Editorial: Conservative Forecasts Can Give False Sense of Security


This is an opinion piece by someone who is only an amateur meteorologist.

The National Hurricane Center (NHC) is the leading authority on tropical weather, and it does a fantastic job. Any time a storm is headed our way it’s important to make sure you are keeping up with official NHC and especially the local National Weather Service (NWS) forecasts either through their websites or social media.

That being said, some products (their word for advisories, discussion, graphics, etc) put out by the NHC can lead to some confusion – even for hurricane veterans.

The NHC is very data driven and also rather conservative with its forecasts. They like to use percentages to show possibility of impacts like rain and wind. Seems simple enough, except those percentages are based on time periods, and they change as storms get closer.

For instance, we have a 50 percent change of hurricane force winds within the next five days according to the NHC’s 11 am update. Yesterday we had a lower chance. Three days ago it was in the 10 percent range. We finally got into the 100 percent chance of tropical storm force winds a few forecast cycles ago.

But the hurricane track and intensity on Thursday/Friday – the time it has been forecast to get here – has barely changed at all. The only major change is how far away from impact we are, yet the percentages changed a LOT.

This can give a false sense of security a few days out from a storm when it is imperative that people begin preparing to hunker down or leave. I’ve fallen for it myself.

On top of that, if you analyze the forecast discussion and track, you’ll find that the NHC is forecasting that Dorian will go right over or right next to Carteret County. We are in the center of the cone of uncertainty, and right now the boundaries of the cone are extending 80 miles out from that center line. This means Dorian will go either between us and Kinston, or between us and about 80 miles offshore.

Go and look at the forecast discussion and you see that Dorian’s wind field is getting larger. Hurricane force winds – 74+ mph – extend up to 70 miles from the center of the storm. Tropical storm force winds extend 175 miles from the center.

The discussion also tells us that Dorian will be passing or going over us Friday morning at a Category 2 strength.

Hurricane hunters data showing wind speed. Purple, light purple and white are hurricane force winds.

The latest hurricane hunters mission shows that these hurricane force winds are present on all sides of the hurricane. It shows that 70 mile stretch of hurricane force winds more to the southeast, and about 40 miles of hurricane force winds to the northwest. There is incomplete data to the west.

So lets do the math according to NHC information. Dorian will be a Cat 2 with winds of 100 mph when it gets to our latitude. Hurricane force winds extend 70 miles from the center to the southeast, about 40 miles to the northeast and probably more than 45 to the west.. Dorian will only go as far as 80 miles west or east of us. 80-70=10; 80-40=40.

So if Dorian actually makes it to the furthest 10 miles of the cone to the west, we won’t get hurricane force winds. If it goes to the furthest 40 miles to the east, we don’t get hurricane force winds.

Remember that’s out of a 160 mile-wide track. So there are 50 miles of horizontal space the hurricane’s center could go to that keep up out of hurricane force winds. If it goes within the 110 mile area around us, we will.

But, we are told there’s only a 50% chance of hurricane force winds in our area. The data seems to show more of a 68% chance, and that doesn’t take into account that the NHC must have decided to put the center line where it is for a reason.

The only way it’s possible for a Category 2 storm to go within that 140 mile area without us experiencing hurricane force winds is for the windfield to contract considerably. Of course that’s possible, but the discussion doesn’t mention anything of the sort. In fact, the windfield has been widening.

We will almost certainly see an increase in these hurricane wind speed likelihood percentages over the next few forecast cycles. The forecasters know it, the data seems to support it, but we aren’t seeing it yet at a critical time for pre-storm preparations.

I want to be clear that I’m not at all saying to ignore the NHC forecasts. They are considered king among forecasters for tropical storms.

But be aware that NHC has consistantly been conservative with forecasts – it’s a widely known fact in the meteorological world. And it appears they’re even conservative with data.

Take a look at the NHC surge inundation forecast versus the Coastal Emergency Risk Assessment tool run by Louisiana State University and Louisiana Sea Grant which is even based partially on NHC forecasting!

Is it just the “what if things drastically change?” factor that brings those predictions down? If so, why is that given so much weight? Why not be overly cautious?

The beautiful thing about the NHC is that it is constantly upgrading its products and adding new products. We are now able to see storm surge predictions and flash flood predictions, and those products will continue to improve.

I hope in the future we see improvements in a different way – more discussion about why the NHC predicts the percentage chance of impact the way it does.