A Winter Storm Primer

A Winter Storm Primer

A Winter Storm Primer

From late November through April, fierce winter storms often assault the continental United States and southern Canada. The effects can be devastating in their cumulative effect because some of these storms are not particularly fast movers, while others, such as a Nor’easter, can develop very rapidly and catch pilots unawares. Either way, they can be very dangerous to aviators.
Winter storms typically follow templates depending on where they originate. Knowing what kind of beastie you’re dealing with can help you cope with many of them, provided you have the necessary equipment on board and a workable plan for diversion. Watching the weather maps and knowing where to look for storm genesis a few days ahead of actual departure will usually allow you to anticipate what kinds of winter operations problems you’ll likely have.
West Coast Winter Storms
Winter Gulf of Alaska storms typically exhibit winds in excess of 50 mph and occasionally will rival the strength of hurricanes with winds as high as 100 mph. When these storms hit the west coast, they interact with the high mountain ranges that run down the coast from the Pacific Northwest to California. The mountains cause the storms to disgorge enormous quantities of rain and snow and the amounts are measured in yards annually, particularly at the higher elevations. For instance, snowfall on Mount Baker in Washington measured 1,140 inches in the winter of 1998-99.
Fortunately, spotting upcoming
West Coast storms isn’t particularly difficult.
Let’s say you have a trip to the Great Northwest in two days. Generally, if you see the semi-permanent Aleutian Low wound up tight in the Gulf of Alaska on the current synopsis chart (if it has a very low central pressure, in other words), you should look at the prog chart for +24 hours and see if the weather guessers show a progeny of the big boy developing just to its east/east-southeast (or if they expect the Aleutian Low itself to
temporarily move to the east).
If there’s a deep low on the move, things are going to get interesting from British Columbia south in +48 hours and you should plan accordingly. Look at where the low is predicted to come ashore and expect a large area to be affected, stretching from the low down the coast about 500 miles. Rain, ice, snow and low IFR conditions aren’t the only problems with these guys. Turbulence is likely to be moderate to severe.
Even if you aren’t headed to the Northwest, check out the situation with the Aleutian Low anyway, since, as we shall see, it’s involved in the creation of a fair amount of the storms that plague the rest of the country.
Mid-Continent Winter Storms
Mid-continent storms are generally known by names that relate to their location of origin: the Alberta Clipper, The Colorado Low or the Panhandle Hook. Let’s take a look at the four most common types; where they typically track and their hazards to aviation.
Alberta Clippers
Not usually related to the Aleutian Low, Albert Clippers originate in the Canadian province of Alberta (surprise). The storm then typically tracks along the upper tier of prairie states until hanging a left and moving back into Canada around Quebec or the Maritimes. Alternatively, it often remains in Canada the whole time. But, when the polar jet is making a deep intrusion into the U.S. midsection, the clipper can pay a visit to southern parts of the upper Midwest, including Illinois, Indiana and Ohio, before heading northeast towards New England.
Alberta Clippers have very little moisture to work with, so 2 to 4 inches of snow on the northern side of their path is the norm. Strong and bitterly-cold north winds are their hallmark and pilots operating from east-west runways would do well to practice their crosswind techniques late in the autumn to be ready for these visitors from the Canadian prairies. Of all the winter storms, these are generally the most benign but turbulence can be intense and white-out conditions can create havoc on an approach (along with a 45+ degree crab angle, particularly if you’re flying an airplane with low mass like a light twin).
Colorado Lows and Panhandle Hooks
As an Aleutian Low (or its offspring) hits the Rocky Mountains, it generally loses its moisture as the circulation is forced up and over the high terrain. Once on the eastern side of the mountain range, what happens next depends on where the remnant of the low ends up.
Depleted Pacific storms often reform around Colorado or a bit farther south. When the polar front’s jet stream loops south, it captures the Colorado Low (or its cousin, the Texas “Panhandle Hook” Low) and heads off towards the northeast. These low pressure areas will deepen as they follow the polar front’s jet stream, picking up moisture from the Gulf of Mexico maritime air mass and drawing it north into the Low’s circulation. As the low deepens, it will frequently cause a central plains event called a “Blue Norther” which drives very strong winds, biting cold and significant snowfall into the southern plains states, complicating life for pilots in Oklahoma, Texas and even eastern New Mexico.
If a winter storm’s genesis is in this area, from west Texas up into eastern Colorado, watch out! Next to Nor’easter, these storms are the most likely ones to cause problems over a large area of the United States and southern Canada. (See the “A Winter Storms Journey” below.)
Ohio Valley Storms
Ohio River Valley storms typically begin life in Arkansas and move through Southern Illinois, Indiana, Ohio, Pennsylvania and into New England. Almost as nasty as the Colorado and Panhandle Hook storms, the saving grace (if there is such a thing with a winter storm) is this type of storm will usually be forecast well in advance and, except for path variations +/- a few hundred miles, the forecast tracks are normally quite accurate. Ice is going to be a problem along and to the south of the track for a few hundred miles. Don’t be fooled into thinking you won’t need a FIKI (Flight into Known Icing) approved airplane just because you’re staying a couple of hundred miles to the south of the storm track – good advice for all winter storms, actually.
East Coast Storms
The path of these storms depends on where they originate and the synoptic set up (large scale weather features). They can move along the western margin of the Appalachians, along the coastal plain, or anywhere within a few hundred miles off shore. The worst place for an aviator to be – especially at low altitude – is to the west of the storm’s center when it’s just offshore.
Normally, the storm’s genesis is somewhere in an arc from the Gulf of Mexico to near Cape Hatteras. When it is still down in Dixie, thunderstorms typically break out but, once it is offshore and north of Cape Hatteras, snow is likely to fall to the left of the storm’s track, with rain more likely to the right.
If a Colorado Low or Panhandle Hook storm runs into a Nor’easter moving north up the coast, the two lows will merge and the storm will “bomb out”, meaning it will become a very powerful storm, very rapidly. The result is the closing of airports all along the eastern seaboard and flying conditions that are impossible for light aircraft (and many heavy ones as well).
An example of a merger of two powerful storms was documented in the movie “A Perfect Storm” which accurately depicted Hurricane Grace merging with an enormous Nor’easter, resulting in the loss of the fishing vessel Andrea Gail as well as a National Guard helicopter. But you don’t need a hurricane to merge with a nor’easter to have virtually un-flyable weather; a potent winter storm will work just as well.
Winter storms come in several flavors but the one thing all have in common is the bad taste they leave in the mouth of a pilot who attempts to fly through them.
A Winter Storm’s Journey
The progenitor for most severe winter storms is a remnant from a piece of energy thrown off by our old friend the Aleutian Low. After it plagues the Northwest, it crosses the Rocky Mountains and, a mere shadow of its former self, it frequently meanders into Texas, where it regenerates.
A “Panhandle Hook” winter storm, so named because of its birthplace in the panhandle areas of Oklahoma and Texas, begins life as a relatively-shallow low pressure area. However, the strong temperature contrast, between the Gulf of Mexico maritime air mass and the frigid continental polar air mass lurking in Canada and the Dakotas, quickly results in the low deepening.
On the first day, the storm begins engorging itself with large amounts of moisture from the Gulf of Mexico. The relatively warm air from the Gulf is carried northward and overruns cold air near the ground. Often, a layer of below-freezing air, normally somewhere between 1,000 to 5,000 feet thick (depending on how far north the warm front has progressed) will give rise to freezing rain as liquid precipitation from the warmer air above falls into the colder air below. This situation creates an excellent opportunity for extreme airframe icing. By the time the storm has wound up tight, the freezing rain can overwhelm even the equipment of FIKI equipped airplanes. Aircraft without anti-ice or de-ice devices are in great danger from an encounter with an energetic winter storm.
Oklahoma, Arkansas and Western Tennessee, located on the south side of the low, become victims of an ice storm with an inch of ice frequently coating the power lines and trees in a few hours. An approach through this kind of freezing rain had better not take very long – even if you can heat the wings and tail feathers with prodigious amounts of bleed air.
In Kansas, Missouri, and Illinois, the precipitation stays all snow and 5 to 8 inches fall within 8 hours. As the storm clears the area, a strong northwest wind blows the foot-deep snow into drifts and the skies rapidly clear behind the departing visitor.
By the second day, the low is fully developed and is making a beeline up the Ohio River valley. A very deep low pressure area now, it begins to draw moisture from the Atlantic Ocean, a full 600 miles away, as well as the Gulf of Mexico. If the Great Lakes haven’t completely frozen over they contribute to the moisture supply as well, resulting in the low deepening even more, plus creating extremely dangerous icing conditions in cloud and close to the low. An airplane can become an ice cube in the lower parts of the atmosphere from ground level through FL250 regardless of boots and prop deice. Even heated-wing jets can get into trouble while descending to land when close to the deep low associated with these storms.
What happens next depends of the path of the storm. If it heads east from southern Ohio, it will likely “bomb out” (greatly intensify) when it hits the Atlantic Ocean. It will then turn northward and become a nor’easter. Life will become miserable for hapless east coast aviators. If it continues on a northeast track from Ohio, however, it will ruin the day for pilots in Pennsylvania, New England and southeast Canada, but not be as disruptive as a full blown nor’easter. So, just for argument’s sake, we’ll assume it becomes a nor’easter.
In a nor’easter, operations all along the east coast are plagued by ATC delays that add hours to our Time-En Route calculations and force wholesale diversions to alternates. If the storm tracks offshore, heavy snow and high winds are the result for the coastal areas. An inland track inundates the Appalachian chain with very deep snow but it becomes mostly a rain and wind event for the coastal areas, albeit with turbulence and ice warnings.
Nor’easter or not, if the Great Lakes icemaker is in full swing, a pilot – maybe several – will have to declare an emergency due to an ice-covered airplane flying somewhere over Pennsylvania, West Virginia or the New England states.
Behind the departing storm, the morning deejays announce the temperature toll: Minneapolis, -21, Chicago, -9, Louisville, +3, Atlanta, +14. Winter has the people in its grip and the grip seems to be extremely tight.
Overhead, the stars shine with a clarity never seen in the summertime. The winds howl and then begin to diminish. A full moon lights up a snow covered landscape and people throw another log on the fire and count the days until spring. The storm has passed. Now, if we could only say the same for winter.•T&T

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