Winter storms unfit for pilots or passengers
The National Climatic Data Center in Asheville, North Carolina, summed up what happened that day and the next: "A complex, wide-reaching winter storm moved from the Mid-Mississippi Valley into the Mid-Atlantic and New England February 14 and 15. The heaviest snow fell in interior regions of the Northeast where amounts over 20 inches were widespread."
Television news showed dramatic images of thousands of passengers stranded at airports, including some extremely angry passengers who had spent up to 10 hours trapped aboard airliners sitting on taxiways or ramps at New York City's John F. Kennedy International Airport because no gates, or other ways to unload passengers, were available.
Anyone who flies just about anywhere in the United States--except in Hawaii and the southernmost states --should expect a similar storm to disrupt life just about any time from early October into April. Fortunately, since the early 1990s the National Weather Service has been pretty good at predicting such storms, although the exact details are often difficult to pin down more than a few hours ahead of time.
The delays at JFK were caused not by snow, but by the intermittent freezing rain that began falling the evening before and continued until the snow started falling the evening of February 14. There were reports of the tires of airliners at some gates being frozen to the ramp.
As with many big winter storms, the Valentine's Day storm affected parts of the United States as far south as the Gulf Coast. The storm's cold front pushed across the Southeast with severe thunderstorms and at least 17 tornadoes. But, unlike for places farther north, the storm's southern extension disrupted flights at any particular airport for only an hour or two.
Meteorologists call such storms extratropical cyclones. Extratropical means that they form outside the tropics or subtropics. They are different from tropical cyclones, such as hurricanes and typhoons. Cyclone refers to a weather system in the Northern Hemisphere with a center of low atmospheric pressure with winds spiraling counterclockwise around and into the low pressure. One of the biggest differences between tropical and extratropical cyclones is that tropical cyclones are warm and humid in all parts of the storm; they have no fronts. Extratropical cyclones include masses of both cold and warm air with fronts separating them.
| Figure 1 |
| Figure 2 |
| Figure 3 |
Figure 1 is a surface chart showing the Valentine Day's storm at 7 a.m. on February 13, 2007, the day before its biggest impact. Its center is over Arkansas, indicated by the red Low. The curvy red lines are isobars--lines of equal barometric pressure. The line around the center shows the pressure is 1,004 millibars on that line and is lower inside the circle, higher to the outside.
The blue line with triangles from the W in Low south into Texas and back north across New Mexico is a cold front where cold air is advancing. The high-pressure center over Canada directly north of western Texas is the center of the mass of cold air that's spreading south. The red line with a half-circle going east from the storm's center is a warm front where warm air is moving north into Kentucky and Missouri. This front changes to a cold front over eastern Tennessee where cold air is moving south.
The dashed blue line running from New Jersey west across the Ohio Valley and Missouri and south into Texas shows where 32-degree Fahrenheit temperatures aloft reach the surface. The green area shows where precipitation is falling; in general the precipitation is snow or freezing rain north of the surface freezing line. The dash-dot blue line across the northernmost states shows the southernmost extent of below-zero surface temperatures. By the time this map was issued, anyone on the East Coast who was thinking of going flying--either as a pilot or a passenger--on Wednesday, Valentine's Day, should have been keeping an eye on this storm.
Over the weekend National Weather Service computer models began indicating that an East Coast storm was in the works for the middle of the week, but were having a hard time pinning down where rain, freezing rain, or snow would fall. Coastal winter storms give forecasters fits because the exact path a storm takes makes a big difference. If the storm's center stays west of the Appalachians as it moves from Arkansas to Maine, the East Coast probably would see mostly rain because the counterclockwise winds around the storm center would bring in relatively warm air from over the Atlantic Ocean. If the storm travels along the coast, forecasting becomes more complicated. Often the heaviest snow from such storms falls on places approximately 60 to 200 miles west of the storm center with freezing rain, sleet, or ordinary rain closer to the center. In other words, the center's exact path makes the difference whether an airport has rain, freezing rain, or snow.
Overnight from Tuesday into Wednesday the storm's center traveled north to where Kentucky, Virginia, and West Virginia meet, with its warm front stretching to the east over the Atlantic. The atmospheric pressure in the old storm center began rising as a new low-pressure center formed on the warm front just off the coast. This occurs regularly with storms on the East Coast. As the old storm center fades inland, the swirling air of the storm's upper part moves east to energize a new storm center.
Figure 2 shows what was going on at 7 a.m. Valentine's Day 2007. The old storm center no longer shows on the map, but the new storm center is off the mouth of Chesapeake Bay. Its central pressure was below 996 millibars--the lower the central pressure the stronger the storm--and it was moving northeastward along the coast with rain and freezing rain near the coast, including at JFK, and heavy snow farther inland.
As the storm moved toward the northeast it continued strengthening until, as seen in Figure 3, by 7 a.m. on February 15, it was centered over New Brunswick and Nova Scotia with a central pressure down to 984 millibars. The closely packed isobars circling the storm indicate its strength.
By the way, the storm's drop in central pressure of 12 millibars, from 996 to 984 millibars, wasn't a very impressive strengthening for an East Coast storm. From time to time, a storm's central pressure will drop by 24 or more millibars in 24 hours. Meteorologists call such storms bombs or meteorological bombs because their explosive strengthening makes them exceptionally fierce.
On February 15, 2007, the leading edge of the cold air, the cold front, is over the Bahamas, moving toward the southeast. The cold air has warmed as it moved across the southeast; moving over the Atlantic Ocean will warm it further until the front fades away.
Just north of the low over eastern Canada we see a purple line with one triangle, like that used to designate a cold front, and a half circle, like the symbol for a warm front. This is the symbol for an occlusion (or occluded front). An occlusion has boundaries between three air masses, with one of the boundaries being aloft. In this case these would be a mass of very cold air, and one of cold air at the surface with relatively warm air aloft. Occlusions form in the last stages of a storm's life.
The Valentine's Day storm illustrates the widespread disruptions that an extratropical cyclone can bring. No pilot, or airline passenger who knows much about weather, would want to fly through a summer squall line of thunderstorms. A general aviation pilot can land somewhere safe and comfortably wait for the squall line to pass. Airline passengers have fewer choices because the disruptions of summer thunderstorms can upset the delicate balance of the airlines' hub systems, leaving passengers stranded. Still for real delays, winter is hard to beat.
Pilots are taught to think as the pilot in command; as the person who decides the safe course to take. But we can apply those same skills if we're flying as airline passengers. On February 12, 2007, someone scheduled for an airline flight out of JFK or another Northeast airport on February 14 could have started considering options, such as beating the rush to reschedule the flight for early on February 13 or for February 15 or 16.
As with the weather decisions we make as a pilot in command, those made as passengers might turn out to be unnecessary. The worsening visibility that causes you to land earlier than planned could clear up. The flight you paid $100 to change could take off on time. What's important to remember is that you can use your weather knowledge to stay safe and comfortable, whether you're planning to fly yourself on a trip or if you'll be a passenger in the back.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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