Thursday, August 17, 2017

Hurricane Harvey (2017)

Storm Active: August 17-19, 23-31

On August 13, a large tropical wave entered the Atlantic from the west coast of Africa. As with many of the previous August waves, thunderstorm activity diminished as soon as it was over water. There was some spin associated with the system over the next few days, but the low pressure area remained elongated. The circulation improved greatly on August 15 and 16, leaving limited shower activity as its main barrier to development. Meanwhile, the system was moving due westward at a steady clip toward moister air, and thunderstorm activity increased significantly by the morning of the 17th. That afternoon, aircraft reconnaissance discovered a closed circulation and tropical storm force winds, prompting the naming of Tropical Storm Harvey.

After formation, Harvey moved at around 20 mph toward the west, steered by a ridge to its north. As with many systems moving at such speeds near the Lesser Antilles, the storm had difficulty maintaining a center of circulation and was rather disorganized. Nevertheless, it brought some localized heavy rain and gusty winds as it passed over the Windward Islands during the morning of August 18. Moderate shear also took a toll on the system as it continued quickly westward; during the afternoon of August 19, it was downgraded to a tropical depression. The system's deterioration continued through the evening and aircraft reconnaissance was unable to detect evidence of a closed circulation that evening. Harvey had opened up into a wave and was no longer a tropical cyclone.

The remnants of Harvey continued toward the west, where conditions for development began to improve again. Early on August 20, they passed near the coasts of Nicaragua and Honduras, bringing some rain to the shoreline. Shower activity increased significantly that afternoon. Before a new circulation could fully develop, however, the system began to interact with the Yucatan Peninsula. While Ex-Harvey crossed over land on August 21 and 22, it produced convection mainly over the Gulf waters to the north. Nevertheless, some spin reappeared on satellite imagery. The system became much more vigorous by the morning of the 23rd, once it was back out over warm Gulf waters. Aircraft reconnaissance soon found a well-defined center of circulation and advisories were reinitiated on Tropical Depression Harvey before noon.

During the afternoon, Harvey slowed to a standstill, and a central dense overcast feature appeared, indicative of a developing system. The central pressure began to slowly decline, but the circulation was still rather disorganized. Overnight, the depression was upgraded to a tropical storm. The upper-level low that was causing some shear over Harvey was weakening, meanwhile, leaving the storm in very favorable atmospheric conditions. Early on August 24, the system began a run of rapid intensification, reaching hurricane strength by early in the afternoon! Though the wind speeds leveled out through the evening, the central pressure continued to drop, presaging a corresponding increase of winds. This occurred in the middle of the night, bringing Harvey to category 2 strength. Around this time, a vigorous outer rain band swept across the Texas coastline. During the morning of August 25, it became evident that the hurricane was experiencing an eyewall replacement cycle (EWRC), where the innermost ring of thunderstorms about a powerful hurricane's eye contracts or dissipates, and a second ring (the outer eyewall) takes over. This put a temporary cap on Harvey's winds, but did not stop the central pressure from dropping steadily.

Meanwhile, the system was still moving at a moderate pace toward the northwest and approaching the Texas coastline. Squalls and even tornadoes occurred over land as the more vigorous rain bands swept across the coast. During the afternoon, Harvey's EWRC was completed, and a large eye cleared out on satellite and radar imagery. This was accompanied by another rapid increase in winds, bringing the hurricane to its peak intensity as a category 4 storm; it had maximum winds of 130 mph and a central pressure of 938 mb when it made landfall in Texas. This was first major hurricane landfall in the United States since 2005, the first hurricane landfall in Texas since 2008, and the strongest in the state since 1961. The storm slowed as it moved north-northwestward inland and weakened. By mid-morning on August 26, Harvey was a minimal hurricane, and it weakened to a tropical storm that afternoon. However, as the storm slowed to a standstill, the greatest concern was rain: the counterclockwise spin of the circulation continually brought new moisture from over the Gulf northward over areas east of the center, dumping feet of rain over a wide swath of Texas. Harvey reversed course and began to meander generally southward by the morning of August 27. Later, the direction of drift turned southeastward. Since part of the circulation remained over water, the storm was able to maintain minimal tropical storm strength through the next day. Meanwhile, the main source of rain was a large band wrapping around the northeast side of the circulation down to the Gulf, pulling moisture over eastern Texas and Louisiana. Early on August 28, Harvey's center reemerged over the Gulf of Mexico.

Some intermittent convection rekindled near the center once it touched warm Gulf waters, but the circulation was entraining dry air from western Texas, and wind shear was high. As a result, the inner core could not redevelop, and Harvey resembled an extratropical cyclone more than a tropical one. It had a comma-shaped tail of thunderstorm activity wrapping from north of the center eastward, so that rain continued over eastern Texas and Louisiana. The cyclone traveled first southeast, and then east over the next day. Winds increased modestly during this period to 50 mph in strong rain bands over water. Finally, on the 29th, Harvey turned toward the north and increased in speed somewhat, making its final landfall in western Louisiana early on August 30. The cyclone weakened over land as it moved north, but dealt a final burst of heavy rain to eastern Texas before becoming a tropical depression that evening. Soon, the system became extratropical, but rainfall continued to push northeastward, bringing moderate amounts of rain to the mid-Atlantic and northeast by September 2. A peak rainfall accumulation of 51.88" occurred in Cedar Bayou, Texas, the highest rainfall total every recorded from any tropical cyclone in the continental United States.

The above image shows Harvey shortly before landfall in Texas.

Harvey's unusual slow movement near and over the state of Texas brought unprecedented rains to south and east parts of the state.

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