5 Changes Coming to Hurricane Season Forecasts
By Jonathan Belles
Mar 15 2017 12:00 AM EDT
As hurricane season approaches, the National Hurricane Center has updated its messaging and forecasts to keep us safe. Some of these improvements, announced last week, have been in the works for a decade.
1. Advisories Will Be Issued Before Storms Form
The NHC will issue advisories for systems that have yet to develop but pose a threat of bringing tropical-storm-force or hurricane-force winds to land areas within 48 hours. These systems will be dubbed “potential tropical cyclones” by the NHC.
The “potential tropical cyclones” will be treated like tropical depressions, named storms and hurricanes. NHC will produce a forecast projected path, watches and warnings and text products, including a full discussion and the forecast advisory every six hours until the threat ends. Advisories will be issued for potential tropical cyclones at 5 a.m., 11 a.m., 5 p.m. and 11 p.m. EDT.
According to James Franklin, branch chief of the hurricane specialist unit at the NHC, “the likelihood of tropical-storm-force land impacts would be at least as important a consideration as the likelihood of tropical cyclogenesis.”
Franklin described this update as an option for the NHC that will allow for more emphasis on impacts. The new option breaks down some of the barriers between certain categories and the ability to warn the public about potentially deadly impacts.
Systems that have a chance of development anywhere in the Atlantic basin will still be outlooked in the NHC’s Tropical Weather Outlook product.
Those tropical disturbances, sometimes called invests, or areas of investigation, are given a forecast chance of formation for the next five days. This will still be done this season, however, systems that will likely impact land with a potentially deadly or damaging threat will be referred to as potential tropical cyclones.
2. Storm Surge Watches and Warnings Are Coming
The NHC posted its first storm surge watches and warnings for the U.S. coastline in 2016 on an experimental basis, but this year those warnings will be issued.
The National Weather Service (NWS) will issue storm surge watches and warnings for the Atlantic and Gulf coasts in the event that significant risk of life-threatening inundation develops from any stage of a tropical, subtropical or potential cyclone.
According to the NHC, ” Storm surge is often the greatest threat to life and property from a tropical cyclone, and it doesn’t always occur at the same times or locations as a storm’s hazardous winds.”
A “storm surge warning” means that there is a “danger of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area, generally within 36 hours, in association with an ongoing or potential tropical cyclone, a subtropical cyclone, or a post-tropical cyclone.
A storm surge watch may be issued as early as 48 hours ahead of possible sea water inundation. These watches and warnings may also be issued when the time available to prepare for a tropical cyclone becomes limited or if storm surge is expected to cut off locations from the mainland.
Along with the watches and warnings, the NHC will also issue a potential storm surge flooding map that will show a reasonable worst-case scenario for a location. It will show how much of a given coast will be inundated and how high water could get.
3. Earliest Reasonable Arrival of Tropical-Storm-Force Winds Will Be Forecast
The NHC will be directly forecasting when tropical-storm-force winds will begin.
The onset of winds sustained greater than 39 mph is a critical threshold after which it becomes difficult or even dangerous to be outside continuing preparations for a tropical storm or hurricane.
This new graphic will display the “earliest reasonable” arrival of such winds, an estimate for when it will become unsafe to continue preparations.
This time will be determined by a wind speed probability model that has been used to calculate what and when the chances that a certain wind speed threshold – 39 mph, 58 mph, 74 mph – will be met in a specific location. The model is run 1,000 times every six hours to account for historical errors.
4. The Cone of Uncertainty Will Be Smaller
Each year, the NHC adjusts the size of its cone of uncertainty based on its average error over the previous five hurricane seasons. The cone of uncertainty refers to the projected path map you frequently see on the internet or television for a given storm.
The cone encapsulates 66 percent of the historical forecast track errors, and does not represent where impacts like surge, wind, flooding or tornadoes will be felt.
For the 2017 Atlantic hurricane season, the NHC will use the average track error for the 2012-2016 hurricane seasons.
Track errors have gone down over the last 10 years and forecasts have gotten better as well. In fact, since 2007, the size of the cone of uncertainty at 120 hours (or five days) has shrunk by more than 35 percent. Since last year, the size of the cone at five days has shrunk by more than 10 percent.
Similar reductions in size were seen from 2016 to 2017.
5. Advisory Graphics Have Been Improved
Although we hope that you’ll visit us during hurricane season, we know that you’ll see the same forecast tracks from the NHC on social media. This year, their graphics will be greatly improved with fresher colors and nicer fonts. The new graphics will also have hurricane- and tropical-storm-force wind fields this year.
Here are old advisory graphics and the new ones.
Hurricane season in the Atlantic, Gulf of Mexico and Caribbean begins on June 1 and continues through Nov. 30.