Wx Watch: IFR Onslaught

Web site of the month

For a look at the upper-air patterns for the North Pacific and Gulf of Alaska, go to the University of Wyoming’s College of Engineering/Department of Atmospheric Science Web site. On the resulting page, under Type of Output, select “GIF.” Under Type of Map, select “Analysis.” Under Level, select “500 mb,” and under Region, select “Northern Hemisphere.” A drop-down menu lets you select a valid time for your chart, and it goes back four months. Finally, click “Get Map” to create an upper-air chart for approximately 18,000 feet, centered on the North Pole. The accompanying chart is from the Web site, and it shows the conditions on December 13, 2007, at 1200Z. Note the prominent trough (in red circle) south of the Aleutians. In a few days, there should be trouble in the Pacific Northwest.

Last December, the weather in the Pacific Northwest made big headlines. There were four cycles of two-day-long stretches of low IFR weather between December 2 and December 11. Heavy rainfall and winds that at times reached 100 mph near the surface accompanied these conditions. Obviously, this weather pattern affected not just pilots and passengers, but the general public as well. Huge mudslides got the most publicity in the general media.

What caused this mayhem? A parade of low pressure systems tracking from the North Pacific ocean and the Gulf of Alaska. These lows, collectively, are produced by a large, semipermanent area of low pressure located in the vicinity of the Aleutian Islands, hence its name—the Aleutian Low.

The Aleutian Low is a winter feature. It occurs when another large semipermanent pressure system—the North Pacific High—moves southward during the winter months. This move, in turn, allows the jet stream at polar latitudes to move farther south. And it’s a trough in the polar jet stream that causes the Aleutian Low to form, and to make storms spin up and track southeastward along short-amplitude waves within the steering winds that ride along the polar tropopause. Very often, this ride leads directly to Washington and Oregon.

How often does a low pressure system visit the Pacific Northwest in the winter? On average, every week, and the weather that precedes and follows them can guarantee days-long stretches of instrument meteorological conditions. On average, the coastal regions of Washington and Oregon experience ceilings and visibilities below 1,000 feet and three statute miles up to 40 percent of the time during December, January, and February. The low pressure complex of December 4, 2007, was especially bad in that its central pressures were low enough to draw a portion of the parent jet stream to such low altitudes—and even to the surface at times!

In addition to instrument meteorological conditions at the surface, icing is another huge factor in flying around the mountainous areas along the Pacific Northwest coast. While it may be raining at lower levels of the atmosphere, severe icing conditions may exist aloft. Anyone traversing the Cascade Range will almost certainly encounter icing at the higher minimum en route altitudes associated with higher terrain. Talk about semipermanent conditions—icing is always a factor when Aleutian Low pressure systems come ashore!

Aviation meteorologists can infer the existence of icing conditions, but it takes a pirep to verify any airmets or other predictions. Trouble is, pilots seldom take the time to issue them. It seems as though either the weather is so bad that few general aviation airplanes are flying over the “icebox” that is the Cascades (which is a wise strategy), or the weather is so good that pilots believe that making a pirep would be a waste of time. Wrong. Pireps are enthusiastically welcomed in any weather condition. Meteorologists can derive much information from temperatures and winds aloft, for example, by noting any variations from computer model projections, then change upcoming forecasts.

This winter, expect more pireps from the skies over the Pacific Northwest. Thanks to a cooperative program between the AOPA Air Safety Foundation and the FAA’s General Aviation Safety Committee, the Seattle Air Route Traffic Control Center, several area Tracons, flight service stations, and the National Weather Service will be actively soliciting pireps during those times when icing and/or low ceilings and visibilities are forecast. So if you fly in the Pacific Northwest, don’t be surprised if controllers aren’t shy about asking for pireps. And don’t you be shy about responding.

From a climatological perspective, flying weather improves east of the Cascade Range. That’s because any dense or precipitating clouds “rain out” as they ascend the windward slopes, then warm via compressional heating as the air parcels bearing them descend downwind of the mountains. This is the “rain shadow” effect, and it prevails over the regions east of nearly everywhere along the United States’ Pacific coastal ranges. To give an example, average annual precipitation totals for the Seattle area are in the 48-inch range (on the Olympic peninsula west of Seattle, precipitation averages 96 to 150 inches per year). East of the Cascades, that amount drops to eight inches of precipitation per year. Use precipitation as a proxy for low ceilings and visibilities, and you can see how VFR flying is the exception during the winter months.

Here’s hoping that the Pacific Northwest weather systems for the rest of the season don’t approach the severity of December 4. To avoid any surprises, check out the Gulf of Alaska’s weather on the Internet see “Web Site of the Month,” below. If there’s a frontal complex on the move, it’ll come ashore in Washington, Oregon, or British Columbia in two to three days.

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