Irma to hit Miami, No, wait...the Gulf coast. Hurricanes throw meteorologists for a loss.
told in “spaghetti” trails. Forecasting hurricanes still agonizingly difficult, even though simulation models are run on some of the world’s most powerful supercomputers. Map by Top Tjukanov.
When you need a weather forecast the most – like when a hurricane is bearing down on you – is when the predictions seem to fail. Why is that?
As Irma came roaring in from the Caribbean with winds of up to 185 mph, weather services which
were able to predict Irma’s birth were unable to predict its behavior.
Days before, we didn’t know where Irma would hit. Maddening to Florida
residents, the path kept moving. They didn’t know which side of the state needed
to evacuate. Miami braced itself for a full assault, only to have Irma veer to
the west.
Image credit: National Hurricane Center.
World’s Biggest CFD Problem
For the last couple of decades, the ECMWF, the European Union’s global
weather model has been the gold standard for accuracy. But it only runs twice a day. And it charges for results.
A US model, the US system, the Global Forecasting System (GFS), runs more frequently –
4 to 6 times a day. It’s known to be good at predicting storms. It keeps getting
better.
The National Hurricane Center, which runs several models, including the ECMWF
and the GFS, to make its “spaghetti charts,” claims increasing accuracy over the years. In 2000, the average miss was 150 miles but now it’s 75 miles.
A two day prediction is the state of the art.
“A hundred miles in a three day forecast is really good,” says Dr. Brian
McNoldy, a researcher at the University of Miami quoted in the New York
Times. Quick to disagree would be millions of people in Florida, when the 100
miles is the difference between one densely populated coast and the other.
The main reason for the uncertainty is the vast amounts of data over a
weather model that is literally the biggest fluid/thermal problem in the
world — and the granularity the scale imposes. A single cell in the global
model has sides that are 40 kilometers (25 miles) long, big enough to swallow
entire cities leaving insufficient resolution to account for local effects. The
widest part of the Florida peninsula would be covered in 6 cells. You will not
know what will happen in your neighborhood, much less to your house.
Weather models also have to deal with ridiculous amounts of input, including radar
images and satellite data.
In a hurricane, not only are parts in motion, but they are travelling fast.
The time slice must be very thin, so more calculations per hour are needed. This
is in contrast to predictions of less volatile weather conditions and forecasts
that are accurate for many days in advance. You can tell on Monday whether the
sun will be out for your hike on the weekend, for example.
We’re Getting Better
Weather researchers insist hurricane prediction is getting better all the
time. A model’s accuracy can be measured in the difference between where it hits
land and and how fast its winds are when it hits, compared to the
predictions a few days out. Predictions of Hurricane Harvey’s (which flooded Houston) landfall varied over a much larger span of coastline
than did Irma predictions, but as the Texas coastline is much less populated than Florida’s,
its uncertainty was less of an issue.
The weather prediction can get a lot better, says Jeff Masters, director of
meteorology at the Weather Underground. He cites a 2007 recommendation that
hurricane research be increased to $300 million a year, presumably
leading to more accurate modeling. While small in proportion to the economic
devastation of a major hurricane hitting a big metro, this is 10 times the amount of research currently
being done in the United States.
With hurricanes getting stronger (this is first time hurricanes of Harvey and
Irma’s intensity have hit so close together), perhaps increased research could
be seen as the prevention that is less expensive than the cure.
