Hurricane Nate

Pre-Landfall 1 | Summary
Posting Date: October 6, 2017, 8:00:00 AM

Meteorological Summary

After causing deadly flooding rains in Central America on Thursday, October 5, then skimming by Mexico’s resorts of Cancún and Cozumel on Friday, Tropical Storm Nate is forecast to intensify while crossing the unusually warm waters of the Gulf of Mexico on Saturday, threatening the U.S. Gulf Coast near New Orleans late Saturday/early Sunday with potential landfall as a Category 1 hurricane.

Tropical Depression 16 formed the morning of October 4 in the southwestern Caribbean Sea and became Tropical Storm Nate by evening. Nate is the 16th cyclone and the 14th named storm in a record-breaking season—the fifth named storm since Aug. 30.

The center of Tropical Storm Nate struck the coast of Nicaragua Thursday, October 5, moving across eastern Honduras by the end of the day, with maximum sustained winds of 40 mph. Nate brought torrential rain that will persist through Saturday, reaching 30 inches in a few locations in Nicaragua, and up to 20 inches in Costa Rica and Panama, according to the National Hurricane Center (NHC). This locally heavy rain has already triggered deadly flash flooding and mudslides, particularly in Nicaragua, where two weeks of saturating rain preceded Nate’s arrival. The storm’s flooding there left at least 15 people dead, with at least seven more in Costa Rica.

The storm crossed into the northwestern Caribbean Sea Thursday night through early Friday. There, fueled by water temperatures in the mid to upper 80s, 2 to 5 degrees Fahrenheit above average, Nate is expected to intensify slightly and graze the edge of Mexico’s Yucatan Peninsula through early Saturday morning, drenching the region with 4 to 8 inches of rainfall, with a potential of up to 12 inches in isolated areas.

Forecast Track and Intensity

The storm is currently moving north-northwest at 21 miles per hour, and is expected to increase in speed. The NHC forecasts Nate will further intensify on Saturday before approaching the Gulf Coast Saturday night/early Sunday as a Category 1 hurricane with tropical storm–force winds extending up to 115 miles, mainly to the east of the center. Landfall forecasts place it near New Orleans, Louisiana, at 1 a.m. Sunday, with the cone of uncertainty reaching as far east as Pensacola in the western Florida Panhandle—an area which missed the worst of Hurricane Irma two weeks earlier.

As of Friday morning, a hurricane watch is in effect from Morgan City, Louisiana, to the Mississippi/Alabama border, in Metropolitan New Orleans, and around Lake Pontchartrain and Lake Maurepas. A storm surge watch is in effect for the area from Morgan City, Louisiana, to the Alabama/Florida border, as well as for the northern and western shores of Lake Pontchartrain. A tropical storm watch is in effect from the Mississippi/Alabama border to the Okaloosa/Walton County Line, as well as west of Morgan City to Intracoastal City, Louisiana.

Southeast Louisiana and the Mississippi Gulf Coast will start experiencing sustained tropical storm–force winds late Saturday. Moderately heavy rainfall of 3 to 6 inches is forecast, with a maximum of 12 inches possible, especially along and to the right of the storm's center. The heaviest rain will be Saturday and Sunday. Isolated tornadoes may also occur, particularly east of the center.

The NHC has warned of "life-threatening" storm surge of up to 7 feet in some parts of Louisiana that were already experiencing “king tides” up to 2 feet above normal tide levels. Surge will be highest on the east- and southeast-facing shores, and destructive waves and rip current conditions are likely. A coastal flood advisory remains in effect for New Orleans until 12 noon on Sunday, October 8.

Unlike Hurricane Harvey, which stalled over Houston to create a seemingly endless inundation, Nate is forecast to move rapidly along. From Sunday into Monday, the storm is expected to cross into Missouri and Alabama, reaching Tennessee by very early Monday morning. Nate is on a similar path to Katrina, but while Katrina was a powerful Category 3 hurricane when it struck New Orleans, Nate’s impact as a Category 1 hurricane is likely to be far less destructive. 

Track map for Tropical Storm Nate as of 11 a.m. EDT, Friday, October 6. (Source: NHC)

Preparations

New Orleans’ mayor declared a state of emergency Thursday, October 5, and urged residents to stock up on supplies. Officials on the barrier island of Grand Isle called for the voluntary evacuation of residents. The New Orleans Police Department has placed over 100 barricades to deter motorists from flood-prone areas. St. Bernard Parish in Louisiana also issued a state of emergency declaration, and called for a mandatory evacuation for areas outside its levee protection system. Florida’s governor declared a state of emergency for 29 counties in north Florida and the Panhandle. Some coastal Alabama counties also issued voluntary evacuation orders, and a state of emergency went into effect across Alabama on Friday morning.

New Orleans has been making preparations for the storm, although the city is still working on fixing drainage pumps, along with the turbines and generators that power them, potentially leaving the city vulnerable to flooding issues. As of Thursday, 108 of the 120 drainage pumps in New Orleans were operational, with three major and five minor pumps not functional. Fifteen National Guard members are stationed to monitor the pumps starting Friday morning—a preventive measure they have been training for since the August 5 failures that led to widespread flooding.

The Bureau of Safety and Environmental Enforcement's New Orleans office stated that as of midday Thursday, six out of the 737 manned oil and gas production platforms in the Gulf had been evacuated, and one of the 18 dynamically positioned rigs in the Gulf has been moved off-site. It is estimated that less than 15% of the current oil production and less than 7% of the current natural gas production in the Gulf has been shut-in.

Exposure at Risk

Single-Family

The majority of single-family residential structures along the U.S. East Coast are of wood-frame construction, although masonry is also common in the Gulf States and along the East Coast. The exterior walls of wood-frame homes can be finished with stucco, wood siding or shingles, vinyl, or aluminum cladding. Interior walls in older homes are usually finished with plaster; in newer ones they are usually finished with gypsum board.

The foundations of single-family homes are often a concrete crawl space, slab-on-grade, or (particularly in older wood-frame houses) spread concrete footings. Florida and the Louisiana coast have a relatively small percentage of basements, so basement flooding may be less of an issue in these regions.

The vulnerability of mobile (manufactured) homes and light metal structures is much greater than that of other construction types, and these types of structures could experience some structural damage.

Large, high-value homes generally exhibit a high quality of construction, often with sophisticated engineering input and often with secondary risk mitigation features. They also tend to be well maintained. They may feature complex architecture with elaborate roof geometries containing multiple gable ends and corners, which tend to mitigate wind loads.

Multi-Family

Apartment buildings and condominiums tend to have a more diversified construction mix than single-family homes. The Gulf Coast states use mostly masonry or reinforced concrete (high-rise) in these buildings. Large, high-rise concrete or steel buildings are often built to strict engineering standards and their vulnerability is similar to large commercial buildings. However, apartments and condominiums often have exterior building components such as balconies, awnings, and double sliding glass doors that are less engineered at the design and construction stages and are hence more vulnerable than the main building structure.

Commercial

As with residential buildings, construction materials used for commercial buildings vary regionally. Around the Gulf of Mexico, concrete, masonry, and light metal are often used, while steel and wood are more common along the East Coast. Low-rise commercial structures are generally similar to single-family homes, while mid-rise and high-rise commercial buildings are similar to large apartment buildings or condominiums. These buildings typically follow stricter standards and are built under the supervision of an engineer, making them less vulnerable to wind damage.

AIR’s analysis of damage data over time has indicated that building vulnerability can change significantly due to changes in building codes and code enforcement, changes in material and construction practices, and structural aging. AIR expects newer structures in the region will perform better than older structures.

Expected Damage

Inundation caused by storm surge is expected to begin Saturday in Louisiana and to become deep and widespread as Nate moves through the area. Structural damage to some homes and businesses is expected, as well as a weakening of some foundations caused by heavy rain and surge, with some homes removed from their foundations entirely. Roads and bridges, including escape routes, could also get washed out, leading to closures.

Nate’s winds are expected to cause damage to roofing and siding materials, porches, awnings, carports, and sheds. Mobile homes, especially those that are unanchored, could experience significant wind damage, and unsecured, lightweight objects could become airborne and cause damage as projectiles, particularly to windows, which could allow water into structures. Large trees could snap or be uprooted, falling on power lines and causing outages. Small craft could break away from their moorings and become beached or damage other craft. Damage to marinas, docks, boardwalks, and piers in the area affected is expected.

In general, engineered structures such as reinforced concrete and steel buildings should experience very little damage, although there may be isolated instances of damage to nonstructural elements, such as to windows, cladding, and roof coverings.

The AIR Tropical Cyclone team will continue to monitor this storm and provide updates as warranted.

Pre-Landfall 1 | Downloads
Posting Date: October 6, 2017, 8:00:00 AM