Most people are familiar with a hurricane's coastal impacts: storm surge, high winds, and heavy rain. Most hurricanes begin to rapidly weaken once they encounter land. The sustained winds steadily decline as the storm runs out of fuel—namely, warmth and moisture extracted from the upper ocean. Occasionally, however, the damage doesn't end along the immediate coast. The extratropical transition of hurricanes over the eastern United States can be quite dramatic. As post-tropical remnants encounter one or more mid-latitude systems (a frontal boundary, or a ripple in the upper-level jet stream, among other features), they can morph into dangerous hybrid cyclones—part tropical, part extratropical—and eventually into full-fledged extratropical cyclones. The result is continued hazardous weather, sometimes far inland, many hundreds of miles from the shoreline. Most frequently, these transitioning weather systems unleash catastrophic floods, especially along the Appalachians.
Table 1 shows a list of significant, post-tropical cyclones that caught their “second winds” over the past several decades. Hurricane Sandy (2012) is the most recent example along the East Coast. This massive superstorm, which achieved hybrid status while over the Atlantic, made landfall over northern New Jersey as a fully transitioned Nor'easter (a type of coastal extratropical cyclone). It was the largest documented storm of tropical origin in the northwestern Atlantic, spanning nearly 1,000 miles in diameter. The tropical system interacted strongly with not one, but two, troughs (southward-diving loops) in the jet stream along the Eastern seaboard. An unseasonably cold Canadian air mass wrapped frigid air deep into Sandy's inner core. Sandy became the second costliest cyclone of tropical origin to strike the United States, behind Katrina. For more on Sandy's amazing story, see “Hurricane Sandy: The Science and Impacts of a Superstorm,” featured in the 2013 March/April issue of Weatherwise.
Table 1. Selected Hurricanes Making Landfall in the United States, with Severe Inland Impacts During the Post-Tropical Phase
|Storm name* (Category and state of landfall)||Date||Region of impact||Inland hazard(s)|
|Hazel (4; Carolinas)||October 1954||mid-Atlantic–Canada||High winds, flooding|
|Camille (5; Louisiana/Mississippi)||August 1969||Central Virginia||Flash and river flooding, mudslides, and debris flows|
|Agnes (1; Florida)||June 1972||mid-Atlantic||River flooding|
|Eloise (3; Florida)||September 1975||mid-Atlantic||River flooding|
|Floyd (2; North Carolina)||September 1999||mid–Atlantic, New England||River flooding|
|Gaston (1; South Carolina)||September 2004||Central Virginia||Flash flooding|
|Ivan (3; Florida)||September 2004||Southeast, mid-Atlantic||Tornado outbreak|
|Irene (1; North Carolina)||August 2011||mid-Atlantic, New England||Flash & river flood, high wind|
|Lee (STS; Louisiana)||September 2011||mid-Atlantic, New England||River flooding|
|Sandy (XTC; New Jersey)||October 2012||mid-Atlantic, New England||High winds, river flooding, heavy snow|
|Arthur (2; Nova Scotia)||July 2014||New England, Atlantic Canada||High wind, flash flood|
* Number in parentheses indicates intensity of the tropical cyclone at landfall (1 = Cat 1, etc.; STS = subtropical storm; XTC = extratropical cyclone).
The inland weather impacts of transitioning tropical cyclones are quite varied, but the common denominator is destruction. They have produced hundreds of fatalities throughout history. Hazel (1954) raced rapidly northward along the spine of the Appalachians, after making landfall in South Carolina, unleashing hurricane-force winds all the way to Toronto, Canada. Camille (1969) surprised central Virginia with a biblical-scope flash flood along the Blue Ridge mountains, burying 160 souls beneath torrents of water, mud, and rock flushed out of steep mountain slopes. Agnes (1972) produced widespread river flooding; curiously, the storm made landfall over the Florida panhandle, but reserved its worst rainfall for central Pennsylvania days later and become the “flood of record” for that state. Eloise (1975) unleashed a remarkably similar flood just three years later. Floyd (1999) inundated eastern North Carolina with 15–20 inches of muddy, brown floodwater, and its rains spread as far north as Newfoundland during landfall in the Carolinas. Ivan (2004) spawned more tornadoes than any post-landfall tropical system in United States history, including a remarkable swarm of over three dozen tornadoes across Northern Virginia, days after landfall in Florida. Lee (2011) gave the mid-Atlantic quite a soaking (up to 21 inches in Colonial Beach, Virginia), even though the storm made landfall over Louisiana then tracked deep into the middle Mississippi Valley.
Before we examine some of these storms in more detail, let's take a look at the key differences between tropical and extratropical storm systems, and how they typically transform from one type to the other.
Extratropical Transition: A Plethora of Changes
While an inland-migrating tropical cyclone initially weakens, it may tap one or more additional energy sources, bounce back, and even rejuvenate (re-strengthen). During the hybrid or “mixed” phase of extratropical transition, cyclones derive energy both from tropical moisture and the potential energy inherent in air mass temperature contrasts—the province of fronts and jet streams. Figure 1 is a graphic portrayal of textbook, extratropical transition.
Figure 1. Post-tropical cyclones (bottom of the figure) can undergo several possible transformations, including decay in the tropics, extratropical transition upon moving into mid-latitudes (middle portion of the diagram), followed by either decay or a brief period of rejuvenation (top of figure).
As shown along the bottom of Figure 1, tropical cyclones (which include both tropical storms and hurricanes) are fairly symmetric storms that consist of numerous spiral rain bands and a central eye. These are warm-core systems; that is, their interior, especially in upper levels, is often 10–15°F warmer than the surrounding tropical atmosphere. The pressure gradient (change in surface pressure with radial distance) is very compact and intense. This tends to concentrate the strongest winds within the storm's inner core, at a radius of perhaps 20–30 miles. However, winds drop practically to zero in the storm's center.
Tropical cyclones typically have a poleward component to their motion. As they move out of the deep tropics, they encounter several changes in their environment. The effect of the earth's rotation (the Coriolis effect) increases with latitude. Thermal gradients are encountered along a frontal boundary, where warm and cold air masses clash. Above these boundaries, wind shear (the increase in wind speed with altitude) is strong. Dry air streams off large continents and becomes entrained into the storm's inner circulation. Sea surface temperature and ocean heat content decrease. The storm may encounter troughs in the westerly jet stream aloft.
Often, the combined effect of these processes is too much for the tropical system, and it rapidly weakens—particularly if the storm was low intensity to begin with. A remnant low, usually a low-level whirl with fairly benign clouds, is all that may remain. Other times, however, the storm's central vortex remains intact, but the character of the storm begins to change. The storm accelerates poleward as it encounters stiffer environmental flow, and wind shear causes the vortex to lean or even become partially open. Dry air evaporates portions of the deep convective clouds. The transitioning storm develops a pronounced asymmetry with respect to its cloud deck and precipitation shield, with much of the condensed and frozen water mass shifting ahead of, and to the left, of the storm track. The wind balance shifts as the Coriolis effect becomes more influential. Interaction with a trough in the jet stream ushers cooler air into the core of the storm. One or more fronts develop, such as the cold and warm front depicted in the top of Figure 1, along which the heaviest precipitation becomes concentrated. The region of wind weakens but also expands. The warm core becomes a cold core. The clouds take on a classic, mid-latitude comma-head shape. In short, the storm becomes an extratropical cyclone.
The environmental changes and storm responses are shown in Figure 1. In some systems, extratropical transition is quite rapid, taking place in as little as 12–18 hours; in others, the process unfolds over two or more days. During extratropical transition, the vortex becomes a hybrid storm, during which it expresses both tropical and extratropical attributes. Hybrid storms do not fit cleanly into either the tropical or extratropical category of vortex.
In 2010, the National Hurricane Center introduced new terminology that describes tropical cyclone remnants—a broad category called post-tropical cyclones. As shown in the bottom half of Figure 1, many tropical cyclones decay in the tropics, forming remnant lows. They never undergo the transition process. Those that migrate into higher latitudes usually undergo some degree of metamorphosis, as opposed to simply fading into oblivion, although the degree of structural change varies considerably across the spectrum of characteristics identified in Figure 1.
Figure 2 shows satellite views before and after the extratropical transition of Hurricane Noel (2007) over the western Atlantic. The intense tropical cyclone phase, seen in the left-side panel, features a central core of vigorous convective clouds that release large amounts of latent heat energy. Latent heat derives from evaporation of seawater. Evaporation extracts heat energy from the ocean. When the vapor later condenses in convective clouds, that heat is released into the atmosphere, warming the center of the vortex. This is the essence of the hurricane “heat engine.”
Figure 2. Satellite view of a tropical cyclone (left) undergoing rapid extratropical transition (right); note the changes in symmetry and overall shape of the storm.
The right-side panel in Figure 2 shows Noel during extratropical transition, which occurred very rapidly. As the storm moved northward, we can see the change in the clouds, as a large, asymmetric comma head developed ahead and to the left of track. Dry, cool air off the United States mainland wrapped around the southern edge of the storm. Key to extratropical transition is a shift in the fundamental energetics of the storm. As cold air is juxtaposed with warmer air, a strong thermal gradient serves as a new source of potential energy. Warm air that is lifted along the frontal boundary and cold air that subsides behind convert potential energy into kinetic energy. Kinetic energy is expressed as spinning wind around the vortex.
During the hybrid phase, a storm draws energy from a frontal boundary, while it continues to extract significant latent heat from the ocean's upper layer. Rarely, a transitioning storm may become super-energized by an unseasonably strong thermal gradient (during the late fall months), to the point where the storm rejuvenates (top of Figure 1), maintaining a steady intensity or even further strengthening. This was the case during the extratropical transition of Hurricane Hazel, discussed next.
Hurricane Hazel's Remarkable Inland Transformation
Hurricane Hazel made landfall along the Carolina coast as a Category 4 hurricane on the evening of October 15, 1954. As the storm moved toward the north, an unseasonably cold and vigorous trough in the jet stream crossed the Great Lakes. The trough created its own extratropical cyclone approaching the northern Appalachians, and a cold front trailed southward from the extratropical storm. Hazel first merged with the cold front across the mid-Atlantic states.
By midnight on October 16, Hazel's post-tropical vortex then combined with the extratropical cyclone over the Great Lakes. The hybrid system moved into Toronto, Canada, where it created considerable damage and loss of life. This large, hybrid system maintained hurricane-force winds hundreds of miles inland. Cold air in the jet stream trough played a significant role in maintaining the intensity of Hazel. In fact, an enormous reservoir of potential energy was generated as cold, Arctic air became juxtaposed with warm, tropical air. This process generated an intense thermal gradient. Sinking, cold air on the back side of the hybrid vortex combined with air ascending in the hybrid system's warm sector converted potential to kinetic energy. This novel energy release amounted to a petajoule—a quadrillion joules of energy! This is equivalent to the yield of several atomic bombs. Added energy counteracted spin-down of the tropical vortex as it left its oceanic heat source. The hybrid storm, in the process of transforming into a full extratropical system, rejuvenated over land.
Because the storm was embedded in fast jet stream winds, it zipped northward at speeds of 50–55 mph. Within 24 hours of landfall in the Carolinas, Hazel traveled all the way to southeastern Canada! Sustained winds near 80 mph lashed cities from Norfolk, Virginia, to Washington, D.C. Gusts in these locations topped 100 mph, with gusts to 90 mph in New York. As Figure 3 shows, high winds were generated to the right of the storm's track. The storm's rapid movement increased wind speeds on the right side of the storm track; this is where swirling winds from the south combined with the storm's motion, which was also from the south. Note the steady progression of the 60 mph isotach (line of constant wind speed) up the East Coast. The period of fierce winds was remarkably brief, just three hours in Washington, D.C., because of the storm's rocket-like transit.
Figure 3. The inland consequences of Hurricane Hazel's extratropical transition—including heavy rains to left of track, and sustained high winds to right of track—continuing all the way to Canada.
A swath of heavy rain fell to the left of track, along the Appalachians (Figure 3). Overall, four to six inches of rain fell, with isolated pockets exceeding six inches. Humid tropical air that was forced to rise up the east-facing slopes of the Appalachians intensified the rain intensity. Whereas the storm's rapid movement increased the wind speed, it limited the total rain accumulation. As discussed next, heavy rain often redistributes to the left of the track in transitioning systems, as exemplified by the metamorphosis of Hurricane Floyd in 1999.
Flood Catastrophe: Generation of Extreme Rains During Extratropical Transition
Some of the greatest flood disasters in the history of the mid-Atlantic and New England have unfolded when post-tropical remnants interact with the Appalachians and pre-existing fronts. Prolonged, heavy rains can lead to widespread river floods (such as with Agnes in 1972 and Floyd in 1999), and also flash floods, mudslides, and debris flows (such as Camille in 1969). The Camille flash flood was described in “Queen of Rains: Hurricane Camille,” featured in the 2005 Nov/Dec issue of Weatherwise.
Hurricane Floyd generated exceptionally heavy rains during its extratropical transition. Floyd struck the Carolinas on September 16, 1999, as a Category 2 hurricane. As shown in Figure 4, the storm tracked northeastward along the Outer Banks and Delmarva Peninsula. An elongated swath of heavy rain, 7–10 inches, expanded from North Carolina to Maine ahead of the storm, inundating 13 states. The rains fell along a coastal front, which developed as the storm swept warm, humid air onshore. Exceptional rains, accumulating 15–20 inches, became concentrated over portions of eastern Virginia and North Carolina. Widespread river flooding ensued across eastern North Carolina, where several mainstem rivers exceeded 500-year flood levels. These streams, including the Tar River, crested 20–24 feet above flood stage. Floodwaters were heavily contaminated with pesticides, fertilizer, and hog waste across this largely agricultural region. Damage to dwellings from the storm's effects was extensive—nearly 75,000 homes—forcing 10,000 people into temporary shelter, with total economic losses approaching $5 billion.
Figure 4. False color, infrared view of Hurricane Floyd, undergoing extratropical transition as the system combined with a strong jet stream trough over the Mid Atlantic. Post-tropical Floyd became a prolific rain producer, spreading devastating flooding all the way from North Carolina to Maine.
As in the case of Hazel, the heaviest rain fell along Floyd's left side. Nearly half of all landfalling tropical cyclones have a left-of-center (LOC) rain distribution. This is caused by the interaction of the tropical vortex and an approaching jet stream trough, located upwind of the storm. Timing is everything. When the trough captures a tropical cyclone, copious water vapor condenses in the trough's “active region” of vigorously ascending air. A steady supply of low-level tropical moisture enters the storm from the south and east. This type of hybrid vortex becomes a highly optimized, heavy-rain-generating machine.
Heavy rains are also generated when post-tropical remnants interact with mountainous terrain. In this situation, there is enhanced uplift of low-level, moisture-laden air along mountain slopes. Figure 5 shows an example of orographically enhanced rain in the days following the landfall of Hurricane Agnes (1972). Agnes struck the Florida panhandle as a Category 1 storm, which quickly weakened over land. The heavy rain was left-of-track, implying (correctly) that a mid-latitude trough enhanced the uplift of moist air. A weather front oriented parallel to the Appalachians, along the Virginia and Maryland Piedmont, further concentrated heavy rains along the track. As Agnes's post-tropical vortex moved off the North Carolina Outer Banks, a new vortex developed over the Appalachians and tracked northward. The second, inland vortex—a purely extratropical feature associated with the jet stream trough—nearly stalled over Pennsylvania. From Figure 5, note how the region of heaviest rains (purple and blue shades) developed where the slow-moving vortex, tropical moisture plume, and steep terrain coincided.
Figure 5. The swath of exceptionally heavy, inland rain produced by post-tropical Agnes, which underwent metamorphosis to an extratropical cyclone over the Mid Atlantic. The heaviest rain was forced by orographic (mountain) ascent of moist air.
A Twist of Fate: Tornadoes During Extratropical Transition
The final act of inland, transitioning tropical cyclones may be a swarm of tornadoes. There is irony that while the parent vortex weakens, small regions of spin can temporarily intensify in the form of rotating thunderstorms (supercells) and tornadoes. This is because ground friction slows the winds in the lowest few thousand feet. The faster flow aloft tumbles over itself, forming giant, horizontal wind rollers in the cloud layer. Updrafts rising through rain band thunderstorms draw up vortex tubes, re-orienting them vertically. Further convergence of air tightens and intensifies these small, low-level cyclones to tornadic intensity.
The great majority of hurricane-spawned tornadoes occur within a few hundred miles of the coastline. The tornadoes tend to be small, transient, and short-track, with peak winds in the 125–150 mph range (many are not even this intense). Formation is favored in the right-front quadrant of the advancing, parent vortex. These tornadoes are difficult to forecast, and by the time warnings are put out, many have already dissipated.
Rarely, moderately strong tornadoes may develop, with winds tipping the 150–200 mph range. This was the case during the landfall of Hurricane Ivan in 2004. Ivan struck coastal Mississippi, then tracked toward the northeast along the spine of the Appalachians. Once inland, extratropical transition reworked the tropical vortex into an extratropical weather system. Over central Virginia, the hybrid storm made an abrupt jog to the southeast, and exited the United States over the Delmarva Peninsula.
As shown in Figure 6, Ivan dropped a remarkable number of tornadoes—117 in all. This established the post-tropical storm as the most prolific tornado breeder in United States history. The tornadoes were spawned episodically, in pockets, beginning on September 15. These swarms commenced with a major outbreak of 40 tornadoes over northern and central Virginia on September 18. The color shadings imply a strongly diurnal timing to the formation of tornadoes; on each day, tornado production occurred during the afternoon hours—the time of peak solar heating. When heating was maximized, the air was most unstable and fueled strong convective updrafts in the hybrid storm's rain bands. The updrafts, in turn, tilted horizontal vortex tubes vertically, consolidating the spin into tornadoes.
Figure 6. Hurricane Ivan's bizarre inland track coincided with a period of extratropical transition; an unexpectedly large swarm of tornadoes (colored circles) erupted over Virginia.
Summing Up: Never Trust an Inland, Tropical Cyclone Remnant!
The bottom line is this: Inland, post-tropical cyclones have wildcards to play. Tropical cyclones that are transitioning to extratropical storms pose genuine forecasting challenges, and no single transition unfolds in the same manner as its predecessors. All types of high-impact weather can result, including sustained hurricane-force winds, flash floods and mudslides, widespread heavy rain and river flooding, and outbreaks of tornadoes. And with the landfall of Superstorm Sandy in 2012, we must add heavy mountain snowfall and 20- to 30-foot waves along the shores of the Great Lakes to the list of inland impacts. The death toll and property damage from the second, inland phase of a hurricane can exceed losses along the coastline. While they do not occur every year, the events of these “second storms” have been etched into the history books, becoming some of the most significant weather disasters in United States history.