It was only when I started looking at the various wind forecasts available on SailFlow that I realized something that I suppose I already knew from sailing. All weather is local. There is more wind on one side of the course. There is a dead patch in the lee of that headland. Etc. Etc.
And if SailFlow is to believed, this is just as true in a hurricane. What winds you are actually going to experience in any given location - your home, your boat anchorage - are highly dependent on the immediate geography, the shape of the land, the height of the hills near that location.
So what does it mean when I look at the National Weather Service forecast for Narragansett Bay and it says ".sun...se winds 45 to 55 kt...becoming s 50 to 60 kt in the afternoon. gusts up to 80 kt." Gusts up to 80 knots? But where? Everywhere on the bay and its surrounding coasts? I think not.
I discovered that the SailFlow website actually has wind forecast weather maps available using several different models, and that for an area like Narragansett Bay, small bays with lots of islands and inlets, the forecast wind patterns vary enormously depending on the "resolution" of the computer model being used. To quote from the SailFlow site...
Why does the grid resolution of the model matter?
The coastal zone is where computer weather models perform the worst. The primary reason for this is the complexities between water and land temperatures as well as friction and topography. When choosing a model to try to predict the weather in this area it is important to use one that can actually "see" the body of water that your going to be on.
Let's look at some examples for Sunday's forecast based on models of different resolution.
First, to orient you, here is Sailflow's chart of the current winds in my area...
You can see one of my favorite sailing spots, Bristol Harbor, in the center of the chart, Mount Hope Bay and Sakonnet River on the right, and Narragansett Bay and Providence River on the left. I live on the eastern shore of Mount Hope Bay.
Now let's look at the forecast for 8am tomorrow morning, using the NAM 12k model (meaning the grid resolution of the computer model is 12 kilometers.)
And also at the same model for 5pm...
But what if we use a model with a much higher resolution for the same times, the WRAMS model which has a resolution of 2k?
And then 5pm...
Could there be a bigger contrast? The low resolution NAM model is showing consistently strong winds (up to 40 knots+) across the area, slackening off slightly as you go further north (away from the ocean.)
Whereas the higher resolution WRAMS model shows similarly strong winds in the middle of the bays, but significantly lower wind speeds over the land and on the sides of the bays in the lee of the land. (Of course, this is obvious to any sailor.) Most importantly for me, it shows the winds on the eastern shore of Mount Hope Bay as never more than 20 knots or so.
NWS says gusts up to 80 knots.
The NAM model says 40+ knots all over this area.
The WRAMS model says no more than about 20 knots near my house.
Who to believe? I know which model I want to believe!
More background on these models and why they differ on the SailFlow website.