What is preliminary report
Miles B. Lawrence
Bertha was an early-season Cape Verde Hurricane that moved across the islands of the northeastern Caribbean Sea as a category 1 hurricane on the Saffir/Simpson scale and made landfall on the North Carolina coast near Wilmington as a category 2 hurricane. Bertha's one-minute winds reached their maximum value of 100 knots on 9 July, while located to the north of Puerto Rico. The last Hurricane to reach this strength, this early in the season. was Alma in 1966 (117K GIF) in the eastern Gulf of Mexico with 110 knots. Bertha is responsible for an estimated eight deaths and $250 million in U.S. damages.
a. Synoptic History
Bertha originated from a tropical wave which moved from Africa to the Atlantic on 1 July. A weak circulation was first detected on satellite imagery on 3 July, centered about 500 n mi south of the Cape Verde Islands in the far eastern Atlantic Ocean. The track of the circulation center begins on 5 July, when the circulation is believed to have reached the surface and become a tropical depression. in the central tropical Atlantic. This track is displayed in Fig. 1 (102K GIF) and listed in Table 1 .
Bertha followed a fairly smooth curved path around the western periphery of the Atlantic subtropical high pressure ridge. This ridge changed little during Bertha's existence and a weak mid-level trough persisted in the western North Atlantic. For three days, the depression moved toward the west-northwest at the fast forward speed of 20 to 25 knots and strengthened to a hurricane with 1-min. maximum sustained winds of 75 knots on the 8th as the center moved across the Leeward and Virgin Islands of the northeastern Caribbean. The center moved between Antigua and Barbuda at 0600 UTC on the 8th, across St. Barthelemy, Anguilla, and St Martin, just north of St. Thomas, and over the British Virgin Islands by 1800 UTC.
The track gradually turned northwestward on the 9th and maximum sustained winds reached 100 knots at 0600 UTC. Bertha was centered 120 n mi north of Puerto Rico at this time, but earlier passed within 30 n mi of this island. The strongest winds were located in the northeast quadrant of the hurricane and most of Puerto Rico experienced only tropical storm conditions, except for Culebra, over which hurricane-force winds might have occurred.
Moving northwestward at a slower forward speed of 15 to 20 knots, the center of Bertha moved parallel to the Bahama islands, passing 40 to 60 n mi northeast of the Turks and Caicos islands, San Salvador, Eleuthera and the Abacos. Again, the strongest winds were located to the northeast of the center, but 65-knot sustained winds might have reached some of the above mentioned islands.
Continuing on its gradual turn, the track became north-northwestward on the 10th and 11th and the center moved parallel to the coast of Florida and Georgia at a distance of 150 to 175 n mi offshore. During this time, the forward speed slowed to about 8 knots. Moving northward and re-accelerating to a forward speed of 15 knots, Bertha made landfall at 2000 UTC on the 12th on the coast of North Carolina, with the center crossing the coast midway between Wrightsville and Topsail Beaches. The hurricane had been gradually weakening since its top speed of 100 knots on the 9th to 70 knots on the 11th. Then, in 12 hours just before landfall, the winds increased to 90 knots. which is the estimated maximum 1-min. wind speed at landfall. Bertha quickly dropped below hurricane strength when it moved inland over eastern North Carolina.
It then moved northeastward along the U.S. east coast, producing 40 to 50 knot sustained winds over land from northern North Carolina to New England and 60 knot winds over nearby Atlantic waters. Bertha was declared extratropical on the 14th when the center moved from the Maine coast to New Brunswick, Canada. The extratropical storm brought 40 to 50 knot winds to the Canadian Maritime Provinces and was tracked to just south of Greenland on the 17th.
b. Meteorological Statistics
Figures 2 and 3 (64K GIF) show a plot, versus time, of the various data used to estimate the minimum central sea-level pressure and the maximum 1-min. wind speed, 10 m above ground. Included are data from reconnaissance aircraft and satellite Dvorak-technique wind speed estimates. Table 2 lists selected surface observations of lowest pressure, peak wind, storm surge and rainfall values. Table 3 lists ship reports of 34 knots or greater that were associated with Bertha. The minimum pressure of 960 mb occurred at 0600 UTC on the 9th and is based on a dropsonde measurement. The best track maximum sustained wind speed of 100 knots at the same time is based on a 700-mb flight-level wind speed of 122 knots. measured 19 n mi east-northeast of the center.
Observations are incomplete from the Leeward and Virgin Islands, but because the circular eyewall was 20 - 30 n mi across, it is believed that hurricane conditions with sustained wind speeds to 75 knots, could have occurred on Antigua, Barbuda, Nevis, St. Eustatius, St. Bathelemy, Anguilla, St. Martin, and from St. Thomas northward through the U.S. and British Virgin Islands. Experience with Hurricane Marilyn in 1995 suggests that even higher sustained winds can occur over mountainous terrain as is found on many of these islands. Winds of 35 to 40 knots were experienced over portions of Puerto Rico
as indicated by the San Juan observations in Table 2 .
A reconnaissance aircraft flight level wind speed of 110 knots in the northeast quadrant of the circulation several hours before landfall is the basis for estimating sustained surface winds of 90 knots on the coast at landfall. The lowest sea-level pressure observed at landfall was 977 mb at Surf City, North Carolina and a value of 974 mb is assumed to be the minimum pressure at landfall.
Storm total rainfall amounts ranged from 5 to 8 inches along a coastal strip from South Carolina to Maine.
Coastal storm surge flood heights, from Florida through New England, ranged from 1 to 4 feet, but values to 5 feet were estimated on the North Carolina coast from Cape Fear to Cape Lookout. A storm surge of 6 feet or a little higher is indicated near Swansboro, where 5 to 6 feet of water was "inside of businesses on the waterfront".(from Newport, North Carolina National Weather Service Forecast Office Preliminary Storm Report ).
Seven tornadoes have been confirmed, and these occurred during the passage of an outer rain band. There were five tornadoes in Virginia, one in North Carolina and one in Maryland.
c. Casualty and Damage Statistics
Twelve deaths have been related, in some way, to Hurricane Bertha. One, in Florida, was from an evacuating military jet crashing into a house. One death from an auto accident occurred in North Carolina and another drowned in rip currents. A surfer died in New Jersey. In Puerto Rico, two died in an automobile accident and another died while surfing. On the French half of St. Martin, one person was electrocuted and one fell off a boat.
The U.S. Virgin Islands, along with North Carolina, has been declared a federal disaster area. Surveys indicate that Bertha damaged almost 2500 homes on St. Thomas and St. John. For many, it was a second hit in the ten months since Hurricane Marilyn devastated the same area.
It is likely that there was beach erosion on the north coast of the Dominican Republic as Bertha passed to the north. The Bahamas were also affected by the weak side of the hurricane, but there are no damage figures available from either of these locations.
The primary effects in North Carolina were to the coastal counties and included storm surge flooding and beach erosion, roof damage, piers washed away, fallen trees, and damage to crops. A survey indicated over 5000 homes damaged, mostly from storm surge. A Federal Emergency Management Agency (FEMA) estimate of the number of persons in South and North Carolina who evacuated is 750,000. Minor wind damage and flooding also spread along the path of the storm all the way to New England.
The American Insurance Association reports an estimate of $135 million dollars in insured property damage, primarily along coastal North Carolina. A conservative ratio between total damage and insured property damage, compared to past land falling hurricanes, is two to one. Then the total U.S. damage estimate is 2 times $135 million or $270 million dollars. No figures are available from the Caribbean.
d. Forecast and Warning Critique
Bertha moved on a fairly smooth track. The average official track forecast errors for Bertha ranged from 80 n mi at 24 hours (32 cases) to 147 n mi at 48 hours (29 cases) to 224 n mi at 72 hours (27 cases). These errors are 15 per cent, or more, lower than the previous ten-year averages of the official track errors and are from 15 to 40 per cent lower than the CLIPER forecast errors for the same cases.
Overall, the track model guidance also performed very well. However, the 0000 UTC Aviation Model run on the 9th, when Bertha was located just north of Puerto Rico, (inexplicably?) showed the track recurving significantly further east than the previous run. All of the track guidance models that use the Aviation Model as a background environment also showed a similar track. This resulted in rather large official track forecast errors on the 9th, with a 613 n mi 72-hour error on the 1200 UTC forecast. The Aviation Model and some of the track guidance models recovered to an excellent forecast only 12 hours later. Fortunately, this guidance problem occurred three days prior to landfall in North Carolina and did not have a significant impact on U.S. warnings or on warnings for the Bahamas.
Table 4 lists the various watches and warnings that were issued. Hurricane warnings were issued from Sebastian Inlet, Florida to Chincoteague, Virginia as well as for the Bahamas and for the islands of the northeastern Caribbean Sea from Antigua through Puerto Rico. Tropical storm warnings were issued from Sebastian Inlet to north of Deerfield Beach, Florida and from north of Chincoteague to Watch Hill, Rhode Island. Almost all of the U.S. east coast was involved with some watch or warning and this is the result of the storm track's expected close passage to the southeast U.S. coast. The hurricane watch for the North Carolina landfall area was issued 65 hours before landfall and the hurricane warning was issued 47 hours before landfall. This is far more than the 36- and 24-hour lead times that the National Hurricane Center strives for and is the result of the forward motion decreasing at a faster rate than expected.
Table 1. Best track, Hurricane Bertha, 5 - 14 July, 1996 (updated 4 August 1996)Source: www.nhc.noaa.gov