Forecasters did a great job predicting the track of Hurricane Iselle as it headed towards Hawaii last week. Even almost a week out when the storm was a thousand miles from the Big Island, meteorologists had the hurricane's track pegged to within 30 miles of where it made landfall.
One of the mantras of weather forecasting is that it's still largely an inexact science, and that's especially true for hurricane forecasting. Last week I wrote an explainer demonstrating why the "cone of uncertainty" is the most important part of a hurricane forecast. In short, the cone is a visual representation of the margin of error in the prediction. The line going through the center of the cone shows where the forecaster expects the center of the storm to travel, and the cone shows where the center winds up 66% of the time based on the previous five years' average.
As uncertainty grows with time, so does the cone.
Hurricane Iselle was no different. Pretty early on it became clear that Iselle posed a serious threat to Hawaii. By Sunday, Iselle was about 1,000 miles away from Hawaii , and the country's fiftieth state entered the 5-day forecast window.
To demonstrate how sound the forecast track was, I used three of the earliest advisories that brought Hawaii into the storm's path — Sunday, Monday, and Tuesday — to show how close forecasters came Iselle's Thursday night landfall right on the money.
Sunday's forecast from the National Hurricane Center was the best, which is amazing because it was so far out. In the eastern Pacific basin, on average, the NHC predicts the location of the center of the storm at the five-day mark to within 212 miles of its actual location 66% of the time.
The NHC's forecast on Sunday, five full days before landfall, nailed the general location of Iselle's center to within 30 miles. That's incredible, especially that far out.
Advisories switched over to the Central Pacific Hurricane Center shortly thereafter, and their forecasts did pretty well. Monday went a little off the rails, falling about 85-90 miles away from where Iselle wound up hitting, but Tuesday brought it back to within 50 miles of the landfall location.
Of course, the elephant in the room is the hurricane's strength. Over the past 20 years, meteorologists have made great strides in dramatically improving hurricane track forecasting thanks to better algorithms in the weather models, but hurricane strength forecasting is a different beat entirely. Models can get a good idea of what storms will generally do, but when it comes down to fluctuations in wind speed, it can be tough to forecast even 6 or 12 hours before the storm changes.
Any number of things can affect a storm's strength, from dry air, ridges of high pressure, fronts, sudden thunderstorm development (or collapse), to interaction with cooler waters or even land. The forecasts didn't anticipate Iselle to suddenly restrengthen in the face of a huge onslaught of dry air a day or two before it hit Hawaii. Forecasters caught guff on social media for saying that Iselle would be a hurricane at landfall, when it weakened to a somewhat strong tropical storm with 60 MPH winds just before it hit the Big Island.
That change from 75 MPH to 60 MPH is relatively negligible, but it means the difference between a "hurricane" and a "tropical storm."
All in all, meteorologists at both the NHC and the CPHC did an incredible job forecasting this hurricane, even given the challenges of its fluctuating strength and speed, along with the uncertainty of how it would interact with Mauna Kea and Mauna Loa on the Big Island.
If you watched the national media (save for The Weather Channel, surprisingly), you got a completely different view of the storm. News networks kept billing Hurricane Iselle and Hurricane Julio, which followed a few hundred miles behind the former, as "double trouble" and "unprecedented" in the recorded history of Hawaii.
The only problem is that it became clear pretty early in the week that Julio would likely pass safely to Hawaii's north. The island chain stayed firmly in Julio's cone of uncertainty, but except for a brief period on Monday, the center was always forecast to safely move north of the state.
Those media outlets would have been okay had they explained the forecast a little more, but nuance isn't exactly their strongsuit. It was up to the public to do research on their own, finding out more information beyond the flawed "two hurricanes are about to slam Hawaii in a history-making event never before seen in human history" angle.
Forecasters did a great job predicting Iselle and the media didn't do a good job covering it. That's the story of pretty much every major weather event, these days.
[Top image via NASA, maps by the author]