What to know about forecast tracks as Idalia looms over Florida
It happens every year: A new hurricane season begins, and the forecast graphics start trickling in.
Online, your friends and family share maps with multicolored blobs. On TV, the strands of spaghetti models twist and turn through the tropics. Forecast information is everywhere. You think you know what it all means — but do you really?
Accurately interpreting hurricane forecasts is a crucial skill in evaluating the risks you face from a looming storm. As with Hurricane Ian in September, information can change quickly and reshape your risk level. Your home could be placed under a hurricane watch, or a storm surge warning, from one hour to the next.
With Idalia threatening to power up to a Category 3 storm before smacking into Florida, let’s review how to interpret forecast graphics.
Tropical weather outlooks have been extended
When the hurricane season kicks into high gear, the National Hurricane Center’s tropical outlooks can look something like a chaotic game of tic-tac-toe. On a map of the Atlantic Ocean to the Gulf of Mexico, colored X’s and O’s often dot the graphic. These symbols represent areas of disturbed weather the hurricane center is watching.
These outlooks, which the hurricane center releases four times a day (at 2 a.m., 8 a.m. 2 p.m. and 8 p.m. during daylight saving time), show the genesis of storms. They are helpful in understanding when and where a storm could form.
Forecasters do not use them for tracking storms. Instead, these outlooks show the likelihood a disturbance will become a tropical cyclone — which can be as low-grade as a tropical depression with maximum sustained winds of no more than 38 mph.
In years past, these outlooks gave the likelihood of a system becoming a tropical cyclone within two days and five days. But this year, the outlooks will be for two days and seven days out, said John Cangialosi, a senior hurricane specialist at the hurricane center.
The hurricane center began experimenting in-house with a seven-day outlook in 2018. Now, forecasters are confident in their ability to deliver these probabilities to the public.
“We’ve done this for a while and also felt that our skill was very good,” Cangialosi said. “So that’s the reason we’re going forward with it this year.”
The hope is that it will provide forecasters, emergency managers and citizens extra time to prepare ahead of storms.
This year, the outlooks will look the same, except the five-day outlook will be replaced with the seven-day outlook.
When a disturbance pops up in the forecast area, a colored “X” will mark the location of the system. The color of the “X” corresponds to the chance the disturbance will turn into a tropical cyclone. A yellow “X” means there is less than a 40% chance, an orange X is a 40% to 60% chance and a red “X” is greater than a 60% chance.
The two-day outlooks number the disturbances and give the probability of formation in 48 hours on the map.
If a disturbance becomes a tropical depression (with sustained winds that are no more than 38 mph) it will turn into a red circle. A tropical storm, which has maximum sustained winds of 39 to 73 mph, forms after a tropical depression. Once a storm reaches maximum sustained winds of at least 74 mph, it becomes a hurricane. Each tropical cyclone’s location can be found on the map.
To learn more information about the disturbances, you can hover over or click on its corresponding symbol.
The longer outlook will probably be most useful for areas farther east, like portions of the Caribbean, Cangialosi said. Tropical systems have an affinity for forming in the Caribbean Sea, where waters are balmy.
Understanding the “cone of uncertainty”
You’ve got the X’s and O’s out of the way. Now let’s talk about the cone.
Once a storm officially reaches tropical depression strength — that’s maximum sustained winds of no more than 38 mph — forecasters start including the infamous “cone of uncertainty” on their graphics.
Every hurricane season, there’s a push from meteorologists to drive home what the cone means, and what it doesn’t mean. There’s good reason for that: Many people get it wrong.
In a study published October 2022, a team of University of Miami researchers surveyed Florida residents about whether they know how to accurately interpret the cone graphic. Of those surveyed, 44% incorrectly thought they could determine the forecast size of the storm, and 40% answered incorrectly that they could identify areas where damage will occur.
Here’s what the cone does tell you, based on historical data: There is a two-thirds chance the storm will stay inside the cone. But there also is a one-third chance it will not. In other words, based on past forecasts, the entire track of the storm is within the cone roughly 60 to 70% of the time.
“It’s a visual depiction of where we think the center of the storm is going to go,” Cangialosi said.
What the cone doesn’t tell you, according to the National Weather Service: The size of the storm, and the impacts both within the cone and outside the cone.
“The cone doesn’t tell you where those impacts are going to be because sometimes storms are really lopsided and they get so much weather on the right side,” Cangialosi said. “You could be way outside the cone and still get a lot of conditions.”
Know the difference: A ‘watch’ vs. a ‘warning’
Without looking it up, do you know the difference between a hurricane watch and a hurricane warning?
How about with storm surge? What’s a “Storm Surge Warning”?
Meteorologists consider watches and warnings a crucial piece of information that can help you better pin down your risk level as storms approach.
A hurricane watch occurs when hurricane conditions, with winds of up to 74 mph or higher, are possible within a specific area. Key word: possible. A hurricane watch arrives two days, or 48 hours, before meteorologists expect tropical storm-force winds will arrive in your area, according to the National Weather Service.
“A hurricane watch means you really need to get your preparations completed,” said Paul Close, a meteorologist with the National Weather Service in Tampa Bay. “Preparedness activities become much more difficult once winds become tropical-storm forced winds.”
A hurricane warning means hurricane conditions (winds 74 mph or higher) are expected within a specific area. The key word now is expected. Meteorologists issue a hurricane warning 36 hours ahead of the anticipated arrival of tropical storm-force winds to make sure there’s enough heads-up for storm prep.
So a watch comes before a warning. Here’s one way to remember it: Think of a sailor on top of a ship’s crows nest, looking out over the ocean with a long telescope. The sailor is watching for threats. Once a threat is found, then comes the warning to the crew below. A watch, then a warning.
On the forecast graphics released by the National Hurricane Center, hurricane watches are denoted in pink, while hurricane warnings are in red. A lot of focus is on the hurricane cones, but watches and warnings are an important detail to track when a storm is looming, according to Cangialosi.
“It’s always best to rely on where your watches and warnings are, and where the impacts are set to be,” Cangialosi said. “Because the cone is just a general depiction of where that center (of the storm) is going to go.”
Take a look at the side-by-side comparison in the image below. Forecasters issued these two advisories three hours apart on Sept. 26 as Hurricane Ian was barreling toward Cuba. Notice the Tampa Bay area: At 2 p.m., the region is shaded in pink. That means we were under a hurricane watch, where hurricane conditions were possible.
But by 5 p.m., forecasters elevated the risk for the region by placing Tampa Bay under a hurricane warning. The graphic on the right now shows Tampa Bay shaded in red. Hurricane conditions went from being possible, to being expected. That one change to the forecast can help better inform your preparation efforts.
Now let’s talk about storm surge
A traditional forecast graphic — the ones you normally see being passed around online by Floridians in the days before a storm — don’t highlight one particular threat that has accounted for roughly half the deaths from landfalling cyclones over the past five decades: storm surge.
To address that, the National Hurricane Center in 2017 started issuing a storm surge watch and warning graphic “intended to separate” the advisories from the previously existing wind watches and warnings, “and serve as a call to action.”
Similar to a hurricane watch, a storm surge watch is the possibility (there’s that word again) of life-threatening inundation from rising water moving toward land within a specific area and generally within 48 hours of a tropical cyclone. Forecasters can also issue a watch earlier if conditions could limit the time needed to make surge preparations.
A storm surge warning means there’s a danger (more certain now) of life-threatening surge inundation generally within 36 hours.
Forecasters release their storm surge graphic roughly 30 minutes after each hurricane advisory goes live. It’s subject to change every six hours, according to the National Hurricane Center. A storm surge watch is denoted in magenta, while a storm surge warning is in lavender.
Right now, the graphic takes into account a myriad of variables, including surge flooding, tides, land elevation, track uncertainties and flooding as a result of levee failures.
But it’s also noteworthy what the graphic doesn’t take into account: freshwater flooding from rainfall and wave action during a high storm surge.
On 11 a.m. Sept. 27, the day before Ian made landfall in Southwest Florida, a storm surge warning stretched along the majority of Florida’s Gulf Coast. The graphic published by the National Hurricane Center depicted the areas under watches and warnings.
“Life-threatening storm surge from Ian is increasingly likely along the Florida west coast where a storm surge warning is in effect, with the highest risk from Fort Myers to the Tampa Bay region,” the hurricane center warned.
Less than 24 hours later, the Gulf of Mexico began swelling toward Southwest Florida. On Fort Myers Beach, scientists would ultimately log up to 15 feet of storm surge. In Lee County alone, the surge killed 36 people.
What to know about models used for forecasting
There are enough forecast models to make your head spin. Yet, when the hurricane season starts, we look to our favorites, scrutinizing thin, colorful lines and their predicted paths.
In reality, there is no one Holy Grail model.
There are a few things to keep in mind when looking at those dizzying spaghetti plots, like where these models come from and what they’re used for.
Meteorologists use models in every day weather forecasting. Some of these models forecast the mundane — from afternoon storms to a burgeoning cold front. Super computers run these models, calculating complex formulas to understand what is happening in our atmosphere.
But in the hurricane season, they are just as useful to forecasters in telling them where a monster storm could end up.
The hurricane center uses what it considers to be the top models for forecasting hurricanes, which come from the United States and globally.
There is an ongoing debate over which is the best weather model. It generally comes down to two: The GFS (the Global Forecast System) or the European model (the European Center for Medium-Range Weather Forecasts).
However, the quarrel over the best model isn’t particularly helpful. Forecasters rely on multiple models, not just one. The hurricane center’s forecast track generally takes the average of five or six of the world’s most accurate weather models to create its forecast.
For forecasters, there’s an advantage to having multiple models because it helps them better understand the uncertainty of a storm.
“If you look at the top six models, pretty much any of those things could come true,” Jeff Masters, a meteorologist for Yale Climate Connections, previously told the Tampa Bay Times. “Don’t put your trust in any particular one. The average of them is usually going to be the best forecast.”
• • •
Rising Threat: A special report on flood risk and climate change
PART 2: Even weak hurricanes can cause huge storm surges. Experts say people don't understand the risk.
PART 3: Tampa Bay has huge flood risk. What should we do about it?
INTERACTIVE MAP: Search your Tampa Bay neighborhood to see the hurricane flood risk.
