All dammed up

That goes double for those planning trips on north-south routes east of the Appalachians. As you scan the surface analysis and prog charts during your preflight briefing, be on the lookout for a distinctive pattern of frontal activity. A look at METARs and TAFs will confirm that the system deserves close attention. And for VFR-only pilots, a wide berth.

We’re talking about a meteorological setup known as cold air damming, or CAD. It’s a situation where geography, pressure patterns, and fronts team up to cover the area east of the Appalachians with widespread instrument meteorological conditions. Cold air damming tends to persist for days and can feature low IMC in dense fog, rain, snow, or freezing rain, depending on the temperatures at the surface and aloft.

The trouble starts when a cold air mass plunges southward from the clockwise flow around an area of high pressure—or a counterclockwise flow around a coastal low—invading the low-lying piedmont terrain east of the Appalachians. Blocked by the mountains, there’s nowhere for this air mass to go but south. A northeast-southwest cold front to the west and aligned with the Appalachians can reinforce the blocking effect. That’s why it’s called cold air damming.

Be on the lookout for a distinctive pattern of frontal activity.As we all know, cold air is denser than warm, and sinks. So, as it travels, the cold air’s leading edge works its way under the lighter, more moist, warmer air mass ahead of it. Locals call this action “the wedge.” But let’s call it by its conventional name: a warm front.

Warm fronts ride up and over the cold air beneath them. This can create a range of problems, especially with cold air damming. The warm front may begin at the surface, but its frontal boundary rises ahead of it. Far ahead of the front’s surface position that wedge of cold air is deep. Deep enough for snow to fall all the way to the surface, assuming that the temperatures in the wedge are subfreezing.

But as the frontal slope’s boundary aloft lowers, any snow created aloft could fall into a shallow layer of warmer air. This warm layer can partially melt the snow, turning it into ice pellets (IP or sleet). Closer to the warm front’s surface boundary, rain falling into temperatures just above freezing can become supercooled (below freezing, but still liquid) and turn into freezing rain.

Dense fog is another possible scenario if any cold rain makes it to the surface and brings temperatures down to the dew point—especially if the ground is already saturated from previous rainfall.

Cold air damming is serious business. It’s not for VFR-only pilots, nor most high-time instrument pilots for that matter. Those heading to destinations in areas affected by cold air damming could find that decent alternate airports are far away. Getting to them may mean climbing up through an icy clag in order to cross the mountains. With cloud bases hugging the ground and tops deep in the teens this is no place for casual trips.

Every once in a while you’ll hear about a GA crash involving Appalachian cold air damming. One involved a pilot who was trying to get from New York to Florida. He ran low on fuel, made several missed approaches in low IMC to an airport in the Atlanta area, then ran out of fuel. If a briefer or website mentions CAD, or if you see a surface analysis or prog chart like the one shown on these pages, best to stand down. Unless you have the experience, and a turbine-powered airplane with the speed, range, ice protection, and ceiling that’ll let you safely take on a trip in a CAD area—or overfly it altogether.