there were three crashes in the Southeast. Two were fatal. At the time, I described the conditions as being caused by a stationary front, which was true enough. But I focused on the accident scenarios more than the weather, and a more complete explanation of the weather certainly would have mentioned cold air damming.

In one accident, a Beechcraft Baron pilot flew from Perkasie, Pennsylvania, to the Atlanta, Georgia, area. The weather was so bad that he missed the approach at his intended destination, diverted to an alternate 38 miles away, shot — and missed — three more approaches, then crashed after running out of fuel.

A Cessna 182 pilot flying from Gainesville, Florida, to Asheville, North Carolina, shot — and missed — two ILS approaches to the Greenville Downtown Airport in South Carolina. He then diverted to the Greenville-Spartanburg International Airport in Greer, South Carolina, where the weather was reported as "sky partially obscured, measured 300 overcast, visibility one-half mile in drizzle and light fog, temperature 41, dew point 40, and a runway visual range of 6,000-plus feet." Fog covered five-tenths of the sky. The Skylane crashed during the approach, but the pilot escaped with minor injuries.

A Glasair pilot died after crashing during an ILS approach to Runway 9 at the Florence Regional Airport in South Carolina, where there was a 300-foot overcast.

All these crashes happened in the space of six hours. So cold air damming is nothing to shrug off. The system stayed in place through the next four days, turning the Southeast into a gloomy, gray mess. Think about that the next time you plan an IFR flight east of the Appalachians this autumn and winter. If you launch, remember to take on enough fuel, and remember that your endurance decreases the moment you take off.


E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.