Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 6:29 AM GMT on February 12, 2013
Tropical Cyclone Report
21 August-1 September 2012
Isaac made landfall in southeastern Haiti as a strong tropical storm. It then traversed eastern Cuba and the Gulf of Mexico before making landfall in southeast Louisiana as a category 1 hurricane (on the Saffir-Simpson Hurricane Wind Scale).
a. Synoptic History
Isaac developed from a vigorous tropical wave that emerged from west Africa on 16 August. The wave was accompanied by a rather broad cyclonic circulation that extended horizontally for several hundred miles as indicated by satellite, scatterometer, and microwave data. A broad area of low of pressure developed in association with the tropical wave on 17 August as it moved slowly westward. The wave showed signs of organization beginning on 18 August but strong easterly shear initially prevented development. The circulation began to become better defined the next day as the wave continued to move westward but convection remained minimal. The system slowly organized as the vertical shear decreased, and based on the exhibition of the convective cloud pattern, the low is estimated to have become a tropical depression near 0600 UTC 21 August while centered approximately 600 miles east of the Lesser Antilles. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added in the final rendition). Northwesterly shear produced by an upper low over the central Atlantic prevented the depression from strengthening immediately after formation, with visible satellite images during the day showing the center to be exposed along the northern edge of some rather disorganized convection. Nevertheless, the cyclone became a tropical storm near 1800 UTC 22 August.
The shear soon changed to westerly, which continued to halt Isaac's intensification. The cyclone circulation was broad and disorganized, with reconnaissance and scatterometer data indicating that winds near the center were relatively light, with all of the tropical storm force winds confined to the southwestern quadrant of the circulation in a band of deep convection over the Lesser Antilles prior to the storm entering the Caribbean Sea. Isaac remained quite disorgranized as it traversed the eastern and central Caribbean Sea, with reconnaissance and satellite observations suggesting that there were at least two competing areas of vorticity within the larger circulation. It appears that a combination of the unusually large size of the cyclone circulation combined with strong low-level shear caused the storm to struggle. Nevertheless, surface and reconnaissance observations indicate that Isaac maintained a well-defined, albeit broad, surface circulation during this time. Isaac began to become a little better organized late on 24 August as it passed to the south of Haiti, with satellite and microwave data suggesting a developing inner core. Based on these data and wind observations from a reconnaissance aircraft, Isaac is estimated to have strengthened to a 60-kt tropical storm around 0000 UTC 25 August. Possibly due to land interaction, the cyclone weakened slightly before it made landfall along the southeast peninsula of Haiti near Jacmel near 0700 UTC 25 August; maximum 1-minute sustained winds are estimated to be about 55 kt when Isaac made landfall in Haiti.
Isaac spent only a short time over Hispaniola, emerging into the Gulf of Gonave around 0900 UTC as a 45-kt tropical storm. Despite the relatively short duration overland, Isaac's inner core was severely disrupted by the mountains of Hispaniola, with microwave data subsequent to the cyclone moving offshore showing a complete lack of an eye, which had been previously observed by aircraft, microwave, and satellite data just several hours earlier. Approximately six hours later, Isaac made another landfall along the eastern coast of Cuba near Baracoa, still at 45 kt. Isaac moved back into the Atlantic waters just a couple hours later and strengthened slightly, back to 50 kt, although the southern portion of the cyclone remained largely overland as the storm tracked west-northwestward toward the Gulf of Mexico; along with moderate southwesterly shear and dry air over the western Caribbean Sea on the subsident side of a decaying trough over the western Atlantic and Isaac's cyclonic circulation, this combination apparently inhibited significant strengthening. Despite some brief attempts at inner core organization per aircraft and radar data from Casablanca and Camaguey, Cuba, Isaac generally changed little in strength as it passed through the Florida Straits over the next 24 hours. Isaac emerged into the extreme southeastern Gulf of Mexico near 0000 UTC 27 August after passing through the Florida Keys. Satellite, radar, and aircraft data indicate that Isaac began to turn northwestward later that day, likely in response to an upstream trough that was developing over the southern United States.
The central pressure steadily fell during the following couple of days leading up to landfall, but Isaac's winds only very gradually increased during this time. It appears that the extremely large size of the circulation, which spanned over 500 miles, inhibited strengthening even as Isaac entered a more favorable upper-level environment in the Gulf of Mexico; in particular, the outer wind maxima was continually expanding as Isaac approached the coast and while the central pressure was falling, a situation that has not been shown to be particularly conducive for hurricane intensification in the past (i.e. Hurricane Ike in 2008, and, more recently, Hurricane Irene in 2011). In addition, analysis of the oceanic heat content in that region (not shown) suggest that Isaac missed the culmination of the Loop Current, which would have likely mitigated the large size of the storm and allowed it to intensify at a somewhat steadier rate. Also, water vapor images during the period suggested that the aforementioned dry airmass in the western Caribbean continued to linger, and eventually gave way to more dry air near the coast. Based on reconnaissance data, Isaac is assumed to have become a hurricane near 1200 UTC 28 August while centered roughly 75 miles southeast of the Mouth of the Mississippi River. As Isaac continued slowly toward the coast of southeastern Louisiana, it assumed its peak intensity of 70 kt near 1800 UTC 28 August; it is estimated that the lowest minimum pressure of 965 mb was attained several hours later, just prior to Isaac's second and final Gulf Coast landfall.
Isaac made its first United States landfall along the extreme lower end of Plaquemines Parish near the Southwest Pass of the Mississippi River near 2345 UTC 28 August with winds of 70 kt. Isaac spent only a brief period of time overland, and because that particular strip of land is rather swampy and narrow, Isaac did not weaken; in fact, the satellite and radar signature actually became better defined subsequent to this landfall; Hurricane Danny in July of 1997 had a similar evolution when passing through this area. After about 0230 UTC 29 August, the convective pattern abruptly eroded, likely due to entrainment of dry air from Texas as well as coastal upwelling caused by the slow movement of the hurricane. Satellite and radar data indicate that Isaac made a second and final landfall along the Louisiana Gulf Coast just west of Port Fourchon at approximately 0800 UTC 29 August, with winds still estimated to be about 70 kt. As Isaac continued inland, it weakened to a tropical storm near 1800 UTC that day while centered about 20 miles northwest of Houma. Isaac dropped to tropical depression status around 0000 UTC 31 August while moving into southern Arkansas. Six hours later, satellite images suggested that Isaac was quickly becoming a remnant low as it accelerated northward toward southern Missouri. The remnant low completely lost its identity early on 1 September. Operationally, it was thought that Isaac's mid-level remnants were the primary focus for an area of disturbed weather that formed across the central Gulf of Mexico several days later; posthumously, however, it was decided that Isaac's primary remnant continued northeast in the prevailing mid-latitude westerlies, with perhaps only a small fragment of the residual energy eventually reemerging into the Gulf of Mexico. This low would ultimately become inconsequential, and did not develop into a tropical cyclone.
Observations in Hurricane Isaac (Figs 2 and 3) include the Dvorak-based satellite intensity technique. A total of 34 aircraft missions were flown into Isaac, providing crucial data in assessing the structure and intensity of the tropical cyclone. These missions were particularly helpful when Isaac was traversing the eastern Caribbean Sea, since subsequent flight data indicated that the center had reformed farther to the south as the storm approached Haiti. In addition, several coastal radars, both in the Caribbean and the United States, were used to track Isaac. These include the Martinique, San Juan, Gran Piedra, Camaguey, and Casablanca radars in the Caribbean. Coastal radars in the United States include the Key West, Mobile, and New Orleans radars. Various microwave data from the Navy Research Laboratory (NRL) website were also useful in depicting the structure and location of Isaac.
Isaac's peak intensity of 70 kt is estimated to have occurred around 1800 UTC 29 August, and is based on a slew of data, including the New Orleans radar, reconnaissance data, and satellite imagery. It should be noted that the aircraft reported flight-level winds late on 28 August generally supported surface winds a little higher than 70 kt, but these were conflicted by nearby surface observations. It is likely that because the convection was focused in primary bands rather than a definite eyewall, attributable to the large size of the hurricane, there was not enough momentum to bring the strongest winds down to the surface from flight-level. Aircraft reports indicate that the central pressure continued to fall after Isaac moved back into the Gulf of Mexico subsequent to its first landfall, but the data also indicate that the maximum winds were not increasing during this process. So while Isaac reached its lowest minimum pressure of 965 mb a few hours later, the maximum winds are not assumed to have increased from the assigned peak intensity of 70 kt at 1800 UTC 28 August.
Isaac is analyzed to have strengthened into a tropical storm near 1800 UTC 22 August based on SFMR data from an aircraft mission, which generally measured believable surface winds of at least 35 kt. In addition, ASCAT data during that time showed similar evidence. It should be noted that at the time of upgrade as denoted in post-season analysis, Isaac's cloud pattern was rather disorganized, as northwesterly shear was keeping the heaviest precipitation (and thus the strongest winds) downstream -- or south -- of the analyzed low-level center. This was due to an upper tropospheric vortex over the central Atlantic.
While Isaac was forecast to strengthen to a hurricane throughout virtually every single forecast, especially across the Caribbean, this did not occur until the cyclone had moved into the northern Gulf of Mexico and neared the Louisiana coast. Isaac is assumed to have -- finally -- reached hurricane status at approximately 1200 UTC 28 August, when the first indication of hurricane-force winds were found from a reconnaissance aircraft. It should be noted that both the central pressure and reported flight-level winds were consistent with hurricane status about 12-18 hours prior to the time Isaac is estimated to have reached hurricane status in the northern Gulf of Mexico. It appears that the lack of an inner core and the large outer core, which was expanding as the hurricane approached the coast, prevented significant strengthening. While unusual, similar evolutions have occurred with landfalling hurricanes in the past in regards to the winds lagging behind the central pressure (i..e. hurricanes Isidore and Ike in 2002 and 2008, respectively). The mechanisms by which this occurs is still unclear, but is likely related to a broad and uneven distribution of the synoptic pressure fields. It should be noted that this is a more common occurrence in the Western North Pacific, where the winds found in a typhoon are frequently lower than the central pressure would suggest; given the vast expanse of seemingly inexhaustible oceanic heat content in that basin, this can probably be attributed to a broader pressure gradient, as alluded to above.
Wind and Pressure
Isaac produced tropical storm force winds over portions of the Lesser Antilles when it moved through that area as a tropical storm. The highest sustained winds reported in the archipelago was 41 kt gusting to 48 kt on Desirade; this occurred around 0700 UTC 23 August. Tropical storm force winds, especially in gusts, spread elsewhere across the Lesser Antilles.
Reliable surface observations along the coast of Haiti are difficult to come by given the dismal economy of that nation which generally inhibits the operation of observation sites or local weather offices with relevant equipment. A wind gust of 27 kt was reported at Port-au-Prince on 24 August at 2000 UTC local time; this was about six hours before Isaac made landfall along the southeast coast of that country.
A sustained wind of 39 kt with a gust to 53 kt occurred at the Guantanamo Naval Air Station (NAS) when Isaac made landfall in eastern Cuba later on 25 August. Northwesterly winds at this site suggest that Isaac made landfall just to the east; winds later veered around to the southwest as the center of Isaac moved off toward the west. The aforementioned sustained wind/gust combination occurred at 2225 UTC in a convective band as Isaac moved toward the Florida Straits.
A sustained wind of 33 kt with a gust to 46 kt was reported in Key West on as Isaac moved through on 26 August. Surface observations at the Key West airport suggest that wind gusts at or near tropical storm strength lashed the city until around 1900 UTC 27 August, by which time Isaac was trudging closer to Louisiana.
Hurricane-force winds were reported along and inland from the southeast coast of Louisiana, with the highest sustained wind report being 65 kt at the Southwest Pass of the Mississippi River. A sustained wind of 58 kt was reported in New Orleans, while a sustained wind of 51 kt occurred at Baton Rouge. It should be noted that Isaac had an unusually large capability to bring down strong wind gusts from the lower and middle troposphere to the surface in strong convective downdrafts, with numerous sites and stations offshore, along the coast, and inland across southeast Louisiana reporting wind gusts to hurricane force, even long after the hurricane had moved inland.
A pressure of 1002 mb was recorded at Port-au-Prince and Guantanamo Bay; in both instances, however, the center of Isaac was west or east of the respective locations, giving credence to lower central pressures in areas closer to the center in both cases.
Select surface observations for land and buoys are given in Table 3 (to be added in the final rendition).
According to island reports, storm surges were minimal across the Lesser Antilles and Puerto Rico, the highest being a report of 1.23 water rise in Puerto Rico. Storm surge estimates are not available for Haiti or Cuba.
Along the west coast of Florida, storm surges were generally between 1 to 3 feet. Inundations of about 3 feet occurred along the panhandle, with Pensacola reporting a maximum storm surge of 3.47 feet; this was the highest reported in the state of Florida.
Primarily due to the large size of the hurricane, Isaac produced devastating storm surges across much of the northern Gulf Coast; a NOS tide gauge at Shell Beach, Louisiana located along the southern end of Lake Borgne reported a maximum storm surge of 11.03 feet. Additionally, a storm surge of 6.35 ft was observed at the New Orleans New Canal Station along the southern shore of Lake Pontchartrain. The storm surge produced considerable flooding across sections of Plaquemines Parish, with estimates as high as 17 feet in some areas. While the highest water levels associated with Isaac stayed away from populated areas, it is presumed that storm surges of up to about 10 feet inundated certain residential areas within the parish. There was also extensive flooding in unprotected areas of Orleans, St. Bernand, and St. Tammany parishes. In St. Bernard Parish alone, Isaac produced water level rises of 8 to 12 feet outside the federal levee system based on tide gauge surveys from several USGS tide sensors in Wood Lake, Delacroix, and Alluvial City. Across eastern sections of Orleans Parish near Rigolets and Lake St. Catherine, storm surge estimates are in the 4 to 8 feet range based on further tide surveys and high water marks. Finally, Isaac is assumed to have produced a total storm surge of 4 to 7 ft across sections of the northeastern shore of Lake Pontchartain. Water from the lake was able to permeate inland, flooding portions of Slidell and other nearby communities. Storm surge from the western portion of Lake Pontchartrain produced 1 to 3 feet of surge in Laplace as indicated by USGS tide gauges in the area.
Isaac also produced widespread river flooding, and the USGS reported that the hurricane actually forced the Mississippi River to flow backward for almost 24 hours. The river rose as much as 8 feet in Baton Rouge.
Mississippi and Alabama did not escape water level rises from Isaac. Mississippi saw the highest water rises between the two states, with USGS tide gauges indicating water level rises between 5 and 8 ft as occurring in Hancock and Harrison Counties; additionally, it appears based on areal high water marks that the surge moved as far inland as north of Interstate 10. Surge values exceeded 6 feet farther east in Jackson County. In Alabama, a maximum storm surge estimated at 4.5 ft was observed at Dauphin Island, which lies offshore from the mainland. Areal sensors on the mainland suggest that storm surges of 1 to 3 feet were experienced along Mobile Bay. A maximum storm surge value of 8.00 ft was recorded by a NOS tide gauge along the coast of Mississippi at the Bay Waveland Yacht Club. A storm surge of 4.63 feet occurred in Mobile, Alabama, at the Coast Guard Sector Mobile Facility.
Select storm surge observations are given in Table 4 (to be added in the final rendition).
While it was moving through the Caribbean Sea as a tropical storm, Isaac produced heavy rainfall across Puerto Rico and Hispaniola, with a maximum storm total of 9.28 inches reported near Ponce. 16.18 inches of rain was reported at Barahona in the Dominican Republic. As of this writing, there have been no reliable rainfall totals received from Haiti. Isaac produced torrential rains across eastern Cuba as well, with 14.08 inches reported at San Antonio del Sur, Guantanamo. Finally, a total of 10.80 inches was reported in El Plátano in the Granma province.
In addition to the aforementioned, Isaac produced considerable rainfall along much of Florida and the northern Gulf Coast due to its slow movement around a blocking high pressure system over the central United States. In Florida, Isaac produced severe flooding in Palm Beach County, with certain neighborhoods in Wellington, Royal Palm Beach, Loxahatchee, and Acreage being isolated due to several feet of water. The highest reported storm rainfall total in the state of Florida from Isaac was 15.86 inches at the Lion County Safari in Loxahatchee. Significant rainfall totals occurred elsewhere across the state, and totals of 10 inches or more were not uncommon across southeast Florida in a narrow band east of Lake Okeechobee. This band included the counties of Martin, St. Lucie, Indian River, and the extreme eastern end of Okeechobee County.
Isaac produced considerable rainfall and flooding over the northern Gulf Coast due to its prolonged slow movement. The highest total reported in Alabama from Isaac was 13.99 inches at Grand Bay located in Mobile County, 22.20 in Pasacagoula, and finally, in Louisiana, the highest was 20.66 in New Orleans. The torrential rainfall caused local river rises, record in some cases. A river crest of 8.5 feet was observed near Gulfport, Mississippi, while a river crest of 4.4 ft was noted near Landon. In addition, Isaac's heavy rains contributed to a 6.5 ft river crest at the East Hobolochitto Creek near Caesar, Louisiana. Major flooding also occurred along the Tangipahoa River in Louisiana near Robert, plateauing at 9.0 ft above flood stage, and 14.8 ft at the Bogue Chitto River near Tylertown, Mississippi.
As is typical for landfalling hurricanes, there were some tornadoes associated with Isaac. A total of 26 tornadoes appear to have been spawned by Isaac, 17 of which occurred while Isaac was a tropical cyclone. While most of these were relatively weak and short-lived, there was a report of two EF2s. Five tornadoes touched down in Florida, all of which were rated EF0. Four tornadoes occurred in Alabama, the strongest being an EF1 that struck near the city of Samson. Six occurred in Mississippi, with an EF2 touching down in Pascagoula; this tornado almost completely removed the roof from a residence, and it also downed large trees. An EF1 tornado that was generated near Skyes, Mississippi injured three people, and had a path length of 6.1 miles. Isaac produced two additional tornadoes across Arkansas, Missouri, and Illinois on 1 September, after the system had dissipated as a tropical cyclone; one of the tornadoes injured a person near the village of Franklin, Illinois. An EF2 tornado produced considerable damage to two hangars and some aircraft at the airport in Corning, Arkansas.
c. Casualty and Damage Statistics
Isaac is blamed directly for 35 deaths across the Caribbean and the United States. It is difficult to distinguish which deaths were direct or indirect in Haiti due to conflicting reports from various agencies within that country. A 51-year old woman was killed in the coastal town of Marigot along the southern coast when a tree fell on her home, while a 10-year old girl was killed in the village of Thomazeau when a wall collapsed on her. Isaac produced five direct deaths in the Dominican Republic, two of which can be attributed to two adult males being swept away in two different river flood events.
Six direct deaths were caused by the hurricane in the United States, two of which occurred in Mississippi. A 62-year old woman in Lexington, and a 52-year old man in Picayune were killed by high winds that toppled trees onto their cars. A 75-year old man was killed in Slidell, Louisiana when he drove his car off an on-ramp on Interstate 10 and into a ditch that was flooded with up to 9 ft of water. In Vermilion Parish, a 36-year old man was killed when he fell 18 feet from a tree while attempting to help his friends move a vehicle before the storm. There were reports of a man dying in a restaurant fire, and a couple drowned in their home in Braithwaite.
Some minor damage occurred in the Caribbean, primarily associated with heavy rains. The rains caused flooding and landslides that caused damage to several homes in Trinidad and Tobago. Heavy rains caused damage in Puerto Rico, where mudslides caused by the heavy rains caused mudslides that caused some road closures and washed out a bridge in the southern part of the island.
In the Dominican Republic, nearly 13,000 people evacuated their homes, while an estimated 864 homes were damaged. A total of 90 towns were isolated by the hurricane due to flooding produced by the hurricane. Roughly 10% of the banana and coconut crops were damaged on the island, and total damage is estimated to be about $30 million (USD).
The United Nations Office for the Coordination of Humanitarian Affairs (OCHA) reported that Isaac killed 24 people across Haiti, and caused 42 injuries and 3 missing persons. Isaac produced storm surge across the southern half of the peninsula from Jacmel to Port-au-Prince. The hurricane destroyed about 6,000 shelters in shelter camps set up after the January 2010 earthquake that struck the region. In addition, Isaac destroyed about 1,000 homes in the country.
In Cuba, nearly 50,000 people evacuated. Isaac destroyed 4 homes and damaged 19 in Baracoa, and 72 homes were flooded by the Sagua de Tánamo River in Holguín province. Strong winds blew down power lines and caused electrical disruptions, although the extent of this damage is not known. Isaac also damaged crops in the region, including the cocoa and coconut crops, spanning over 1,100 acres.
Isaac caused about $970 million in insured damage across the United States according to insurance estimates, with total uninsured losses presumed to be about $2.35 billion. Minor to moderate flooding and some power outages were experienced in Florida, but the majority of damage came from Isaac's impact to the northern Gulf Coast. Isaac produced considerable storm surge flooding across the northern Gulf Coast, particularly in Louisiana. For the first time, the Hurricane and Storm Surge Risk Reduction (HSDRRS), which was implemented subsequent to the widespread economic damage wrought by hurricanes Katrina and Rita in 2005 was tested for the first time by a serious hurricane. The project proved to be successful in protecting most areas within the system, and storm surge flooding appears to have occurred primarily in areas outside the protection of the federal levee system. Around 59,000 homes were damaged by the hurricane. Although the majority of houses that were damaged were located in Jefferson Parish, the most extensive damage occurred in St. John the Baptist and Plaquemines Parish. Severe flooding from storm surge was observed in Plaquemines Parish, where a non-federal levee situated on the east bank was overtopped by storm surge; the resulting flooding inundated areas within the levee system from Braithwaite southward to White Ditch. The force of the water was strong enough in Braithwaite to pick up cars, homes, and various debris and plant it atop Highway 93 and onto the Mississippi River levee. Over 100 people had to be rescued from flooded homes and rooftops within the community. Later on, city officials intentionally breached the non-federal levee to allow water to escape from the town. The city of Laplace in St. John the Baptist Parish experienced storm surge from nearby Lake Pontchartrain, which caused nearly 4,000 people to be rescued, and also a closure of Interstate 10. 1,000 people had to be rescued in Lafourche Parish due to storm surge that overtopped the Bayou Boeuf levee. Additional flooding occurred elsewhere within adjacent parishes, both from rainfall and primarily storm surge.
A total of 901,000 homes and businesses lost power in Louisiana, which is approximately 47% of all customers in the state. Isaac caused considerable crop damage, with about 90% of the sugarcane crop destroyed by the hurricane. In addition, there were reports from state officials that about 565,000 pounds of oily debris from the Deepwater Horizon disaster site were brought to the surface and deposited on the coast.
Damage also occurred in sections of Mississippi and Alabama. Around 6,000 homes in south Mississippi appear to have sustained some degree of damage according to the state governor, which resulted in about 70 road closures. Storm surge from Isaac deposited 4 feet of sand on the western end of Dauphin Island, but overall damage in the state was minor. Isaac unearthed the shipwreck of The Rachael, a vessel from the early 20th century that ran aground in Gulf Shores, Alabama, purportedly due to rough seas in 1930.
Isaac also caused damage across portions of Arkansas. Numerous streets, homes, and other structures were flooded in Pine Bluff. Heavy rains from the once powerful Isaac caused damage to crops in the state, including the soybean, cotton, corn, and rice. Isaac's winds also caused some power outages in the state, and around 20,000 customers lost power during the storm. Total damage in the state is estimated to be about $30.5 million (USD).
d. Forecast and Warning Critique
The formation of Isaac was anticipated very well. The tropical wave that would eventually give birth to Isaac was first mentioned in a Tropical Weather Outlook that was issued early on 18 August, about three days prior to formation. At the time, the system was assigned a low chance (less than 30%) of developing into a tropical cyclone within 48 hr. Probabilities were increased to "high" (exceeding 50%) early on 19 August, and remained in this category until genesis. In fact, genesis probabilities reached 100% at 0600 UTC 21 August, the time of genesis as denoted in post-season analysis. This type of strong wording is extremely rare in developing systems.
Isaac proved extremely difficult to track. While track forecasts up to the 96 hr mark were generally good, the 120 hr forecasts proved most challenging. Isaac was consistently forecast to strike well to the east of the Mississippi River Delta; this was due to, in a similar situation to Tropical Storm Debby three months earlier, there was a notable contrast between the two most reliable global computer model systems: the GFS and the ECMWF regarding Isaac's ultimate trajectory. The former continually anticipated that Isaac would make landfall in Florida somewhere between the Big Bend and Pensacola, while the ECMWF was more insistent on a landfall along the northern and central Gulf Coast, generally in Mississippi or Alabama. As it turns out, the GFS gradually shifted westward with time, joining the ECMWF's initial prognostications of a Louisiana landfall. As this happened, confidence increased within official forecasts regarding the threat to the northern Gulf Coast. Isaac was forecast to make landfall along the Louisiana coast about 48 hours before landfall actually occurred; this gave satisfactory time for hurricane watches and warnings.
Intensity forecasts fared a little better, although there was still a notable high bias, particularly in the early forecasts, which generally showed Isaac becoming a hurricane over the Caribbean Sea. In addition, although it was correctly forecast that Isaac would become a hurricane over the Gulf of Mexico, the timing and onset of this process was difficult to predict. In particular, the rate of intensification over the Gulf of Mexico was overestimated; in fact, a forecast that was issued around 0600 UTC 26 August, two days before landfall, showed Isaac becoming a 100 kt major hurricane and making landfall along the extreme western Florida panhandle. This was due, in part, to model prognostications, especially from the GFDL and HWRF, of a large and powerful hurricane striking the United States Gulf Coast. It appears that these models and the official forecasts failed to fully appreciate the negative role that the enormous size of the Isaac vortex would play in regards to intensification. This is still a subject of considerable research in the meteorological community.
Watches and warnings associated with Isaac are given in Table 5 (to be added in the final rendition).
Figure 3. Visible satellite image of Hurricane Isaac at peak intensity at 1800 UTC 28 August. It should be noted that the maximum winds were attained several hours prior to the lowest central pressure.
Figure 4. Rainfall totals from Hurricane Isaac beginning 25 August and ending 3 September, 2012. Image courtesy of the Hydrometeorological Prediction Center (HPC).
The National Hurricane Center (NHC) provided a fair bit of the data detailing the damage and fatalities associated with Isaac, especially across the Caribbean. Data from the National Data Buoy Center (NDBC) was used to compile the marine observations provided in Table 3 (to be added in the final rendition). The rainfall data given in Figure 4 was provided by the Hydrometeorological Prediction Center (HPC).
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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