The severity of a tropical storm depends on multiple factors.  However, the ferocity of tropical storms is formally categorized on the basis of wind speed as measured by the Saffir-Simpson Hurricane Wind Scale. This scale categorizes a hurricane's intensity on a scale ranging from one to five based on the storm's maximum sustained wind speed. Levels of potential property damage are associated with each of the five categories. Hurricanes categorized 3 or higher are considered major hurricanes. This scale does not indicate the amount of surge or rain expected from a hurricane, only wind.

The illustration below describes the severity categories for tropical storms associated with New York City. Category 4 hurricanes in the New York City region are possible, but unlikely. Category 5 hurricanes are not anticipated to occur in the New York City area, because they are not meteorologically sustainable over the Atlantic Ocean north of Virginia.

Although the Saffir-Simpson Hurricane Wind scale is a practical way of measuring hurricane strength, other factors contribute to a hurricane's impact on a given location -- the storm's size (proportional to the radius of maximum winds) and speed of its forward motion. A larger, slower-moving storm, for example, may cause greater damage than a smaller, faster-moving storm with high winds because a single location might be battered by winds from a slower-moving hurricane for a longer period of time.

The wind’s fetch -- the distance that the wind blows across the water’s surface -- and its duration also affect a tropical cyclone’s severity.  Wind fetch is determined by the radius of maximum winds and forward speed of the tropical storm. The severity of waves and height of the storm surge is directly affected by how far and how long a specific storm’s winds blow across the water.

A Hypothetical Storm Approaching New Jersey

The bearing of a storm —the direction that it is moving — when it reaches New York City also contributes to the severity of its impact.  Its bearing determines the wind direction, which affects the height and severity of the storm surge. The local geography of the coastline amplifies storm surge in the New York City region. The New York Bight – the nearly 90-degree angle formed by the shorelines for Long Island and New Jersey – can direct a storm surge directly into New York Harbor. Storms with a westward bearing often generate higher storm surge risk. For New York City, the worst-case hurricane track is a storm making landfall just to the south along the coast of New Jersey. In this scenario shown in the illustration below, the city is in the right-front quadrant of the storm as the storm funnels the surge directly into Raritan Bay and New York Harbor.

This scenario is precisely what happened in 2012 during Hurricane Sandy and why the storm had such a disastrous impact on New York City. Hurricane Irene made landfall over Brooklyn in 2011, but was a very different tropical storm than Sandy. At landfall, Hurricane Irene’s bearing was north-northeast, which did not create the direct surge impacts that Hurricane Sandy inflicted on New York City just over one year later.

Nor'easters do not have a universally recognized classification system, but their strength and severity are influenced by factors similar to those discussed regarding the severity of tropical storms.

When a Nor’easter produces a heavy snowfall, the Northeast Snowfall Impact Scale (NESIS) is used to measure the intensity of wintry precipitation. The NESIS characterizes and ranks high-impact Northeast snowstorms – those with large areas of snowfall accumulations of 10 inches and greater – on a scale of one to five.

The National Climatic Data Center developed this scale to indicate the degree to which Northeast snowstorms might affect the transportation systems and economy of the region and thereby impact the rest of the country. The NESIS index incorporates population data and meteorological measurements to gauge a nor’easter storm's overall impact.

The National Hurricane Center (NHC) forecasts storm tide heights for tropical storms and hurricanes on a probabilistic basis (a range of likely storm tide levels) and updates its forecasts regularly as specific storms approach landfall.

The NHC calculates return periods for hurricanes for various locations along the East Coast of the United States. These return-period calculations represent the average amount of time between the passages of two hurricane eyes within a 50-nautical-mile (57.54-mile) radius of a given location.

According to these NHC probability models, New York City should expect to experience a lower-category hurricane on average once every 19 years and a major hurricane (Category 3 or greater) on average once every 74 years.

The National Oceanic and Atmospheric Administration (NOAA) provides timely storm tide forecasts for non-tropical storms (including nor'easters) on a deterministic basis (a single value for each tide gauge location). New York City typically experiences several nor'easters of different intensity every year. Most of these nor’easters are relatively weak but retain the potential to produce significant rainfall or snowfall hazards that can cause minor-to-moderate damage across the area. The probability of severe nor'easters affecting New York City is low, but they do strike occasionally.

Some academic institutions, including Stevens Institute of Technology and SUNY Stony Brook, provide forecasting services similar to NOAA. Although conducted purely for research purposes, their forecasts are often valuable supplements to official forecasts.

The vulnerability of different neighborhoods and areas of New York City to hazards associated with tropical storms varies significantly. To predict storm surge and to guide the City's planning for coastal storms, New York City Emergency Management (NYCEM) utilizes outputs from a National Hurricane Center computer model called SLOSH (Sea, Lake, and Overland Surges from Hurricanes).

The SLOSH model calculates surge heights for storms that move in different directions and vary in strength from Category 1 to Category 4. Calculations are based on the different wind speeds associated with Category 1-4 storms, the radius of maximum winds, the storm’s forward speeds, storm bearing, and tides. The SLOSH model calculates surge levels for specific locations assuming that each location is hit by the most intense part of a storm. The SLOSH model uses all of these factors to generate a worst-case scenario for storm surge at specific locations in New York City, it does not relate to probability of occurrence.

The map below shows areas of the city likely to experience inundation from different categories of storms, based on the SLOSH model calculations. The extent of inundation and storm-surge depths indicated on the map reflect the worst-case scenario for Category 1-4 hurricanes in different parts of New York City.

The likely inundation depths listed under each location on the map reflect the difference between the height of the storm tide projected for each hurricane category and the land elevation of that specific area in New York City. The red areas on the map represent the areas of the city that could be inundated by storm surge in a worst-case scenario from a Category 1 hurricane.

NYCEM utilized SLOSH data to develop evacuation zones for the New York City Coastal Storm Plan (CSP). Evacuation zones are based on a SLOSH output called Maximum Envelope of Water (MEOW). MEOWs show the maximum surge inundation from a set of hypothetical storms of fixed size at specific landfall locations with varied intensity, forward speed, storm direction, and tide anomaly.

The CSP evacuation zones employ a range of possible scenarios from the MEOWs, in contrast to the SLOSH maps that only display one worst-case scenario storm surge, the MOM (Maximum of MEOWs) for each category.

Evacuation zones presume a universal worst-case scenario, as if a storm were to make landfall in all parts of the city at once, creating the maximum amount of potential surge inundation in each location. Evacuation orders are issued for an entire zone due to the inherent uncertainty in the forecast, even though a storm will not create worst-case effects in every part of the zone.

The City updated its evacuation zones in June 2013. Unlike the prior set of evacuation zones, which were in place when Hurricane Sandy struck in 2012, the storm's bearing is now a more significant input for the calculation of the new zones.

SLOSH data is not used for nor’easters. However, the National Weather Service provides water level forecast ranges referenced to MHHW and MLLW for tidal gauges around the New York City metro region ahead of an impending coastal flood event. Then, NOAA’s Office for Coastal Management Sea Level Rise Viewer and NWS New York Impact Catalogs is used to determine the potential impacts from that storm.