Tracking hurricane Sandy: As storm 'zigs,' it's also changing dramatically
Hurricane Sandy has lurched westward as it heads toward landfall late Monday. It's also swapping energy sources as it becomes extratropical. The process sometimes gives storms a boost of power.
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As of 11:00 a.m., the center of Sandy was located some 213 miles southeast of Cape May, N.J.
Sandy's pending shift to an extratropical cyclone comes as the storm trades heat sources.
Tropical storms and hurricanes draw their energy from warm seawater in the tropics. The water evaporates and rises. As the water vapor rises, cools, and condenses to form the storm clouds, the heat needed to evaporate the water in the first place returns to the atmosphere. The heat reinforces the cloud-building process, especially at the storm's core. Thus, warm cores are the hallmark of tropical cyclones.
Outside the tropics, extratropical cyclones are large-scale low-pressure systems that bring the typical storms that travel across mid and high latitudes. These storms draw their energy from the contrast in temperature between warm air moving north from the tropics and cold air coming down from the north.
Hurricane Sandy is slated to merge with an extratropical cyclone as it makes landfall, with its core gradually shifting from warm to cold, as it changes heat sources from ocean water to the atmosphere itself.
This shift can have a profound change in the location of rain and winds in the storm, forecasters say. With tropical cyclones, surface winds reach their highest speeds closest to the core, and to the right of its center as seen from above the storm. Rainfall in a tropical cyclone appears in pinwheel-like bands spreading out from the core.
As the storm goes extratropical, maximum winds can appear far from the storm's center. And the most intense rain tends to migrate to a large patch to the left of the storm's track, while the highest winds and waves tend to appear to the right of the track.
Moreover, a tropical cyclone working its way through the merger with another storm can strengthen for a time, feeding off of heat brought by the extratropical cyclone it's joining.
Once Sandy makes landfall, forecasters expect the storm to weaken rapidly. The center of the system is projected to work its way north through Pennsylvania and New York through Thursday morning, then head northeast through Quebec and out into the Gulf of St. Lawrence over the weekend.