Published: August 26,2017
In just 57 hours, Harvey grew from a regenerated tropical depression over the Gulf of Mexico into a Category 4 hurricane, an incredible example of how rapid intensification can occur in hurricanes when atmospheric and oceanic conditions are ideal for development.
Harvey's rapid intensification was fueled by two main large-scale ingredients: favorable winds aloft and high oceanic heat content. There may have been other factors at play, but for now, let's examine the big picture.
(MORE: Full Coverage on Harvey's Impact)
Key Ingredient 1: Low Wind Shear
Wind shear stunts tropical storm and hurricane development. It was absent in the region where Harvey rapidly intensified.In the western Gulf of Mexico, where Harvey exploded, wind shear was very low. Wind shear is a change in wind speed and/or direction with height, which can stunt tropical storm or hurricane development by blowing convection away from the low-pressure center.
The National Hurricane Center said it expected Harvey to be a hurricane at landfall after it regenerated on Wednesday, citing a "relatively low-shear environment" as one factor. However, the intensity forecast was well short of projecting major hurricane strength (Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale), which further illustrates the challenge of forecasting intensity fluctuations in some tropical storms and hurricanes.
Outflow in the upper atmosphere provides exhaust for a hurricane. (Image: wundeground.com)As Harvey made its charge toward hurricane strength Thursday, the atmospheric environment featured a growing area of high-pressure aloft thanks to the low wind shear environment. The upper-level high provided outflow to the hurricane by acting as an exhaust vent, allowing warm air at lower levels to easily rise, which aided in the rapid intensification.
"Water vapor images indicate that the cyclone's outflow is expanding," the NHC noted in its Thursday morning forecast discussion.
Given the ripening atmospheric environment, the NHC's Thursday morning forecast called for Harvey to reach Category 3 strength – about 27 hours before the hurricane hit that threshold. Harvey then climbed up to Category 4 strength several hours later.
Key Ingredient 2: High Oceanic Heat
Oceanic heat content on Aug. 26, 2017. Yellow, orange and red shaded area had higher oceanic heat content. (NOAA)As you've probably heard before, warm waters fuel hurricane growth. However, hurricanes also prefer the warm water extends to several hundred feet deep, and this can be quantified by examining the oceanic heat content.
The region of the western Gulf of Mexico where Harvey passed through Friday featured a pocket of higher oceanic heat content.
The NHC highlighted this ingredient in its forecast Thursday morning: "Harvey will be moving over a warm eddy of high oceanic heat content in the western Gulf of Mexico in about 24 hours. As a result of these conditions, several intensity models, including the ICON intensity consensus, are now explicit showing Harvey reaching major hurricane intensity."
A quick lowering of atmospheric pressure is an indication of rapid strengthening in hurricanes, which is what occurred Friday as Harvey moved over this warm patch of ocean water.
"Harvey passed over a warm ocean eddy with high heat content for over six hours Friday morning, and the extra energy the eddy provided allowed Harvey’s central pressure to fall a spectacular 15 (millibars) in just two hours, from 967 mb at 4 a.m. CDT to 952 mb at 6 a.m. CDT," said Jeff Masters in a Weather Underground Category 6 blog Friday.
The pressure continued to drop significantly in the hours before landfall, bottoming out at 938 mb. That ranks among the 20 lowest atmospheric pressures for a landfalling hurricane in the U.S., according to Stu Ostro of The Weather Channel.
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