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Irene's 1-in-100 year rains trigger deadly flooding

By: Dr. Jeff Masters, 4:00 PM GMT on August 29, 2011

Hurricane Irene is gone, but the huge hurricane's torrential rains have unleashed one of the Northeast's greatest flood disasters. Videos of rampaging rivers in Vermont, New York State, New Jersey, and surrounding states attest to the extreme nature of the great deluge Hurricane Irene brought. Numerous rivers and creeks throughout the Northeast crested above their highest flood stages on record over the past 24 hours. The previous records were mostly set during some of the great hurricanes of 50 - 60 years ago--Hazel of 1954, Connie and Diane of 1955, and Donna of 1960. Vermont, where 3 - 7 inches of rain fell in just twelve hours, was particularly hard-hit. Otter Creek in Rutland, Vermont crested at 17.21 feet, 3.81' above its previous record, and more than 9 feet above flood stage. In northern New Jersey and Southeast New York, where soils were already saturated from the region's wettest August on record even before Irene arrived, record flooding was the norm. According to imagery from metstat.com, Irene's rains were a 1-in-100 year event for portions of eight states.

Figure 1. Water levels on Vermont's Otter Creek in Rutland, Vermont crested at 17.21 feet, 3.81' above its previous record, and more than 9 feet above flood stage. Image credit: USGS.

Here are a few of the rivers in the Northeast that set all-time flood height records over the past 24 hours, which I found using our wundermap with the USGS rivers layers turned on:

Mettawee River, Middle Granville, NY
Hoosic River, North Bennington, VT
Saxton River, Saxtons RIver, VT
Schoharie Creek, Gilboa, NY
Esopus Creek, Coldbrook, NY
Passaic River, Millington, NJ
Rockaway River, Boonton, NJ
Pompton River, Pompton Plains, NJ
Millstone RIver, Blackwells Mills, NJ
Assunpink Creek, Trenton, NJ

And here are the unofficial maximum 24-hour rainfall amounts each state received from Irene, as compiled by our weather historian, Christopher C. Burt:

North Carolina: 14.00" Bunyan
Virginia: 12.52" Ft. Eustis
Maryland: 12.96" Plum Point
Delaware: 8.50" Federalsburg
Pennsylvania: 8.00" Goldsboro
New Jersey: 10.20" Wayne
New York: 11.48" Tuxedo Park
Connecticut: 8.70" Burlington
Massachusetts: 9.10" Savoy
Vermont: 7.60" Walden
New Hampshire: 6.09" 5SE Sandwich
Rhode Island: 5.37" Warren
Maine: 6.11" Phillips

Newark, NJ broke its all-time 24-hour precip record with a total of 8.92" (8/27-28)--old record 7.84" on 8/27-28/1971. Also, New York City, Philadelphia, and Newark now have August 2011 as their rainiest month in recorded history. Overall damages from Irene could range from $5 billion to $10 billion, according to Kinetic Analysis, a risk assessment firm that specializes in natural disaster impact. This would put Irene between 13th and 24th place on the list of the most damaging hurricanes ever to hit the U.S. Irene is very likely to have its name retired. The multi-billion dollar price tag from Irene puts the year 2011 in first place for the greatest number of billion-dollar disasters in one year, ten. The previous record was nine such disasters, set in 2008.

Irene cuts though barrier island, isolates North Carolina's Outer Banks
Irene's large surge surge and pounding waves breached North Carolina's Pea Island in two places Sunday, cutting U.S. Highway 12 and isolating the Outer Banks from the rest of the world. According to Dr. Rob Young of Western Carolina University, Irene did little damage to the ocean-front homes along North Carolina's Outer Banks, but there was significant soundside flooding impacting many areas of Duck, Kitty Hawk, Collington Village, and Roanoke Island. The $30-plus million dollar beach nourishment project at Nags Head survived the storm, although there has been some loss of sand along the beach.

Figure 2. Hurricane Irene's storm surge and winds carved two new channels through Pea Island on North Carolina's Outer Banks. This cut, near the town of Rodanthe, is the smaller of the two cuts. Highway 12 connecting the Outer Banks to the mainland will need to be re-built across these two cuts. Image credit: Western Carolina University.

Tropical Depression 12
Tropical Depression Twelve formed this morning from a strong tropical wave that had moved off the coast of Africa. TD 12 is undergoing some moderate wind shear of 10 - 20 knots due to strong easterly winds, which is keeping most of the storm's heavy thunderstorm activity limited to the west aide. This shear should relax as TD 12 pulls away from Africa, and allow the storm to become Tropical Storm Katia. Ironically, today is the 6-year anniversary Hurricane Katrina's landfall on the Gulf Coast, and Katia is the name that was selected to replace Katrina after that name was retired. Forecast tracks from the long-range GFS and ECMWF models suggest that Bermuda and Canada might be the only land area threatened by TD 12, but it is too early to be confident of this.

Figure 3. Tropical Depression Twelve at 8:10 am EDT August 29, 2011, as seen by the MODIS instrument on NASA's Terra satellite. Image credit: NASA.

A break in Texas' drought coming
One other area we need to watch later this week is the Gulf of Mexico. A significant shift in the atmospheric circulation is predicted for the region, with the ridge of high pressure that has brought Texas its record heat and drought predicted to shift eastwards and allow a flow of moist, tropical air into the state. A low pressure region is forecast to develop in the Gulf near the coast of Texas on Wednesday or Thursday, and this low will need to be watched for tropical development. The shift in the large scale weather pattern does not signal a permanent end to the Texas drought, but it should bring welcome rains and cooler temperatures to the Lone Star state beginning on Thursday. This will be a relief to the residents of Austin, where the temperature topped out at 112°F yesterday--the hottest day in Austin's recorded history, tied with September 5, 2000. By Labor Day, hot and dry weather will settle back in over the state, but the new ridge of high pressure will be weaker, and temperatures will not be as hot as this week's.

Jeff Masters
Tuttletown Gristmill
Tuttletown Gristmill
Tuttltown Gristmill looking South down the Plattekill.
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Bad Break

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.