WX BRIEF

Preparing for wintry blasts

While winter weather can cause widespread flight delays, proper planning can reduce the disruptions.


Current Icing Potential (CIP) map from adds.aviationweather.gov shows an area of moderate to heavy icing and supercooled liquid droplets (SLD) over the Great Lakes in association with a developing cyclone cold front. In this composite, the worst icing is below 15,000 ft due to the low freezing level as winter approaches. By midwinter such cyclones will develop much further south.

Unfortunately, since warm fronts are shallow, a descent may not help shed the ice-but the clouds associated with warm fronts tend not to be too thick, and a climb will often put you on top and out of icing danger quickly.

From a convective standpoint, winter cold fronts tend to not be as energetic as their summer counterparts. However, they can still contain significant convection and produce heavy precipitation and even thunder.

Because the layer of cold surface air is deeper along the cold front, precipitation often reaches the ground as snow, even if it began as rain aloft. The most significant icing danger along the cold front is often found along the front aloft, where the outside air temperature (OAT) may hover right around freezing and a heavier coat of glaze may accrete.

If the system is particularly strong, the cold front can deposit heavy snow at rates faster than plows can clear runways. Snow rates of several inches in an hour have been reported-and behind the front there are often strong winds which can drift snow across runways and taxiways very soon after they are cleared.

Some of the most dangerous aviation weather conditions are, however, encountered poleward of the system. Aloft, the warm air continues to flow around the low, which means low cloud decks and precipitation.

By the time the air aloft reaches these regions, it has generally cooled to a point where precipitation will fall primarily as snow. However, the proximity of the high pressure to the low in this region means a strong pressure change over a relatively short distance.

That pressure gradient translates into fast and gusty winds. Snow and strong winds are the 2 main ingredients in blizzard conditions. Airports beneath this sector of the cyclone are likely to have IFR conditions in blowing snow, with winds that may make landing impossible.

Planning for winter cyclones A look at the surface weather chart will provide an instant view of whether any winter cyclones are present. If they are, observe the positions of the cold and warm fronts. If they are still in a relatively straight line through the low, the storm is in its formative stages.

Four major source areas for winter cyclones (red, light blue, green and yellow) and their respective general tracks. General position of the winter jet stream is shown with a dotted blue line.

If the fronts form more of a chevron shape, the cyclone is in its open wave stage and is likely getting stronger. The presence of an occluded front (normally shown by a purple line or an alternating red/blue line) means that the system has occluded and is likely as strong as it's going to get.

The longer the occlusion, the further into that stage the cyclone has progressed, and the more likely it's in its waning stage. A low that is poleward of the polar front is considered a cut-off low, and is generally the last vestige of a mature cyclone.

If you notice a developing or open wave cyclone, look at the upper-level charts for 300 and 500 millibars. The 300-mb chart will show the position of the surface low in relation to the jet stream. If it's downwind of a trough or ridge, the storm is likely to continue to develop. The 500-mb map will show 2 important things.

First, it will show the steering currents that will direct where the cyclone will move. The surface low will generally move in the direction of the 500-mb flow above it, and at around half the speed.

Second, many 500-mb maps also show a measure of vorticity. This is the amount of spin in the air, and is related to the divergence of the air aloft. The stronger the vorticity upwind of the surface low, the stronger the low is likely to become.

Lastly, there are the 24 and 48-hr prognostic charts. These are forecasts made by meteorologists and they are heavily based on the output of several computer weather models. These will provide a good idea about the near-future location and strength of the system, as well as any areas that may be affected by adverse weather.

However, it's important to remember that computer models are not infallible-wherever you see a developing winter weather system, you should pay close attention to weather conditions. Winter cyclone tracks In North America, the majority of winter cyclones take 4 tracks, and 2 of them originate in the Rocky Mountains.

Alberta Clippers are storms that move quickly out of the northern Rockies, flowing out of a ridge into a jet stream trough over eastern North America. Depending on the depth of the trough, these cyclones can speed across the Great Lakes or dip all the way down into Texas.

The second major source region is the central Rockies. These storms tend to spin up around Colorado and move eastward across the central plains of the US. Such cyclones are often responsible for the major winter blizzards that affect DEN (Intl, Denver CO).

Texas and Oklahoma represent the 3rd source area, as lows there may develop downwind of a sharp shortwave trough and move northeast into the Great Lakes region. These storms are often characteristic of early winter, before a larger ridge-trough pattern emerges in the jet stream.

The southeastern US is the final major storm genesis region. In winter, a deep and persistent trough often forms in the jet over the eastern US, placing the source region immediately downwind of the base of the trough.

The resulting cyclone strengthens and moves along the jet up the east coast of North America, reaching maturity as it strikes New England as a nor'easter. Winter cyclone development regions in other parts of the world share characteristics similar to those of North America.

In general, they are downwind of jet stream ridges associated with major north-south mountain ranges, or are in areas just downwind of long wave troughs in the jet. Not all bad Winter weather is not all adverse. In fact, most winter conditions are ideal for aviation.

If you are well away from the polar front, especially poleward of it, you are likely under the influence of a relatively strong high pressure. All that cold, dense air is suppressing convection and keeping the skies clear.

Furthermore, flying in and out of higher altitude airports can be a joy. Density altitudes in winter often operate in reverse, making the apparent altitude of the airport several thousand feet below its actual elevation.

Closer to sea level, aircraft performance can be stellar, with short takeoff rolls and outstanding climb rates. In addition, the dense air and lack of vertical motion often curbs the level of aerosols present in the atmosphere.

As a result, visibility can be unlimited, with distant terrain and obstacles staying clearly defined. Overall there is more good weather than bad during the winter, even in the middle latitudes.

However, the bad weather can be some of the worst for aviation and can affect it for hundreds of miles around and for days on end. But, when we realize that the characteristics of most cyclones are nearly the same, we can plan for them and reduce their impact on our flying.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University and was a scientist with NASA's Global Change Master Directory. Shein holds a commercial license with instrument rating.

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