Hurricane Irene Summary

October 2012: This post is for hurricane Irene from 2011. For the latest information about the upcoming Hurricane Sandy, please refer to the 10/26/12 Hurricane Sandy discussion.

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Tropical cyclones affecting the NYC tri-state is a relatively common occurence. Tropical storms occasionally move up the East Coast, either in the process of transitioning into extratropical cyclones or in the form of remnants, bringing rain into the NYC tri-state area. The last tropical cyclone to directly affect the area was Tropical storm Hanna in 2008, bringing moderate rain across the area as it made landfall in central Long Island.

Major tropical cyclones are more uncommon, however, as the last tropical cyclone to bring a significant impact in the tri-state area was Hurricane Floyd in 1999. Irene’s intensity was not an unusual event, as it made landfall as a strong tropical storm as Floyd did, but what made Irene unusual was its track as it made landfall right over New York City, a very uncommon event with tropical cyclones affecting the area. Irene produced very heavy rainfall in the area as well as strong wind gusts, which resulted in widespread power outages and major flooding in the area, making Irene the most damaging tropical cyclone to affect the area since Floyd.

Irene History:

Irene formed from a tropical wave in the eastern Caribbean on Saturday, August 20. At that time, Irene was expected to move over the major Caribbean islands and staying as a tropical storm. Despite the original forecast, Irene moved WNW and steadily intensified, making landfall on Sunday in eastern Puerto Rico and intensifying into a hurricane shortly afterwards. Irene intensified into a category 2 hurricane on Monday is it stayed just north of Hispaniola.

Irene slightly weakened on Tuesday to a category 1 hurricane, but quickly intensified into a category 3 hurricane as it moved through the Bahamas on Wednesday and Thursday. During that time, Irene’s pressure dropped but the wind speed stayed the same, and as a result, it began to grow into a very large hurricane. As Irene turned to the north early on Friday, it unexpectedly weakened into a category 2 hurricane as dry air and higher shear began to affect the system. Due to its large size, it was very slow to weaken despite these unfavorable conditions, and made landfall on Saturday afternoon as a strong category 1 hurricane in eastern North Carolina. Irene slightly weakened as it moved up the Mid Atlantic coast, making landfall in NYC on Sunday morning as a 65 mph tropical storm.

Irene’s Impact In East Coast/NYC

*Irene at 5 PM, when starting to affect the area*
Irene’s outer bands began to reach the Carolinas by Friday afternoon, when Irene was east of Florida and moving north. These bands became stronger and more widespread as Irene moved up the coast, making landfall in eastern North Carolina on Saturday afternoon. Due to Irene’s very large size, its outer rain bands already reached parts of the NYC area by Saturday morning, when it was still south of North Carolina. Irene’s steady rain slowly moved up through New Jersey during the afternoon hours of Saturday, and steady rain developed in the area by 4-5 PM. In Long Island and southern Connecticut, the rain varied in intensity until the overnight hours; most places experienced cloudy skies while scattered yet heavy thunderstorms developed offshore and moved into those areas. From NYC and further west, however, a steady moderate to heavy rain developed, and intensified through the late evening and early overnight hours.

*Irene at 12 AM, when the peak of the storm began in the area*

The worst of Irene took place from 12 AM Sunday until Irene’s center made landfall in NYC. A steady wind-driven heavy rain fell across most of the area, especially from NYC and further west, and a band of tornado warned storms previously over central NJ moved into the area. After the storm, two EF-0 tornadoes were confirmed in the tri-state area. Winds steadily increased, with gusts peaking in the 40-65 mph range across most of the area. The highest gusts ended up in NYC and Long Island, where gusts were in the 60-70 mph range in some places, and a 91 mph wind gust was even observed in Sayville, NY.

Rainfall from Irene ended between 7-11 AM, when Irene made landfall in NYC. Cloudy skies continued throughout the day, but winds picked up once again for a few hours in the afternoon hours as Irene’s center moved north of the area, with gusts returning into the 40-60 mph range in parts of the area before calming down in the evening.

Irene’s Rain, Wind And Tornado Impact

Rain Amounts: Major flooding resluted from Irene in parts of the area, as very heavy rain fell in a short period of time. Using the National Weather Service rain reports in parts of the region, I made an estimated rainfall map for the Mid Atlantic region which was posted to the left. In general, 6 to 11 inches of rain fell in northern NJ and SE NY and 4 to 7 inches fell in NYC. Long Island and S CT saw more variable amounts; as little as 1-2 inches fell in eastern Long Island/SE CT, with as much as 5-7 inches in western Long Island/SW CT.

Wind: Irene also produced strong wind gusts in the area. Most places saw wind gusts in the 40-60 mph range, with parts of NYC and Long Island seeing gusts in the 60-70 mph range. A peak gust of 91 mph was observed in Sayville, NY (Long Island).

Tornadoes: Tropical cyclones often tend to produce tornadoes, and Irene was no exception. Several tornadoes were confirmed from the Carolinas up the Mid Atlantic coast, and two tornadoes were confirmed in the NYC area as well. An EF-0 tornado was confirmed in Queens, and another EF-0 tornado was confirmed near West Islip, NY.

Record Wet August: August was already a very wet month prior to Hurricane Irene, with monthly rain totals as high as 11-12 inches in JFK and Central Park. Hurricane Irene brought heavy rain across most of the area, nearly double the August rainfall prior to Irene in some places. As a result, rain records were broken across the area, with parts of the area ending up in the 15 to 20 inch range for rain totals this month, making this August not only the wettest August on record, but the wettest month in record as well in New York City. Below are a few selected rain totals this month in the area.

18.95″ – Central Park
17.24″ – JFK
16.11″ – Teterboro, NJ
14.30″ – Sussex, NJ

Forecasting Irene’s Track And Impact

*GFS model run from Wednesday, 8/24, showing Irene east of NYC*

Irene was a difficult storm to forecast, and uncertainty with Irene’s impact in the area continued up until the worst of the storm began. When Irene first formed, the majority of the models took Irene over the Caribbean islands, such as Hispaniola and Cuba, and into the Gulf of Mexico. By Irene’s 2nd day as a tropical cyclone, however, some of the model guidance, such as the GFS model, shifted east towards Irene moving up the East Coast. Not all models caught on as quickly, and the UKMET and the GFDL models took another few days to adjust from a Gulf of Mexico track to an East Coast track.

Once it became apparent that Irene would move up the East Coast, the new question was how far east/west Irene would end up. The models split at that point; the ECMWF model took a western track, taking the storm right over New Jersey and Pennsylvania consistently for a few days, the GFS model took the middle solution with a track ranging from Long Island to Cape Cod, and the GGEM model was the eastern solution, mostly showing the storm staying east of NYC. At one point, the model consensus kept Irene east of Long Island; an image of a GFS run on 8/24 from the NCEP Model Analyses and Guidance link, which I posted above, shows Irene staying east of NYC and affecting central/eastern Long Island with the heaviest rain.

*NAM model run from Friday, 8/26, showing Irene over NYC*

On 8/25, the model consensus dramatically shifted to the west; in fact, by the evening model runs, the model consensus took Irene west of New York City, led by the ECMWF model which was consistent with such a solution for a while. There were still models that took Irene over Long Island, such as the GFS and NAM, and by the morning of the 26th, a day before Irene began affecting the area, the models finally reached a consensus showing Irene moving over western Long Island/NYC.

In addition to the track, there was also some uncertainty on Irene’s intensity and timing. Originally, Irene was expected to intensify into a category 4 hurricane, but an eyewall replacement cycle prevented Irene from doing so. Once it completed this cycle, its pressure quickly dropped, but its wind speed did not increase, and instead, the pressure drop resulted in Irene growing to a much larger size. Some models showed Irene intensifying once moving north of North Carolina and reaching NYC, including the NAM model posted above; this solution was unlikely, however, considering that Irene was being affected by dry air, higher wind shear and land interaction while moving into an area of colder sea surface temperatures. The timing was also an issue with forecasting Irene, as the models trended faster with Irene’s impact in the area as Irene came closer. At first, Irene’s rain was expected to continue in NYC until Sunday evening, when in reality, the rain already ended before noon.

Forecast Verification for Irene:


Rain Verification: I posted a comparison image above; to the left is my rain forecast for Irene from my August 26 Irene discussion, and to the right is the estimated rain total map I made using the storm reports from the National Weather Service, filled in to show the Mid Atlantic. To the west of Irene’s center, the heavier rain ended up slightly west of my forecast, but otherwise, the rain totals were close to the forecast. To the east of Irene’s center, including Long Island and eastern New England, however, my forecast was too high, as totals ended up below the forecast. I expected Long Island to see 4-8 inches of rain, when in reality, totals ended up as low as 1-3 inches in the east to as much as 4-7 inches further west. In New England, totals were slightly lower than expected in some areas, as 3-8 inches of rain fell across most of New England (CT, RI, MA and NH) with locally higher/lower amounts, when my forecast map showed these areas receiving at least 4-9 inches of rain. The rain forecast for Vermont did verify, as 4 to 8 inches of rain fell in most of the state, which is the same as my forecast map showed.

Track Verification: In each of my updates from August 22 to August 26, I provided a forecast map for Irene with each updates. While Irene did end up in the cone of uncertainty with each update, the forecast track did not fall exactly in line with Irene’s actual track. The August 22 forecast had Irene too far west, making landfall close to South Carolina. The August 23 forecast went too far east, showing Irene staying just east of Long Island. By the August 24 update, I went with a further west solution, taking Irene over central Long Island, and went close to Irene’s actual landfall, showing a western Long Island/NYC landfall for the August 25/26 updates. From each evening update, my August 25 update had the closest verification rate, while from the intermediate updates posted each morning in the Tropics page, the August 25 morning forecast track had the best overall verification, with the forecast track almost identical to Irene’s actual track.

Summary: Even though Irene was not as strong as originially thought when it made landfall in NYC, Irene caused widespread damage along the East Coast from the Carolinas to New England. Wind damage uprooted trees and left millions without power. Excessive rains from Irene combined with what was already a record wet August in some places caused widespread flooding in the NYC area and in New England as well. While Irene was not the most damaging tropical cyclone to ever affect the East Coast, it will be remembered for its widespread flooding and wind damage, and is another addition to what has been a year of extremes in the NYC area.

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Irene Poll: A few days before Irene reached the area, I opened a poll regarding Irene’s impact in NYC for the viewers to vote. Below are the results of this poll:

What will NYC see from Hurricane Irene? (78 votes)

4 votes – Nothing, Irene will stay to the east
1 vote – Light rain, up to 1/2 inch
19 votes – Moderate rain, 1-5″, some wind
54 votes – Very heavy rain, 5-12″, strong winds (Correct answer)

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