Winter weather can be dangerously fickle — without giving any radically overt manifestations of adverse conditions. In the warmer months of the year, we can see radar imagery and lightning plots that warn of oncoming storms as part of the preflight briefing process. In flight, we can see clouds gather, darken, or build, and watch lightning strikes or radar signatures if we have lightning detection devices or airborne weather radar aboard.
With Old Man Winter, any cloud in freezing temperatures could ice you up. There's no need for fronts to be nearby — though this would certainly make things worse. If the clouds are deep enough, the terrain is mountainous, and suitable escape routes are not guaranteed, then staying on the ground may be the best option (see " WxWatch: Winterwise," December 2002 Pilot, and " WxWatch: Ice Advice," January Pilot).
Those of us lucky enough to have airplanes with ice-protection systems (even better, airplanes with equipment packages certified for flight into known icing) have better shots at "getting through" when icing is a factor. But sometimes no one gets through. Not high-powered turboprops, not business jets, not the airlines.
For this reason, it's good to know the warning signs of an impenetrable winter weather system, and its typical tracks and hazards. As a case study, let's look at the winter storm of December 4 and 5, 2002. This was a classic east-of-the-Mississippi snowstorm, with widespread areas of freezing rain thrown in for good measure.
The storm's parent lows first organized early on the morning of the fourth, in the Gulf of Mexico. A stationary front and a warm front extended to the east of these lows: the stationary front over the Florida panhandle, and the warm front across Kentucky and the Carolinas. Remember that. Some of the worst Eastern storms have their origins in the South — not the frigid Dakotas or icy western Canadian provinces.
Meanwhile, a center of high pressure anchored a very cold air mass over the mid-Atlantic states. The northerly winds blowing from this high sent very cold air southward. On its way, this cold, dense air sank to low altitudes, hugging the ground.
The battleground was set: Warm Gulf air would run up and over the low-level cold air for two days. And it would be a big battleground, bringing the first snows of the season. The affected areas would range from Vermont to Florida, Kansas to the entire Atlantic Coast.
The evening of the fourth was marked by instrument meteorological conditions (IMC) over six states. The widespread IMC stopped almost all air traffic in North Carolina and Virginia. Why?
The low-level air was subfreezing, but the air above 3,000 feet agl or so — the air of the warm front, riding up over the cold — was well above freezing. The clouds in this overriding air mass were loaded with Gulf moisture, and they produced a lot of rain. The warm rain fell into the subfreezing air near the surface, became supercooled, and fell near the surface as freezing rain. This was no ordinary winter storm in many locales. It was an ice storm.
Consider this METAR for Raleigh-Durham International Airport at 11 p.m. on the fourth: Wind 030 at 7 kt; visibility 6 sm; light freezing rain and mist (-FZRA BR); 700 overcast; temperature minus 3 Celsius; dew point minus 3 Celsius; altimeter 30.33 in Hg (inches of mercury).
Farther north, the 11 p.m. report for Washington Dulles International Airport, with its wider temperature-dew point spread and high cloud bases, showed that it was in a drier air mass: Wind calm; visibility 10 sm; 8,000 broken, 9,500 overcast; temperature minus 2 Celsius; dew point minus 10 Celsius; altimeter 30.43 in Hg. These conditions wouldn't last long.
Boosted and steered by a trough of high-speed winds above the 18,000-foot level (winds at FL180 were 85 kt out of the southwest over central Virginia; 90 kt out of the west over central New England), the whole system of lows and fronts moved pretty much as predicted. But the movement was slow.
All the players — the warm front, the cold high pressure pressing south — were still at work, but by the morning of the fifth the system had expanded. The weather depiction chart for 9 a.m. showed IMC all over the mid-Atlantic and southeastern states. Part of this intensification was caused by nighttime fogs, when temperatures dropped to the dewpoint. Another cause was the lift created in the upper atmosphere by high-speed winds.
The freezing rain was still a big, big factor in North Carolina, as indicated by an urgent pirep (code prefix: UUA) submitted by the brave (?) pilot of a Piper Navajo at 6:27 a.m. He reported moderate to heavy clear ice during the climb from 1,500 to 3,500 feet. This shows just how shallow that cold low-level layer was. The pilot made his report in IMC at 6,000 feet msl, where it was a reported 8 degrees Celsius. The Navajo had climbed above the warm front's vertical boundary.
Morning at Dulles brought low IMC in heavy snow. That's because the warm front's vertical boundary was at a much, much higher altitude. As advertised by a low-level significant weather prognostic ("prog") chart the day before, low pressure over the Deep South was wandering to the North Carolina coast. The circulation around this low brought moist, dense clouds from the Atlantic to the states north of Virginia. It also meant low-altitude winds out of the north and northeast, fed by that extremely cold high — still a player in this drama, but now pushed to a position over Cape Cod. The 9:27 a.m. Dulles METAR — Wind 340 at 8 kt; visibility one-quarter sm; Runway 1R visual range 2,400 feet variable, 3,500 feet in heavy snow (+SN) and freezing fog (FZFG); 400 overcast; temperature minus 4 Celsius; dew point minus 5 Celsius; altimeter 30.16 in Hg. By the end of the day, some five inches of snow would fall at Dulles.
Airmets and sigmets warned of this storm's icing hazards. Sigmet Papa 6, issued at 8:44 a.m. on the fifth, advised of "occasional severe mixed/clear icing in clouds and precipitation below 9,000 feet. Conditions due to multiple freezing levels below 8,000 feet ...."
By the end of the day on the fifth, the snowfall from the flow wrapping around the coastal low, and the freezing rain falling through the warm front, had moved to New England. It would linger there for another day, then the whole weather complex would move offshore. Slowly at first, then rapidly, flying conditions improved in its wake.
A system this big was hard to miss, and there were plenty of historical examples of similar weather setups. Experts knew this storm complex would be a dangerous one, and predicted frontal movements, terminal aerodrome forecasts (TAFs), and other warnings were spot-on. Everywhere you checked those days — flight service, The Weather Channel, the local and national news — this system was heavily advertised.
Like meteorologists, pilots should also know the signs of a bad East Coast winter storm. Cold high pressure to the north, pushing south and damming up against the Appalachians? Low pressure in the Gulf of Mexico? Warm front from the Gulf moving north? Warm air atop cold? Strong winds above 18,000 feet, out of the southwest? A low off the North Carolina coast? A "yes" to all these questions gives you fair notice: Freezing rain and snow will likely track to the northeast. If you're in the way, it's time to throw another log on the fire, and get your airplane into a hangar.
E-mail the author at [email protected].
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