National Hurricane Center (NHC) Products
The National Hurricane Center (NHC) is a division of NOAA and the National Weather Service, which has a team of forecasters who specialize in forecasting tropical cyclones and their impacts. The NHC covers the entire Atlantic Ocean and the eastern half of the Pacific Ocean. This includes the Caribbean Sea, the Gulf of Mexico, and the waters surrounding Hawaii.
i. Tropical Weather Definitions
To better understand the National Hurricane Center's messaging, one must be familiar with the terminology used in tropical cyclone forecasting. Here are the most important terms to know:
- Tropical Cyclone: A well-defined region of low pressure that is formed and maintained by copious tropical showers and thunderstorms. Unlike baroclinic cyclones (which have a well-defined cold front and warm front), tropical cyclones do not have well-defined fronts.
- Tropical Disturbance: A region of tropical weather that has at least some potential for organizing into a tropical cyclone.
- Invest: A tropical disturbance that hurricane models are running for (usually because it has a high likelihood of developing into a tropical cyclone).
- Potential Tropical Cyclone: An area of low pressure that is expected to become an organized tropical cyclone within 48 hours.
- Tropical Depression: A tropical cyclone with maximum sustained winds under 39 mph.
- Tropical Storm: A tropical cyclone with maximum sustained winds between 39 mph and 73 mph. A cyclone that reaches this intensity is assigned a name (e.g. Fred). Tropical storms can cause property damage through strong straight-line winds, but the main concern with tropical storms is the potential for significant flooding.
- Hurricane: A tropical cyclone with maximum sustained winds of 74 mph or higher. A cyclone that reaches this intensity usually forms an "eye", or a quasi-circular region of clear sky near the cyclone center. Hurricanes are then categorized based on their wind speed to better communicate the potential threat that it poses.
- Major Hurricane: A hurricane with maximum sustained winds over 110 mph. Cyclones that are classified as "major hurricanes" can produce devastating or catastrophic damage if they strike a populated area. Major hurricanes that track near land can also prompt the issuance of an extreme wind warning.
- Saffir-Simpson Scale: A rating system designed to assess a hurricane's potential impacts if it strikes a populated area. The scale ranges from 1 (least intense) to 5 (most intense). More information on this scale can be found here. At the moment, there is no official criteria for anything above Category 5 intensity.
- Post-Tropical Cyclone: An initial tropical cyclone that now resembles a baroclinic cyclone with a well-defined cold front and warm front. Even though the cyclone's characteristics may have changed, some post-tropical cyclones can produce hurricane strength winds and very heavy rainfall.
ii. Tropical Cyclone Hazards
- High Winds: Often, the main concern with a tropical cyclone is the potential for damaging winds. Major hurricanes can cause extensive devastation and leave areas uninhabitable for weeks. On the lower end, high winds usually cause power outages, which can be widespread and long-lasting. Downed trees and tree limbs can also damage nearby structures and block roadways. On the higher end, high winds can devastate neighborhoods and isolate them due to road blockages and long-lived pervasive utility disruptions.
- Storm Surge: The intense winds and low pressures within a tropical cyclone can cause significant increases in water height. In extreme cases, hurricanes can raise water levels over 10 feet above ground level. Storm surge is primarily a hazard to those along the immediate coastline, but the impacts can extend several miles inland. Coastal regions are frequently evacuated as storm surge poses a major risk to people who are caught in or near the high waters.
- Rainfall: Tropical cyclones host showers and thunderstorms that can produce extreme rain rates. Large quantities of rain falling in a short amount of time can cause flooding. Slow-moving or stationary tropical cyclones can cause catastrophic flooding as numerous persistent bands of tropical showers track over the same areas.
- Tornadoes: Tropical cyclones often produce tornadoes, but such tornadoes are usually weak (EF0-EF1) and short-lived. However, some cyclones can favor the development of strong (EF2+) tornadoes, especially as the cyclone tracks toward higher latitudes (further away from the equator).
iii. Analysis
One of the products WarnUS conveys is the National Hurricane Center analysis of active organized tropical cyclones. Using observational data and model outputs, the NHC will determine a "best-guess" on the cyclone's geographic location, minimum central pressure, maximum sustained winds, and movement (direction and speed). Be advised that the impacts of a tropical cyclone often extend well beyond the cyclone's center.
iv. Forecast Track
The National Hurricane Center also publishes a path the cyclone center is expected to follow over the next 5 days. This also includes a forecast of how the cyclone's intensity will vary over the next 5 days. This is another feature that WarnUS can plot on its full map. Again, be advised that the impacts of a tropical cyclone often extend well beyond the cyclone's center. It is also important to note that forecasts can change on short notice, and the cyclone's actual track can deviate several hundred miles away from the expected track. If a cyclone is expected to dissipate within 5 days, the forecast track will be cut short.
v. Tropical Alerts
The National Weather Service and the National Hurricane Center utilize a number of different alerts to raise awareness and prompt action from emergency managers and the general public. The main alerts are described below:
- Tropical Storm Watch: Tropical storm conditions are possible in a particular area within the next 48 hours. Persons within a tropical storm watch should begin preparations for winds of up to 73 mph and significant flooding.
- Tropical Storm Warning: Tropical storm conditions are likely in a particular area within the next 36 hours. Persons within a tropical storm warning should be finishing preparations for winds of up to 73 mph and significant flooding. Note that tropical storm watches and warnings are issued even when a hurricane is active, because the winds far away from the hurricane center are often at tropical storm intensity.
- Hurricane Watch: Hurricane conditions are possible in a particular area within the next 48 hours. Persons within a hurricane watch should begin preparations for winds over 74 mph and significant flooding. Be alert to potential evacuation orders and tune into local news outlet for updates.
- Hurricane Warning: Hurricane conditions are likely in a particular area within the next 36 hours. Persons within a hurricane warning should be finishing preparations for winds over 74 mph and significant flooding. Be alert to potential evacuation orders and tune into local news outlet for updates.
- Storm Surge Watch: Life-threatening storm surge flooding is possible within the next 48 hours. Persons within a storm surge watch should prepare for major coastal flooding and the possibility of being evacuated further inland.
- Storm Surge Warning: Life-threatening storm surge flooding is likely within the next 36 hours. Persons within a storm surge warning should finish preparations for major coastal flooding. Check with local officials to see if you're under an evacuation order.
- Extreme Wind Warning: Straight-line winds capable of inflicting catastrophic damage are imminent or occurring. Persons under an extreme wind warning should immediately seek shelter in an interior room on the lowest floor of a sturdy building.
Tropical cyclones often produce tornadoes. If a tornado warning is issued for your area, immediately seek shelter in an interior room on the lowest floor of a sturdy building. The tornadoes produced by tropical cyclones are usually weak (EF0-EF1), but can strike with very little or no advance warning.
Additional information:
More information about National Hurricane Center products
NOAA hurricane safety and preparedness page
NWS hurricane safety and preparedness page