Season: Although tropical systems including hurricanes have developed in every month of the year, the "Official Season" for the Atlantic Basin is from June 1 to November 30 (May 15 to November 30 in the Eastern Pacific).
Atlantic Basin (AB): Includes the Atlantic Ocean, Caribbean Sea and the Gulf of Mexico.
Tropical Wave: A convergence of wind, but no central circulation (winds don't rotate completely around a central core), in the AB that usually moves from East to West, pushed by the Easterly Trade Winds near the Equator.
Tropical Depression: A weak central circulation (cyclone) with sustained (1 minute average) winds 38 mph or less. Winds rotate around the center in a counter-clockwise fashion in the Northern Hemisphere.
Tropical Storm: A cyclone with sustained winds between 39 and 73 mph. The storm is then given a name which it maintains either as it grows into a hurricane or weakens back to a depression.
Hurricane: A cyclone with sustained winds of 74 mph or greater. Major: 111 mph or greater. A hurricane often has a central eye which is a relatively calm center; that eye can change shape and size and sometimes there is more than one. Around the eye is the wall cloud which usually is the most intense portion of the storm. Extensions called spiral bands spread out like spokes on a wheel and frequently have significant thunderstorms and tornadoes within them.
Quasi-tropical: The cyclone has a mix of tropical and extra-tropical characteristics.
Extra-tropical: The cyclone maintains circulation, but has lost its tropical characteristics. Primarily the warm core of the hurricane has changed to the cold core of an extra-tropical cyclone.
Landfall: This marks the location where the center of the storm hits a land mass. Of course, the size of the storm means that destructive winds, tornadoes, torrential rainfall, etc., can be felt well in advance of the center.
NATIONAL HURRICANE CENTER:
Located in Miami, FL, the NHC issues 120 hour tropical cyclone track and intensity forecasts four times per day for all storms in the AB and more frequently as the storm approaches landfall. They use a battery of guidance models ranging in complexity from simple statistical models to three-dimensional equation models. Some of these models are designed for tracking, some for intensity, and some for both.
The Center then has its trained meteorologists and oceanographers produce the "Official Forecast" for guidance. On the basis of that, there may be watches or warnings promulgated.
A tropical storm or hurricane Watch is issued when tropical storm or hurricane force winds are possible within 36 hours. A Warning is issued for the 24-hour period.
Tropical systems require warm waters (at least 80 degrees F), light winds aloft, and converging winds at the surface. Storms in the early part of the season usually form in the Caribbean and Gulf of Mexico. For August and September, the eastern Atlantic becomes more intense. Then the frequency returns to the GOM and Carib.
The Saffir-Simpson Scale is used to categorize hurricanes to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale. For other damage see HAZARDS section below.
Category 1: Winds 74-95 mph. No real damage to building structures; damage primarily to unanchored mobile homes, shrubbery and trees.
Category 2: Winds 96-110 mph. Some roofing material, door and window damage of buildings. Considerable damage to shrubbery and trees. Considerable damage to mobile home, poorly constructed signs and piers.
Category 3: Winds 111-130 mph. Some structural damage to small residences and utility buildings. Damage to shrubbery and trees with foliage blown off trees and large trees blown down Mobile homes and poorly constructed signs are destroyed.
Category 4: Winds 131-155 mph. Some complete roof structure failures on small residences. Shrubs, trees and most signs blown down. Destruction of mobile homes. Extensive damage to doors and windows and lower floors of structures near the shore.
Category 5: Winds greater than 155 mph. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Most shrubs, trees and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Major damage to lower floor of all structures located less than 15 feet above sea level.
Wind: See Saffir-Simpson Scale above.
Storm Surge: This is the swell of water that accompanies tropical storms, to a minor degree, and hurricanes, to a larger degree, depending on intensity and speed. Storm surge values are highly dependent on the slope of the continental shelf in the landfall region coupled with the angle of the storm.
Cat 1: 4-5 ft. Some coastal road flooding and minor pier damage.
Cat 2: 6-8 ft. Coastal and low lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings.
Cat 3: 9-12 ft. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland up to 8 miles.
Cat 4: 13-18 ft. Low lying escape routes may be cut by rising water 3-5 hours before arrival of center. Major damage to lower floors of structures near the shore. Terrain lower than 10 feet above sea level may be flooded.
Cat 5: 18+ ft. Low lying escape routes are cut by rising water 3-5 hours before arrival of center. Major damage to lower floors of all structures located less than 15 feet above sea level and within 500 yards of the shoreline.
Rainfall Flooding: There is no direct correlation between intensity of the storm and the amount of rainfall. That is due to factors such as the speed of the storm, the terrain it passes over, and other meteorological factors. In general, the slower the storm, the greater the rainfall in any one location. The more upslope, the greater the rainfall (Appalachians). With dry air intrusion for East Coast storms, there are usually two maxima: the front right quadrant and the farther west.
Tornadoes: Although frequent, tornadoes are usually weak (F0 and F1).
Hail: Since tropical systems are relatively warm from the surface to the highest levels, hail is not a normal problem. However, when the storm becomes more quasi- or extra-tropical, the threat of hail damage increases.
Seiche: On occasion slower moving tropical systems will maintain and easterly wind component which piles up water on the western side of the ocean, bays and back bays. When the winds shift to westerly, the water moves from the west to the est. This can cause extensive flooding on the western shores of Delmarva and New Jersey.
Named Storms: An average of 11 to 12 per year.
Hurricanes: An average of 5 to 6 per year; Major: An average of 2 per year.
Hurricane Landfall: Over a 3-year period approximately 5 hurricanes strike the US coastline from Texas to Maine. The three most likely targets are Florida, Texas and the Carolinas. Within 75 miles of Ocean City, there is a 4.2% probability of any hurricane and a 0.9% probability of a major hurricane per year. By comparison, Cape Hatteras is 21.3% and 5.3% and Miami is 26.3% and 11.1%.
DELMARVA FACTS AND CONCERNS
In recorded history there have only been three hurricane eyes passing over or immediately adjacent to the peninsula, the last being in 1878. (Well, lets make that 3 and 1/4 as a partial eye of regrowing Floyd in 1999 may have clipped Fenwick Island, but the hurricane force winds were already well offshore.)
However, there have been a number of "close calls" where the center was nearby and which caused significant damage and flooding on the peninsula. In addition, there have been numerous tropical storms and even tropical depressions that greatly affected Delmarva.
Some of the notable over the past century are:
UNNAMED, 1933: Center hit Va.-N.C. border as Cat 3 and traveled northwestward to Richmond, north to Washington, D.C., and northeast to Burlington, Vt. Created OC Inlet. Reports of hurricane force winds and significant damage over much of the peninsula.
GREAT ATLANTIC HURRICANE, 1944: Center passed near Cape Hatteras as Cat 3. Cape Henry, Va. had 134 mph sustained winds. Extensive coastal and maritime damage.
CAROL AND EDNA, 1954: Carol stayed offshore about 150 miles on trek from Charleston, S.C. to Long Island. Two weeks later, Edna followed a path just to the east. Flooding rains were main problem.
HAZEL, 1954: Made landfall at Cat 4 near NC-SC border. Winds up to 150 mph in NC, 78 mph at DC.
CONNIE AND DIANE, 1955: Like Carol and Edna, it wasn't the wind, it was the rain. Connie dumped up to 12" and a few days later Diane dumped 10-20" in pretty much the same area from NC to MA.
DONNA, 1960: Donna is the only hurricane of record to produce hurricane-force winds in every state along the eastern seaboard from Florida to Maine.
CAMILLE: Although only a tropical depression when passing from WV eastward through VA and southern Delmarva, she dumped 15 to 31" rains in just a few hours, causing 256 deaths, 113 in VA alone.
FLOYD, 1999: Floyd regained some strength in the Atlantic after crossing through the middle of Delmarva, but it was mainly the torrential rains that hit us. Reports of over 12 inches of rain in less than 12 hours came from our weather watchers and official sites. Rainfall ranged from less than 4" from Lewes to Chincoteague to over 12" west of a Nanticoke-Seaford-Newark line. 50 of the 56 deaths nationwide were attributed to flooding.
ISABEL, 2003: Isabel weakened to a tropical storm shortly after landfall along the VA-NC border as she moved northwestward. Although rains were significant on the peninsula (5-9") and winds approached hurricane strength, it was the seiche (see above) that caused the most significant damage and danger on Delmarva. Isabel is also known as the "Jeff Porter Storm" as it was his first day on the job.
Like "tornado", the word "hurricane" congers up images of death, destruction and fear. And that is true in many locations, but not significantly here on the shore. We are blessed that most storms that hit us have already made landfall and are weakened. We are blessed that most Atlantic storms start the "curve to the northeast" south of us. We are blessed that we don't have mountains that enhance rain and concentrate flooding waters. And we are blessed that much of the soil is sandy so water can penetrate rather than collect.
Despite all that, however, there have been some very destructive storms here, and every tropical system has its perils. Historically, the major problems are excessive rainfall (especially if the ground is already wet or saturated), coastal flooding and the rebound of seiches in the Chesapeake Bay.