Saturday, August 27, 2011

All Weather Is Local (I Hope)

I expect that I'm not the only person in the north-eastern US who has been watching, and listening to, and checking out online the weather forecasts over the last few days. As Hurricane Irene approaches of course I want to know how bad the winds will be. What can we expect?

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?

First 8am...


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.

Hmmm.

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.

13 comments:

Baydog said...

In any case, as RW Rawles says, it's gonna be windy enough to blow dogs off the dock.

Tillerman said...

But maybe not windy enough to blow dogs off chains.

Turinas said...

This post totally did my head in. All I know is the more I try to understand this wind thing the less I have a clue.

This and boat electrics are my top 2 things I need to learn about

Tillerman said...

Me too. I suspect that what I've written here is old news to many sailors. But I hadn't realized how much these models differ so much before.

JP said...

Good points - its always best to know as much as possible about the assumptions behind inputs to simulations when assessing the output.

At work have a similar discussion about terrain databases - but even with the highest res data (i.e. lowest grid size) the standard deviations on the outputs can be large.

There ought to be a background button for data on the internet - such as for surveys how was the sample selected, how exactly were the questions phrased etc otherwise it is hard to know how to interpret them

Sam Chapin said...

I hope t-man has his house all shuttered up and trees trimnmed, battery radio, generator on line and food and water etc, etc.

Andrew said...

You will let us know which model was right I trust. We're all hoping for the same one, hold on tight.

Tillerman said...

Definitely think the WRAMS 2k model was much better at predicting the day's weather at our specific location.

There is higher ground to our east so even though a weather station on the other side of the bay recorded a gust of over 70 knots at 9:30am this morning we never experienced anything like that at our house from the easterlies this morning.

The wind did go south in the afternoon as predicted and I expect it will go west as the evening progresses, but it's definitely been slacking off during the afternoon so it seems the worst is over.

We did get a couple of power cuts during the day but the power is back now, and there was no major tree damage in our immediate vicinity.

Pandabonium said...

Happy to know you, uh, weathered the storm.

Forecasters know that you are wise enough not to venture out on your Laser during a hurricane/tropical storm, so see no point in fine tuning your local forecast.

Carol Anne said...

Glad to hear you weathered the storm all right and that you didn't lose power for very long. One factor that was cited in the television news for the storm not being so powerful by the time it got to New York and New England was that it bumped into the coast a few times on the way north, and each time it bumped into land, it lost power.

Sam Chapin said...

Good to here things are OK.

John in PDX said...

Wow,
The funniest shot we saw was Al Rocer in foul weather gear in front of some huge waves - superimposed from the studio. No risk of Al getting wet. We loved the news dorks in full foulies and in the background guys with no shirts walking down the street.

I am so glad that we are far enough away from the big media markets that we don't make national.

80 mph in Oregon - no problem - it doesn't make national news. Bring in the garbage cans.

6.0 Earthquakes - no big deal. The furnace and the water heaters are anchored in concrete.

Snow is our big one that you easties would fall over laughing about. They regularly close schools because they think it might snow (it usually doesn't).

Pat said...

And Phoenix didn't close schools for the haboob, though there was a bit of sweeping up afterward. We're glad to hear that your location had some protection. Would it be interesting to compare the official recorded wind strengths for your area with what you actually experienced?

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