WX BRIEF

Winter weather refresher

Learning synoptic scale charting for storms can eliminate many frigid air headaches.



Occlusions and stationarity

Freezing level maps show quickly at what altitudes 0°C (32°F) is likely to be found. When combined with the position of storm systems, such maps can help pilots determine where they are most likely to encounter icing aloft.

When a cold front overtakes a warm front, the result is an occluded front. On the one hand, an occlusion—usually displayed as a purple or alternating-red-and-blue line extending from the low—means that the storm system has cut itself off from its intake of warm surface air and will start its process of dying out.

But, on the other hand, the occluded stage is often when the low is at its deepest and the storm its strongest. What's more, the occluded front means that cold surface air completely surrounds the low, and the warm air that is being drawn up over the warm front and lofted by the cold front is now free to wrap completely around the top side of the low, dropping snow and ice pellets into a sector of the storm that contains some of the strongest pressure gradients, and thus the highest winds.

The occluded storm is often seen on satellite images as a comma-shaped mass of clouds. Under the comma's head, to the immediate north and northwest of the storm, is where the most ferocious blizzard conditions can be found. Winds beneath the comma head can easily gust above 70 kts and visibility in blowing snow can quickly be reduced to zero, recovering freshly plowed runways in a matter of seconds.

Finally, not all winter weather is restricted to these winter storm systems. Before and after the low makes its appearance, there exists the polar front. When it is not being twisted by transient lows, it is more or less a stationary front—meaning it has had no significant movement in a 3-hr period. Unfortunately for us, this can mean poor winter weather for days on end.

Despite its lack of movement, a stationary front is still a front, and has air of different temperatures and moisture conditions on either side. Warm and humid air may still flow up against the colder air to the north, generating clouds and freezing drizzle or snow flurries.
It can be worse when the front is stationary along the coast.

As occasionally happens along the midlatitude coasts, cool, humid air may flow onshore to the north of the front, while warm, humid air is flowing up over the front from the south. This configuration creates a situation similar to a warm front but stationary—with fog and freezing rain as temperatures hover near freezing and humidity near 100%.

Overall, one of the best clues to winter weather is your proximity to the polar front and the overlying jet stream. A look at satellite imagery, a surface chart and a jet stream map can quickly alert you to most potential adverse winter weather conditions and make your call to the briefer far more informative.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg Unversity. Shein holds a commercial license with instrument rating.

 


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