WX BRIEF

Winter weather woes

Cold season flying includes preparing for both severe and subtle adverse conditions.


Jet stream level winds associated with a late October storm system sweep a cold front across the Appalachians. A trough exists in the flow over the Great Lakes, which is driving the surface system, but the strongest wind speeds are ahead of the trough, indicating that the system will likely weaken and move away rapidly to the northeast.

If there is a surface low just ahead of the trough, then a winter cyclone is present and likely to affect aviation over that area and upstream over the next few days.

Satellite and radar imagery will also reveal these areas and give a pretty good idea of the extent of any adverse weather, such as precipitation or low ceilings. However, a visible satellite image can often be difficult to interpret because snow on the ground can be indistinguishable from the equally reflective cloud tops.

Enhanced infrared imagery can help, because in the infrared spectrum, reflectance from snow and from cold cloud tops can be differentiated, allowing cloud patterns to be identified more easily. Forecast graphics quickly show a pilot how a winter weather system is expected to evolve over subsequent days.

Because the storm systems normally tend to behave according to well-understood atmospheric dynamics, short-term forecasts of their development and movement are relatively accurate, more often than not.

A deiced Learjet 60 is directed to taxi by an FBO lineman at LED (Pulkovo, St Petersburg, Russia) while the remnants of a blizzard make for a slushy departure.

If forecast charts are not available, examination of the current 200 or 300-mb pressure map will give a general idea of whether the system will enhance or decay. Simply look at the trough above the system.

If the peak winds are behind the surface low, it will likely enhance, but if the strongest winds are above or ahead of the low, it will probably begin to occlude and decay. Metars and Pireps along a route of flight are invaluable for determining not only current but also recent weather conditions—and, by comparison, a pilot can see quickly how conditions are changing.

Surface weather reports also can give a good idea about ceilings, precipitation (and precipitation potential), winds and visibility around a storm system. You can estimate a freezing level from surface air by subtracting 3.5° F (or 2° C) from the surface temperature for each 1000 ft of altitude. This will give you a relatively good idea of potential altitudes for icing danger.

Of course, discussing the weather with a professional briefer will also give you some of the best information about how much winter weather you are likely to encounter. And any Pireps you file will help your fellow pilots determine what weather they may face.

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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