Preliminary Report
Hurricane Bob
16 - 29 September 1991

1991 PRELIMINARY REPORTS

Tropical Storm Ana (TS)
Hurricane Bob (3)
Hurricane Claudette (3)
Tropical Storm Danny (TS)
Tropical Storm Erika (TS)
Tropical Storm Fabian (TS)
Hurricane Grace (2)

Hurricane Bob was the first hurricane to strike the northeast United States since Hurricane Gloria in 1985.

 

a. Synoptic History

Satellite imagery and synoptic analyses indicated that Bob originated from the remnants of a frontal trough. Showers and thunderstorms increased along this frontal trough just south and southeast of Bermuda on 12 August. The area of disturbed weather moved toward the southwest and west during the next few days. Surface analyses at 1200 and 1800 UTC on 15 August indicated that a weak (1015 mb) low had formed within the area of disturbed weather, a couple of hundred nautical miles to the east of Bahamas. However, animation of high resolution visible satellite imagery during that period revealed only a broad, poorly-defined cyclonic circulation of low clouds near a band of convection with little curvature.

The area of disturbed weather was more organized on satellite imagery the next day. In addition, a closed circulation at a flight level of 1500 feet, and a maximum wind of 37 miles at 0143 UTC were reported by the U.S. Air Force Reserve unit aboard the first reconnaissance flight into the system. Based on these data, it is estimated that the area of disturbed weather became the third tropical depression of the Atlantic hurricane season at 0000 UTC on 16 August, while centered 175 nautical miles east of Nassau in the Bahamas. Early-light visible satellite pictures on the 16th showed further development, including a more-curved convective band wrapping around the west and south quadrants of a well-defined low-level cloud-system center that was moving west-northwest at about 6 miles.

Both the reconnaissance data near 1800 UTC on 16 August, which included a 59 mile wind at the 1500-feet flight level and a surface pressure of 1005 mb, and satellite intensity estimates from the NESDIS Synoptic Analysis Branch (SAB) and the NHC support upgrading the depression to Tropical Storm Bob at that time. Bob was then centered 120 nautical miles northeast of Nassau. The storm continued strengthening and began moving more toward the northwest under the influence of the deep layer mean flow.

On 17 August, satellite imagery showed increased convective banding around a central dense overcast. An Air Force reconnaissance plane at 1719 UTC encountered 82 mile winds at the 1500-feet flight level, and a surface pressure of 987 mb was reported. The onboard Air reconnaissance Weather Officer estimated surface winds at 75 miles. Based on these reports, the best track shows that Bob strengthened to a hurricane at 1800 UTC on the 17th, while centered 205 nautical miles east of Daytona Beach, Florida. The hurricane began turning toward the north and then northeast at an increasing forward speed. The steering flow was the result of the combined effects of the subtropical high pressure ridge over the Atlantic and a mid to upper-level trough over the southeastern united States.

Deep convection became more concentrated near the center of the hurricane and a well-defined eye appeared in satellite pictures late on August 18. Bob continued intensifying and the eye became even more distinct as it passed 25 to 30 nautical miles east of Cape Hatteras early on the 19th. An Air Force reconnaissance plane at 0412 UTC on 19 August encountered 137 mile winds at the 700 mb flight level. At 0621 UTC a surface pressure of 950 mb was measured. These reports are the basis for showing the maximum surface wind of 115 miles on the best track at 0600 UTC on the 19th while Bob was located about 90 nautical miles east-southeast of Norfolk, Virginia. Bob was a Category 3 hurricane on the Saffir/Simpson Hurricane Scale at this time and was moving toward the north-northeast near 23 miles.

Bob continued to move toward the north-northeast, steered by the flow between a mid to upper-level cut-off low near the Great Lakes, and a strong Atlantic subtropical high pressure ridge. In this steering flow, Bob moved parrellel to the United States mid-Atlantic coast and headed toward New England on a track similar to the tracks of Hurricanes Carol and Edna 1954.

Bob weakened while accelerating the north-northeast over waters which became significantly cooler off of the mid-Atlantic coast. The eye was partially filled with clouds when it passed just east of Long Island, New York. The west side of a weakened eyewall passed over Montauk Point on the eastern tip of the island. The eye passed over Block Island at 1720 UTC and moved over Newport, Rhode Island near 1800 UTC. Aircraft reconnaissance personnel were unable to report an eye just prior to this landfall, because less than 50% of the center was surrounded by an eyewall. By the time of landfall on Rhode Island, Bob was moving toward the north-northeast at 32 miles with maximum sustained winds of a Category 2 hurricane.

Bob crossed Rhode Island and Massachusetts, with the center moving between Boston and Scituate. It then moved over Massachusetts Bay. The hurricane continued to weaken and began losing tropical characteristics as it passed just offhore of the southern coast of Maine and made landfall as a tropical storm near Rockland, Maineat at 0130 UTC on 20 August. Bob turned more toward the northeast and crossed Maine and New Brunswick. Bob exited New Brunswick near Chatham at 1200 UTC on the 20th and became extratropical over the Gulf of St. Lawrence by 1800 UTC. It then crossed northern Newfoundland, and the central North Atlantic along 50-55°N, before moving southeastward and dissipating near the coast of Portugal on 29 August.

b. Meteorological Statistics

Air Force Reserve Unit aircraft provided most of the reconnaissance on this hurricane from near the time of initial development into a tropical depression east of the Bahamas, until just before final landfall on the coast of Maine. A NOAA plane made a flight into Bob as the hurricane passed the Outer Banks of North Carolina.

Table 2c lists selected hourly observations made by NOAA's National Data Buoy Center (NDBC) platforms near the path of Bob. The NOAA buoys report an 8-minute average sustained wind, and the Coastal-Marine Automated Network (C-MAN) reports give 2-minute average sustained winds. The Diamond Shoals Light C-MAN (35.2°N 75.3°W) reported a minimum pressure of 961 and maximum sustained winds of 98 miles with gusts to 112 miles at 0200 UTC on 19 August. The Buzzards Bay C-MAN (41.4°N 71.0°W) reported a minimum pressure of 971 at 1800 UTC on 19 August and maximum sustained winds of 77 miles with gusts to 89 miles at 1700 UTC. Since these were reported hourly observations, they are likely not the extreme values which occured at the sites. In fact, the continuous wind data from the Diamond Shoals C-MAN received well after the event showed a peak gust of 123 miles.

The wind measurements from Block Island of 105 miles and 100 miles were reported as peak gusts. However, both of these values were at the upper limit of the equipment range and were reportedly maintained (and probably exceeded) for a short period (< 1 minute). An observer on Block Island reported the eye overhead at 1720 UTC on 19 August, with the approximate duration of light winds from 30 to 40 minutes.

As the eye moved over Newport, the Navy Ship USS VALDEZ, anchored in Narragansett Bay, reported a pressure of 964 mb at 1815 UTC on 19 August. A cooperative observer in Adamsville, Rhode Island, just east of the track reported a pressure of 964 mb at 1820 UTC. These values are close to the 965 mb pressure reported from an Air Force reconaissance plane at 1737 UTC just prior to the Newport landfall.

As Bob emerged over the cooler waters of Massachusetts Bay, the maximum sustained surfcae winds continued to decrease. NOAA buoys 44013 (42.4°N 70.8°W) and 44007 (43.5°N 70.1°W), along with the Mantinicus Rock (43.8°N 68.9°W) and Mt. Desert Rock (44.0°N 68.1°W) C-MAN stations, all reported maximum sustained winds below hurricane force, justifing the downgrading of Bob to tropical storm before final landfall near Rockland, Maine.

1. Storm Surge Data

At the time of publication of this report a post-storm high water mark survey was being conducted by the New England Division of the U.S. Army Corps of Engineers. Initial survey results suggest that the highest storm surge values occured in Massachusetts and Rhode Island, where wind driven water was funneled into Buzzards and Narrangansett Bays. Visual and measured high water marks, which contain both storm surge and astronomical tide heights and in some cases the effects of breaking waves, were also highest there. (Note: storm surge values derived from National Ocean Survey (NOS) tide gages listed in Table 2a have the effects of tide hight and wave effects removed). Specifically high water marks at the upper end of Buzzards Bay ranged from 8.5 to 15.3 feet. Further south on the east shore at Woods Hole a storm surge of 5.8 feet was calculated from NOS tide gage data. Similarly, on the west shore of Buzzards Bay, a storm surge of 5.8 feet was calculated from tide gage data at the New Bedford hurricane barrier. In Rhode Island, high water marks between 10.2 and 16.5 feet were measured near Sakonnet Point. These values include wave effects because of its exposure on the coast. A storm surge height of 6.5 feet occurred ath the Providence hurricane barrier tide gage. This gage, which is located at the west end of the Sound, had a storm surge of 6.7 feet. The surge occurred almost 2 hours after landfall when the hurricane was abeam of Boston.

2. Rainfall Data

Rainfall totals ranged up to nearly 8 inches along the path of Bob. The largest totals include 5.3 inches at Cape Hatteras, 7.04 inches at Bridgehampton (on Long Island), 7 inches at the Groton Emergency Operations Center in Connecticut, 5.43 inches at Brimfield, Massachusetts, and 7.83 inches at Portland, Maine. Several rainfall amounts between 2 and 5 inches occurred elsewhere over portions of New York and New England.

3. Tornadoes

Six confirmed tornadoes were reported in associated with Bob, 4 in North Carolina and 2 in New York on Long Island. Thirteen additional unconfirmed were reported, including 9 in wooded areas on Hatteras Island, 2 in Rhode Island, and 2 in Massachusetts.

c. Casualty and Damage Statistics

There were 17 deaths attributed to Bob; 6 in Connecticut, 3 in Maine, 2 in New York, 2 in New Hampshire, 2 in Nova Scotia, 1 South Carolina and 1 in North Carolina.

The American Insurance Association preliminary estimate of insured property damage for the United States is $782 Million. This includes $4 Million for North Carolina, $75 Million for New York, $40 Million for Connecticut, $115 Million for Rhode Island, $525 Million for Massachusetts, $2 Million for New Hampshire, and $21 Million for Maine. The addition of flood claims, uninsured property damage, and the cost of cleanup increases the total damage estimate from Bob to $1.5 Billion. Without adjustments for inflation, Bob would rank 5th or 6th on the list of 20th century United States hurricanes. With adjustments for inflation Bob will rank 13th or 14th on that list. These damage figures make Bob the most recent of a string of hurricanes that that were very costly to the northeast United States. Other prominent, destructive hurricanes there include Gloria in 1985, Agnes in 1972, Donna in 1960, Diane in 1955, Carol in 1954, an unnamed hurricane in 1944, and the New England Hurricane of 1938.

Power was knocked out to an estimated 2.1 million homes and businesses primarily on the Outer Banks of North Carolina, on Long Island, and over portions of New England.