Wx Watch: Winter Warm Fronts

Your preflight briefing mentions that a warm front is due to arrive along your planned route of flight. Oh joy, you may think, anticipating balmy temperatures, ice-free skies, and a smooth ride beneath a high overcast that conveniently eliminates that bothersome sun glare. Well, think again. While it's true that warm fronts involve the replacement of colder air masses with warmer ones, there are a few big reasons why winter warm fronts should set off your preflight briefing alarm.

The first involves that transport of warm air. Sure, warm air will travel horizontally (or advect, to use proper meteorological phraseology), but first it arrives aloft, and long before surface temperatures reflect the warm front's presence.

What does this mean? Think about the textbook illustration of a warm- front cross section. It's a shallow wedge of dense, ground-hugging, retreating cold air, overlain by a mass of warmer, less dense, moist air riding up and over it. As the moist air rides up along the frontal slope — the front aloft, if you will — it cools, condenses, and saturates the frontal area with plenty of water vapor. In other words, clouds. Stratus, nimbostratus, and stratocumulus clouds to be more precise.

These clouds are usually layered and nonconvective in the colder months of the year. That's because frontal lifting and convection aren't as great in warm fronts. Why? The warm front's slope is shallower than a cold front's. This promotes gradual lifting, not the more violent vertical displacements of the faster-moving, steeper frontal slopes of the cold front.

So now we have all this warmer air aloft, saturated to the point where rime icing — a feature of stratus clouds, because their water droplets tend to be smaller — is a real probability. Moreover, the icing airmets you see advertised for areas under warm frontal influence will be widespread. That's because a warm front's effects can range as far as 800 nautical miles ahead of the surface warm front's position. That's another by-product of the warm front's shallow slope — and another warning: Don't be fooled into thinking that warm-front effects are centered around its position as plotted on a surface analysis chart. More warm-front action extends miles ahead of the surface front, up at cruising altitude.

Widespread icing conditions aloft are one danger of winter warm fronts. But there's something worse: supercooled large-droplet icing and freezing rain. This happens when rain or drizzle falls from the warmer levels aloft into the colder temperatures of the retreating cold air mass beneath the warm front. The droplets remain supercooled — below freezing, but not frozen — until they strike a below-freezing object (like your airplane). These large droplets run back along the wings and other airframe components, then freeze and coat it with a tenacious layer of clear ice.

Freezing rain and large-droplet icing are most likely when flying in temperatures close to zero degrees Celsius/32 degrees Fahrenheit. At temperatures below minus 10 degrees C/14 degrees F the supercooled droplets are smaller, and tend to create rime ice accretions. While rime or mixed clear and rime icing is bad news, it's the large-droplet and freezing rain that can strike hard — and rob lift in a matter of seconds.

There are other big downsides of warm-front icing:

• Warm fronts are slow movers and can keep you on the ground for days.
• Low ceilings and visibilities in fog, rain, or snow are possibilities in zones ahead of the warm front's surface position. Rain-saturated or snow-covered ground, coupled with cold temperatures and light winds, can fill low-level air with so many water droplets that conditions can go to low-IFR values (ceilings below 500 feet, visibilities below one statute mile).
• Expect multiple freezing levels. The warm front's shallow slope aloft isn't a flat plane, and the vertical distribution of temperatures isn't linear, either. This means that temperatures can rise and fall with altitude. A wedge of below-freezing temperatures can protrude into above-freezing layers of the atmosphere, creating several freezing levels, and alternating layers of clear, mixed, and rime icing.
• Immediately ahead of a surface warm front, the cold air mass may only be 1,000 or 2,000 feet thick, and loaded with freezing rain or freezing drizzle. Cold high pressure to the north of a warm front can help feed low-level subfreezing air into surface boundary layers, especially in the lee of mountain ranges, where the cold air dams up against higher elevations. This is called cold-air damming, and an instrument departure in this kind of setup can be deadly — even though above-freezing temperatures may be tantalizingly close.

To sum up, remember that warm and cold are relative terms. Don't expect above-freezing temperatures aloft just because a warm front is advertised. You could reach the "warm" levels in the warm frontal region, then see minus 5 degrees C on your outside air temperature gauge, and be stuck in icing conditions in stratus clouds thousands of feet thick and hundreds of miles wide. Of course, a thorough preflight check of icing airmets, area forecasts, winds and temperatures aloft, pireps, and METARs and TAFs should give you a fairly clear idea of what to expect. If you're at all concerned, then the best strategy might be to throw another log on the fire and wait it out.

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