Tuesday, August 21, 2012

Hurricane Isaac (2012)

Storm Active: August 21-September 1

A tropical wave moved off of the African coast on August 16, quickly developing widespread shower activity. A circulation slowly became defined in association with the system as it moved west over the open Atlantic over the following few days.

The low jogged to the north on August 18, and became exposed to some dry air the next day, limiting the convection near the center. However, the circulation continued to deepen, and early on August 21 the system was organized enough to be classified Tropical Depression Nine.

Nine's center was exposed by shear several times during the day, but deep convection persisted sufficiently for it to be upgraded to Tropical Storm Isaac that evening The general satellite appearance of the cyclone improved on August 22, but dry air completely prevented consolidation of the center and the concentration of deep convection. Meanwhile, Isaac crossed over the Lesser Antilles, bringing heavy rain and tropical storm force gusts.

The system consisted of two hemispheres of convection for much of the evening of August 22 and August 23, one northeast of the circulation, and the other, also the more intense, to the southwest. These areas of heavy rain were quite broad, but the winds near the center remained very light. Early on August 23, Isaac center reformed to the south of its former position.

During the day, Puerto Rico experienced rain and scattered downpours as Isaac passed well to the south. The convection had by this time amalgamated into a single mass, but the center was still slightly displaced from the deep convection. The system's motion shifted toward the west-northwest that night, and modest strengthening began as conditions in Hispaniola steadily deteriorated, with tropical storm force winds sweeping through the island beginning on August 24. That day, Isaac generally increased in organization, as the numerous vortices associated with the low-level center finally combined, and the circulation became more defined.

The cyclone made landfall in Haiti very early on August 25, packing winds just below hurricane force. Though land disrupted the development of deep convection in association with Isaac, its circulation remained largely intact and the storm weakened only slightly that morning as it emerged back over water. However, the center experienced a westward shift a few hours later, bringing it near the coast of eastern Cuba. Despite Isaac's northwestward motion away from Hispaniola, much of the moisture associated with the system remained entangled in the mountains of the island, causing drenching rainfall to continue over most of that day.

The cyclone moved roughly parallel to the northern coast of Cuba overnight at a faster clip than before, and so had difficulty recovering convection near its center. Additionally, Isaac's circulation remained somewhat entangled with a larger low-pressure system over the northwest Caribbean, which had an additional weak center south of Isaac, near Jamaica. Not only did this system cause dry air and moderate shear to invade the southern portion of Isaac's circulation, but the two lows, under the Fujiwhara effect, began to orbit slightly about a common center, causing the tropical storm's motion to shift toward the west early on August 26.

The atmospheric conditions steadily improved during the day, and Isaac experienced some strengthening as it approached the Florida Keys. A distinct eyewall finally formed later that day, and the windfield of Isaac broadened, causing tropical storm conditions to sweep over much of southern Florida and northern Cuba during that day. Overnight, the circulation jogged to the north, and appeared to be closer to the deepest convection than previously, but the maximum winds remained nearly constant at strong tropical storm intensity. Though the winds did not change, the central pressure dropped rapidly overnight and into August 27, falling below 985 mb that afternoon as Isaac moved northwest into the central Gulf of Mexico.

No longer under the influence of the low pressure area to its north or the ridge to its northwest, Isaac slowed considerably later that day, though maintaining a general northwestward motion. The outer circulation of the cyclone was impressive, with prominent outflow and powerful rain bands, one of which swept across the northern Gulf that morning. However, the internal structure of Isaac never fully coped with the dry air invading the circulation. A broad eye occasionally formed, but the eyewall was never closed completely. Despite this, the winds increased just enough during the late morning hours of August 28 for Isaac to be upgraded to a hurricane.

Isaac strengthened slightly even as it encountered land; the system reached its peak intensity of 80 mph winds and a pressure of 968 mb that night. At around 7:45 pm, Isaac made landfall in southeastern Louisiana. However, the cyclone barely moved overnight, and in fact briefly emerged over water early on August 29 after wobbling westward. By the morning, the hurricane had resumed a slow northwest motion, bringing it over Louisiana again, slightly farther westward than before.

Isaac weakened very slowly over the following day, losing hurricane status that afternoon. The cyclone moved slowly northwest, its lack of forward speed causing flooding rains throughout Louisiana, Mississippi, and Alabama, with localized amounts over 20 inches. The system maintained tropical storm strength through the morning of August 30, at which point it was moving through northern Louisiana.

It weakened to a tropical depression that afternoon, and became asymmetric, as is typical of systems tracking inland. Most of the thunderstorm activity remained on the north side of the center, though more scattered rain bands to the south and east still persisted. The center moved over Arkansas later that day, and crossed in Missouri on August 31. It took a turn to the east later that day, and dissipated as it combined with a front on September 1.

The remnants of Isaac and the tropical moisture it carried northward caused shower activity throughout the Ohio River Valley and the northeast during the following few days.

Isaac's legacy throughout the Caribbean and U.S. was heavy rainfall. Particularly near its final landfall in Louisiana, the cyclone moved very slowly, and therefore had the opportunity to dump flooding rains over a huge swath of the southern U.S. However, it also provided needed rain to states farther north.

Isaac as a minimal hurricane. Even at peak strength, it had a poorly defined inner core.

Track of Isaac.

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