Plane & Pilot
Tuesday, April 1, 2008

The Ugly Side Of Spring


Winter hasn’t released its icy grip yet


springWhether Punxsutawney Phil sees his own shadow or not, winter is losing its death grip. But it isn’t dead yet. Widespread icing still exists during the transition months of March and April. Gulf moisture, warmer temperatures and an overactive jet stream guarantees that convective SIGMETs will begin to spring out of hibernation. With temperatures slowly on the rise, you need to tailor your briefings to focus on key weather products that track the vernal transition.
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springWhether Punxsutawney Phil sees his own shadow or not, winter is losing its death grip. But it isn’t dead yet. Widespread icing still exists during the transition months of March and April. Gulf moisture, warmer temperatures and an overactive jet stream guarantees that convective SIGMETs will begin to spring out of hibernation. With temperatures slowly on the rise, you need to tailor your briefings to focus on key weather products that track the vernal transition.

While adverse weather, such as dense fog, low ceilings and turbulence, play a role in the decision to go or stay, icing and convection are the heavy hitters this time of year. As wintertime stratus morphs into springtime cumulus, the freezing level and NEXRAD image tend to compete for attention. It’s common to be worried about thunderstorms one day and icing the next. Obviously, thunderstorm and icing forecasts are critical. Specifically, you’ll need to pay close attention to the freezing level as well as convective outlooks.

spring
Photo by Rochelle Buley

Freezing Level
The lowest freezing level has the most impact of any single meteorological factor during the early spring months (http://adds.aviationweather.gov/icing/frzg_nav.php). Day to day, it changes more during early spring than at any other time of year. It’s not unusual to see the freezing level drop 5,000 feet or more within a 24-hour period at any one location. A round-robin instrument flight on Monday might not be possible on Tuesday because the lowered freezing level now places you into possible icing conditions. Even though the freezing level is something every pilot needs to know all year long, pay special attention as spring emerges.

On your preflight analysis, get comfortable with the big picture first. That’s what meteorologists refer to as the synoptic view. Start with the mean sea-level-pressure surface analysis and forecast. Get a sense of where the adverse weather is located and where it may be moving. Take note of all of the surface low-pressure areas and surface frontal zones, especially those with occluded fronts. When a low-pressure system begins to occlude, it’s near its peak intensity.

While it’s important to identify the location and movement of frontal systems, the mean sea-level chart provides a dearth of information; it tells only part of the synoptic picture. Often the story at 500 mb (18,000 feet) provides many more clues about the weather you might expect to encounter, including how the freezing level might change along your route.

Here’s a perfect example. As you fly south, the freezing level typically rises. If the freezing level is forecast to be about 12,000 feet in Portland, Ore., a flight to Los Angeles, Calif., should be a no-brainer. Worst case, the freezing level while en route to Southern California should be at least 12,000 feet or higher. You certainly wouldn’t expect to find the freezing level in Los Angeles to be at 6,000 feet.





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