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post #1 of 6 Old 01-18-2011 Thread Starter
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GFS model v. NAM model

I was looking at the weather - PassageWeather - Sailing Weather - Marine Weather Forecasts for Sailors and Adventurers

I noticed that GFS and NAM give very different wind directions for this Sunday between Lake Worth and Freeport. Can anyone shed some light on how to read this forecast, or it still too early to tell? Thanks.


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post #2 of 6 Old 01-18-2011
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If you want to look at the information direct from the NWS, go to Model Analyses and Forecasts.

Generally speaking, the NAM model is a bit more precise as it's a higher resolution model than GFS. This is, of course, just a general rule of thumb. GFS can be used to give you an idea of longer term forecasts but I wouldn't use it for short term. Because of the higher resolution, the NAM model generally does better predicting precipitation and its effect on atmospheric destabilization. This destabilization is what then translates into wind speed and direction and would explain why you are seeing some of the differences.

Since you are looking at Sunday, I would probably say that the GFS forecast might be more indicative of what you will be seeing, but if there's any precipitation involved neither model is going to be overly accurate. That far out the weather is pretty much a crapshoot - especially these days. GFS tends to have fronts moving at a higher speed than what actually happens (not sure why, but these models are all computer generated so I'm sure it's algorithmic), so once again - Sunday is a long time out for any modeled forecast.

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post #3 of 6 Old 01-18-2011
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I am sure labatt is correct. NAM has a higher resolution that GFS

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GFS model

GFS stands for the Global Forecast System. It is run by National Centers for Environmental Prediction (NCEP) which is a unit of NOAA (National Oceanic and Atmospheric Administration), NWS (National Weather Service), USA. The GFS is run four times per day (00 UTC, 06 UTC, 12 UTC, and 18 UTC) out to 384 hours. Since July 2010 it is run with resolution of 27 km out to 192 hours (previously 35 km) and then with lower resolution for up to hour 384. GFS datafiles that are currently available from NOAA have resolution of 0.5 degree (about 50 km) so this is the real resolution of products that you can see here or on other websites that use the same source. Windguru only uses 180 hours of forecast, because beyond this it's more "crystal gazing" then forecast I think . You see that the model resolution is not that great but it covers the whole Globe!

NAM model

The North American Mesoscale (NAM) is a regional mesoscale model using enhanced terrain and improved parameterization of surface and precipitation processes. It is also run by NCEP, formerly it's name was Eta, but in early 2005 it was renamed to NAM. Windguru uses 84 hour forecast data in 3 hour step which covers Northern America with average resolution of 0.11 degree (about 12 km). Updates four times per day. Much higher resolution than GFS should give more precise short term forecasts.

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post #4 of 6 Old 01-19-2011
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You may find it interesting to review NWS's own catalog of known "biases" of the different models. See for example Model Performance Characteristics

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post #5 of 6 Old 01-19-2011
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What everyone should be doing is studying weather and learning to read the clouds the sea and why is the barometer is falling or rising. Thus doing your own local forecast. This is in addition to getting weather reports from other sources.
Combining your home grown skills with the additional weax reports will help you read the weax so much better, to the point where the squalls, micro bursts and thunderstorms don't surpise you or find you unprepaired.
A minor detail: I've had about seven bone breaks & fractures. I know when there is a cold front moving in rapidly. This is a painfully acquired tool. I don't recommend it to anyone... Right now am dealing with a slowly healing broken ankle.

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post #6 of 6 Old 01-19-2011
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Quote:
Originally Posted by Boasun View Post
What everyone should be doing is studying weather and learning to read the clouds the sea and why is the barometer is falling or rising. Thus doing your own local forecast. This is in addition to getting weather reports from other sources.
Combining your home grown skills with the additional weax reports will help you read the weax so much better, to the point where the squalls, micro bursts and thunderstorms don't surpise you or find you unprepaired.
A minor detail: I've had about seven bone breaks & fractures. I know when there is a cold front moving in rapidly. This is a painfully acquired tool. I don't recommend it to anyone... Right now am dealing with a slowly healing broken ankle.
Agreed - this is somewhat like the bow thruster thread. Use the tools available to you and remember that sometimes some of those tools will not be available. Keep lots of arrows in your quiver. After getting caught once by an unexpected low, I will not sail without a barometer. My watch has one.

I have found that many of the models are misleading when they cannot account for local weather conditions, like the Qualicum and Squamish in the PNW. They are good for more open waters.

But the old Mark I eyeball is good for weather as well as navigation.

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