April 20, 2018

“Cone of Uncertainty” Shrinking, and Other Changes
to NHC Forecasting Tools for 2018


The National Hurricane Center (NHC) announced the changes to its suite of forecasting products for 2018, which are the online tools that forecasters, officials, and the public use to track and gauge the intensity of incoming tropical cyclones.

Hurricane season begins on June 1, but the NHC makes tweaks and adjustments to its online products annually, before the season arrives, to enhance forecasting and information sharing when a storm approaches.

Here are the major highlights from the list of changes that was announced earlier this week. A complete list of updates can be found online at https://www.nhc.noaa.gov/news/NHC_new_products_and_services_2018_FINAL.pdf.

The “Cone of Uncertainty” will be smaller.

One of the most notable updates is that the tropical cyclone forecast cone, (also known as the “Cone of Uncertainty”), will be smaller this year. The cone represents the probable track of a storm over the course of three or five days, and is adjusted every year, based on forecast errors in the past. Frequent NHC website visitors will recognize the cone as a collection of concentric circles that indicate where a tropical cyclone may move in the next few hours and days. In 2018, the swath of area that is covered by the cone will be smaller, as the NHC utilizes the average track error for the 2013-2017 seasons.

This is a bit of a trend, as track errors have been going down for a decade. As such, since 2008, the size of the “Cone of Uncertainty” has shrunk by 35% for the five-day forecast.

The NHC “Arrival Time of Tropical-Storm-Force Winds” graphics will no longer be in trial mode.

First introduced in 2017, the Arrival Time of Tropical-Storm-Force Winds was a new experimental tool that displayed when sustained tropical-storm-force winds would likely arrive in a given area. It was developed to provide guidance on when people should consider having their preparations completed before a storm.

These trial graphics are becoming operational in 2018, and will be a permanent product going forward. The graphics display both the earliest reasonable arrival time of sustained tropical winds, as well as the most likely time of arrival.

The NHC Public Advisory will now discuss forecast information beyond 48 hours.

The NHC Public Advisory is a text product that contains a list of all current coastal watches and warnings, and which gives pertinent storm information, including general forecast and hazard info (like storm surge, wind, rainfall, tornadoes, and surf.) The forecast information contained within the advisory will now include information beyond 48 hours.

Previously, these advisories were limited to a discussion of a tropical cyclone’s track and intensity forecast through 48 hours. This change will allow public advisories to discuss the track and intensity forecast routinely through 72 hours, and will also provide flexibility to discuss the forecast through 5 days when conditions warrant.

NHC will begin issuing 48-hour hurricane-force-wind radii forecasts.

NHC will begin issuing hurricane-force-wind radii forecasts at the 48-hour forecast time. These wind radii will be provided in the Tropical Cyclone Forecast/Advisory Message (TCM).

Previously, the NHC provided hurricane-force wind radii forecasts out to 36 hours. The NHC Forecast/Advisory will now include a forecast of tropical-storm-force wind radii out to 72 hours and hurricane-force-wind radii out to 48 hours.

These radii forecasts provide the predicted maximum extent of these winds in each quadrant of the storm. The addition of hurricane-force-wind radii at 48 hours will improve the deterministic and probabilistic guidance for areas at risk from hurricane-force winds, and can help forecasters facilitate the issuance and placement of a hurricane watch.


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