Winter Hurricanes N Oct

THREATANALYSIS

BY ED BROTAK

Heavy snowfall and high wind from monster storms called bomb cyclones periodically disrupt aviation operations.

Winter Hurricanes n Oct. 29, 2011, U.S. weather maps Massachusetts received 32 in (81 cm) of snow, a showed a benign-looking 1007-millibar record for so early in the season. Locations that (29.74-in Hg) low pressure area off the escaped heavy snow still had to deal with strong coast of North Carolina at 1200 coordi- winds. Onated universal time (0800 local). As the low Nantucket, Massachusetts, observations re- moved up the East Coast, it began to deepen ported sustained winds of 40 kt with gusts to 60 explosively. With abnormally cold air being kt. Aviation operations along the entire north- pulled in from Canada, rain changed to snow at eastern corridor were dramatically affected. At many locations. Newark (New Jersey) Liberty International Air- Some reporting stations recorded snowfall port (EWR), heavy rain changed to heavy snow rates of 2 to 4 in (5 to 10 cm) per hour and during the day and visibilities dropped to 1/4 visibilities at local airports dropped to near mi (400 m). For a time, all flights were canceled.

U.S. National Aeronautics and Space Administration National Aeronautics U.S. zero. The town of Peru in extreme western John F. Kennedy International Airport (JFK)

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and LaGuardia Airport in New York developing extratropical cyclones. landing extremely difficult, especially City had five-hour delays. By the morn- These are the extreme winter storms. if there is a crosswind. Just above the ing of Oct. 30, a powerful 977-millibar To officially qualify as a bomb, a low ground, the extreme wind shear gener- (28.84-in Hg) low sat just south of Nova must deepen at least 24 millibars in ates severe turbulence. Airplane crews Scotia. The pressure had fallen 30 mil- 24 hours. Meteorologists even call the in flight having to deal with strong libars (0.90 in Hg) in 24 hours. development/intensification process headwinds face significant delays in bombogenesis. their arrivals. Consecutive ‘Bombs’ What causes a low to “bomb out”? Less than a year before this storm, an Phenomenal Origin The answer is that the same atmospheric even more powerful one had affected These super storms develop in the processes that produce typical extratrop- the same U.S. region. Having left snow colder months at higher latitudes, usu- ical cyclones are at work, but they are in as far south as Georgia and Alabama ally those from 40 degrees toward the overdrive. To briefly review the physics on Dec. 25, 2010, an already potent North Pole. They are the result of intense involved, atmospheric pressure is just low pressure area (992 millibars [29.29 temperature contrasts and powerful jet the weight of a column of air above a in Hg]) started moving up the coast streams. They also are often associated point. Surface pressure will fall when air on Dec. 26. By the next morning, a with copious amounts of precipitation, is removed from above a point. This oc- 962-millibar (28.41-in Hg) low sat just and, because these are cold-season phe- curs when the wind speed increases with off of Cape Cod, Massachusetts. nomena, the precipitation is often in the height or when air is spread out over a All three major airports servicing form of snow. Regardless of amounts or larger area. This is called divergence. New York City — Kennedy, LaGuardia types of precipitation, bombs are always So, to explain what is happening at and Newark — were closed on Dec. 27 prolific wind-makers. Winds of 50 kt are the surface, we must look aloft. Diver- and did not reopen until the morning common, and winds in excess of 100 kt gence aloft is found in certain regions of of Dec. 28. More than 1,400 flights were have been recorded. the jet stream and ahead of upper-level canceled. At the height of the storm, In aviation, these storms produce a troughs of low pressure. On the east JFK reported a visibility of zero in wide variety of problems and hazards. side of these troughs in the Northern heavy snow with winds gusting to 49 kt. The dangers of heavy snow to aviation Hemisphere, the airflow is spread out, Across the Hudson River, EWR operations are well documented and creating the divergence needed to lower reported visibility of 1/8 mi (200 m) for include reduced visibility, aircraft icing the surface pressure (Figure 1). With hours with snow accumulating at rates and slick runways. The strong winds of- a favorable jet stream configuration, of 2 to 3 in (5 to 8 cm) per hour. Winds ten pose an even greater threat. Besides development can be rapid. Exacerbating gusted to 39 kt. Newark wound up with the high sustained winds at the surface, the problems caused by these storms 24 in (61 cm) of snow and JFK with 16 excessive gusts are also common. is the fact that they tend to slow down in (41 cm). Farther up the coast in Mas- Typically, gusts can run 20 kt or when they are intensifying rapidly. They sachusetts, winds continued to increase greater than the mean wind. This is can even become stationary. The prob- through the night and into the morning indicative of strong vertical wind shear. lems associated with these storms can of Dec. 27. Nantucket reported gusts of Higher winds just above the surface last for a day or more at some locations. 50 kt from the southeast ahead of the are mixed down in the gusts. For the As previously noted, strong wind low and from the northwest following December 2010 storm, a sounding associated with these storms is a major the storm’s passage. Logan International taken just outside New York City in the risk factor to aviation operations. This Airport in Boston measured 18 in (46 evening of the 26th showed winds of 50 wind is generated by differences in cm) of snow. Meteorologists call such kt just 1,000 ft above the surface, with pressure, with air always trying to go extreme winter storms bomb cyclones or winds increasing to 77 kt at 3,600 ft. from higher to lower pressure. The simply bombs. For the October 2011 storm, verti- greater the difference in pressure, the Fred Sanders, a professor of cal wind profiles taken near Nantucket faster the winds. On a surface weather meteorology at the Massachusetts indicated winds of 60 kt within 1,000 map, this is indicated by the pressure Institute of Technology, coined the ft above ground level. Winds of this gradient, that is, the distance between term bomb in 1980 to classify rapidly strength at a terminal make takeoff and the isobars or lines of equal pressure. If

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these lines are close Bomb Cyclone Surface Analysis together, we have a tight pressure gradi- Legend ent, and strong winds Overcast Sky obscured can be expected. 40-kt wind blowing from west Low pressure areas, Warm front Cold front especially ones that Occluded front are deepening rapidly, Isobar Canada 964 964 millibars (28.47 in Hg) have very tight pres- Heavy snow sure gradients and re- 18 18° F/ 8° C temperature 15 15° F/ 9° C dew point sulting strong winds.

Familiar Features People often con- sider these storms as “winter hurricanes,” 18 and at times they can 15 resemble tropical L cyclones. They can 964 Atlantic Ocean have eye-like features, 968 small clear areas 972 at the center of the 976 storm. They can also 980

have active convec- United States 984 tion. The December

2010 storm featured 996 992 988 “thundersnow” Note: Examples of data from 1200 coordinated universal time on Dec. 27, 2010, show a deep center of low pressure (L) along the northeastern coast of the United States. Selected weather stations show the winds and one (center) includes temperatures (ASW, 10/10, p. 18) and precipitation. in the New York/New Source: Ed Brotak and U.S. National Oceanic and Atmospheric Administration Jersey metropolitan region. Convec- Figure 1 tive bands also were the east coasts of continents are active breed- noted, similar to the so-called “spiral bands” in ing grounds. The mild air over the warm waters tropical cyclones. For bombs, the bands have contrasts greatly with the bitterly cold arctic air been associated with the strongest winds and that moves off the continents at high latitudes. heaviest snowfalls. These storms are always The extreme temperature difference between associated with surface fronts and strong jet air masses — in addition to energy derived stream winds aloft. from condensation in the clouds — fuels the Although surface winds may not be quite as cyclone. So bombs are technically extratropical, strong as in the strongest tropical cyclones, the not tropical, because they get their energy from lows in the case of bomb cyclones cover a much the contrast of air masses, not just from warm bigger area. Tropical cyclones are relatively small, tropical waters. averaging a few hundred miles across, but the di- Interestingly, tropical cyclones occasionally ameter of the winter storms is often much larger, can turn extratropical in the late fall, espe- at times approaching 1,000 mi (1,600 km). cially at higher latitudes. Sometimes, tropical Typically, bomb cyclones begin to develop cyclones will “bomb out” and become major over water. Warm currents typically found off storms again. This commonly occurs in the

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Pacific Ocean in October and Novem- Minnesota. This was the lowest days or sometimes even a week in ber when typhoons in the western Pa- recorded pressure ever in the central advance. These models work best for cific become major winter-type storms United States and is comparable in the upper levels of the atmosphere, so in the North Pacific. pressure to a Category 3 hurricane. the upper-level troughs which pro- The magnitude of the storm is duce the surface cyclones are usually International Locations illustrated (p. 47) by the fact that five well handled. In particular, the model Areas most commonly affected by these U.S. states and considerable portions looks for injections of very cold air into storms in the Pacific Basin include of southern Ontario and Manitoba existing troughs. This will intensify the eastern Asia from Japan northward, in Canada experienced wind gusts trough and the jet stream winds. coastal Alaska and British Columbia, exceeding 60 kt. At Chicago’s O’Hare The roles of moisture and other and occasionally the Pacific Northwest International Airport, winds increased low-level features are also dealt with of the United States. On the Atlantic throughout the day on Oct. 26 and more efficiently now than a few de- side, the affected areas tend to be the continued blowing hard on the 27th. cades ago. In fact, it was a bomb off the U.S. East Coast from North Carolina Sustained winds of 20 kt with gusts Mid-Atlantic coast — the infamous northward, through the Canadian to 44 kt were recorded. Hundreds of Presidents’ Day Snowstorm of February Maritimes, coastal areas of Greenland flights were canceled at O’Hare alone. 1979, which dropped an unforecast 20 and Iceland, and sometimes as far east Pierre Regional Airport in South to 30 in (51 to 76 cm) of snow — that as Western Europe. Dakota endured winds gusting to more prompted much of the research that Bomb cyclones also develop over than 40 kt for a 32-hour period. identified this class of storms and re- the Mediterranean Sea in the colder These winter storms are even stron- fined the techniques to forecast them. months. And powerful extratropical cy- ger at higher latitudes. The Bering Sea Forecasts are not perfect, and occa- clones are not limited to the Northern and Gulf of Alaska have many power- sionally, a storm will be missed. Warn- Hemisphere. New Zealand and south- ful cyclones in the winter, but on Oct. ings for the December 2010 blizzard ern Australia, as well as southern South 25, 1977, a low-pressure area rewrote were sent out only 24 hours in advance. America, can be affected. A bomb that the record books. Dutch Harbor in the The various forecast models all pre- hit southern Brazil and Uruguay in Aleutians recorded a pressure of 926 dicted major cyclogenesis, the process August 2005 produced winds of 80 kt, millibars (27.35 in Hg), the lowest pres- of development or intensification of a with a peak gust of 100 kt. sure ever recorded in North America cyclone, but some predicted that the But warm water is not essential for for an extratropical system. Winds at storm would move up the East Coast bombogenesis, and strong cyclones also nearby Adak gusted more than 80 kt for while others took it harmlessly out to can develop over the interiors of conti- 12 hours with a peak gust of 110 kt. sea. In situations where forecast models nents. Inland cyclones, often deprived Cyclones in the North Atlantic don’t agree, meteorologists are left in a of a significant moisture source, have Ocean have gotten even stronger. quandary as to which model to believe. less precipitation. Therefore, the avia- The storm that severely damaged the A bomb-induced snowstorm in the tion problems associated with rain and, luxury liner Queen Elizabeth II in U.S. northeastern megalopolis during any especially, heavy snow are often greatly 1978 had a central pressure drop of Christmas–New Year holiday week — or reduced. However, the hazards pro- 60 millibars (1.77 in Hg) in 24 hours. similar major air travel period in a dense- duced by strong winds are still present. In January 1993, a storm in the North ly populated region in any country — can On Oct. 25, 2010, an already- Atlantic had a central pressure of 913 be expected to have major consequences. potent low pressure area moved into millibars (26.96 in Hg). If it had been a U.S. forecasters decided to wait in the the U.S. western Great Lakes. Fueled tropical system, it would have quali- 2010 example until the models came into by a 165-kt jet stream and a 40-degree fied as a Category 5 hurricane, the better agreement, which, in this case, left F (20-degree C) temperature differ- strongest storm ranking. little lead time for warnings. 

ence across the cold front, the low Bombs, by their nature, can develop Edward Brotak, Ph.D., retired in 2007 after 25 “bombed out.” By late on the following very quickly. Fortunately, today’s com- years as a professor and program director in day, a 955-millibar (28.21 in Hg) low puter forecast models are pretty good the Department of Atmospheric Sciences at the was located over extreme northeastern at spotting major cyclone development University of North Carolina, Asheville.

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