Who to Believe

A friend turned me on to this weather website: www.ventusky.com. I must say it is a beautiful sight and my buddy who cruises a lot says it is the most accurate site he uses. So I logged on and was surprised (but not shocked) to see a marked variance relative to predicted wave heights. NOAA predictions appear to be much higher than Ventusky. Who do you believe and why?

What I see is what I believe. I focus more on wind speed, direction, and wind & wave data over the previous 24 hours.

Ted

Funny, because we already tend to find NOAA is somewhat conservative in their wave height predictions. I can't tell you the number of times reports of 2-4 were really 4-6. We were surprised a couple of years ago (though we really shouldn't have been). We were sitting on the dock waiting for a hurricane to pass well offshore. Wind was around 20-30. NOAA reported wave height as 3-5, which we thought was fairly low and definitely doable. When we got into the thick of it, it was 8 ft and breaking. More inline with what we expected but way off from what NOAA was reporting.

At that time I had a planing boat and knew measurements from waterline to eye level was 8 ft. When you're in the trough and staring at or up to the crests it's easy to tell wave height.

O C Diver said:

What I see is what I believe. I focus more on wind speed, direction, and wind & wave data over the previous 24 hours.

Ted

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Me too.

Ventusky

O C Diver said:

What I see is what I believe. I focus more on wind speed, direction, and wind & wave data over the previous 24 hours.

Ted

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I agree with Ted, but I will add that I use ventusky a lot to go offshore here in the keys, waves have been as advertised. But I do go by wind speed and direction first.

Wifey B: None of the above. Look at forecasts sort of like hurricane tracking where you see all the different models. Well, thing of all forecasts like that and consider several. If one says 4' and another says 6' then hope for 4', expect 6' and don't be surprised by 8'.

I don't believe anyone any more. But I will tell you if I ever meet someone who works for NOAA I'm going to punch them in the mouth !

Egregious said:

I don't believe anyone any more. But I will tell you if I ever meet someone who works for NOAA I'm going to punch them in the mouth !

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Well, the closest we have here is Richard of Dauntless fame. Hope he's prepared to duck if you come his way.

And why would you punch NOAA? They are forecasts with a high degree of anticipated error. Not promises. Better than 20 years ago, but still very far from perfect.

We primarily use Windy, but we also check several other sources and try to make the best of the information to makes a decision. For the most part (like 95%) Windy has been spot on, but one time last trip We and ASD got caught in nasty stuff crossing Dixon entrance. The Canadian weather was the most accurate, and they have a history of being conservative.

So we have learned to take all the information, compile it and hope for the best. As noted above though, sometimes you have to be in the thick of things to know for sure.

Most of the web weather sites are using some of the same computer models. This Ventusky uses HRRR, Icon, GFS and GEM. Which model they use and how they present the data is one of the differences between the sites. The models usually don't take into account local influences. Here's an explanation from Ventusky 's site on temperature:


Data: ICON, DWD (resolution 7 km)
Updated: 12:00 PM UTC (next update: 6:00 PM UTC - prepare)
Current time: 5:00 PM, 2017/10/12 (UTC−04:00)For this output data, temperature is shown for 2 m above ground. The calculations take into account the terrain (elevation), but with lower resolution than in reality. Therefore the models cannot differentiate, for instance, the temperature on a mountain peak or on a city square scorched by the sun. The general rule is that the centres of large cities are 1 °C to 3 °C warmer than the surrounding area or natural landscapes. Significant temperature differences over a small area are primarily caused in the winter by an inversion. A short yet noticeable cooling can also occur after a local summer storm.

We have access to grib files when we're out and once a week we get this email:

This grib file is extracted from a computer forecast model. While such computer data can provide useful guidance for general wind flow, there are limitations which must be understood. What you are receiving is a weather prediction generated by a computer run by NOAA/NCEP (GFS, WW3 models) or the US Navy (comaps, nogaps) and downloaded and processed by Saildocs (a service of Sailmail). The network is complex, and any computer network is subject to hardware and software failures or human error which can effect accuracy or availability of data. In particular, if our servers were not able to download a current data file then the grib-file may be based on old data. The file information is shown above and also contained in the file itself.
Also remember that grib data is not reviewed by forecasters before being made available. You are getting a small part of the raw model data that the forecasters themselves use when writing a forecast, and it is your responsibility to make sure that the data is consistent with your local conditions and with the professionally-generated forecasts (e.g. text bulletins and weather-fax charts).
Grib data also has limitations along shore, where local effects often dominate and may not be adequately modeled. In addition these models cannot provide adequate prediction for tropical systems, frontal activity or convergence zones. For example, while global models can provide useful data on the likely track of hurricanes, they grossly underestimate the strength of hurricanes because of their small size compared to the model grid. For hurricane/cyclone forecasts, carefully monitor the appropriate warning messages and do not rely on grib data from any source.
That all said, grib data can provide useful guidance not available elsewhere. Understand the limitations and use the data carefully. Grib data should be considered supplemental to other forecasts, and not be relied upon in lieu of professionally-generated charts or forecasts.

We go with a minimum of 2 sources when we're out. If they don't reasonably agree, we look at why knowing weather forecasting/modeling is not an exact science. For major passages we usually go with a professional.

BandB said:

Well, the closest we have here is Richard of Dauntless fame. Hope he's prepared to duck if you come his way.

And why would you punch NOAA? They are forecasts with a high degree of anticipated error. Not promises. Better than 20 years ago, but still very far from perfect.

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YEP!

And most weather sites get the info for their predictions from NOAA as well as a few other.

In my experience the weather guessers never look out the window. How can what I am seeing be so different than what they see?

It seems like various areas around the US coasts are easier and harder than others. I will say that the predictions for Harvey from development to landfall to the Houston rain event were absolutely dead-on. Obviously larger weather systems should be easier...day-to-day the Texas coast is about 75/25 for accuracy.

Don't forget, thought, that sea state forecasts are giving the 'significant' wave height, which is the average of the highest 1/3 of the waves, and you will statistically see some during a sample that are much higher, sometimes as much as twice or more. And which ones are you going to remember?

Nice link, will add it to my collection on my decision tree. We found http://stormsurf.com/locals/pens.shtml was helpful in a recent Gulf crossing from Venice to Governors Cut, was spot on, gives an 180 hour wave forecast, great for us 8 knotters!

I have yet to see any forecast stay consistent past about 4 days.... except once in awhile.

Especially wind forecasts which are critical for waves ......so while a 180 hr forecast might be fun to hope for and plan around...I would never take one too seriously.

Kinda like looking at hurricane forecasts out past 5 or 6 days.

Often the day of a trip the bouy reports are already making the wave forecast wince.

psneeld said:

I have yet to see any forecast stay consistent past about 4 days.... except once in awhile.

Especially wind forecasts which are critical for waves ......so while a 180 hr forecast might be fun to hope for and plan around...I would never take one too seriously.

Kinda like looking at hurricane forecasts out past 5 or 6 days.

Often the day of a trip the bouy reports are already making the wave forecast wince.

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I've seen several stay materially on target beyond. For instance, when I looked at Windy on Sunday it showed decent conditions for FLL for this weekend and showed the mass of bad seas well north of here. I can't say that they had the wave heights exact or that they do even now. I'm also sure that the mass of bad has probably moved south or north from what they had. However, in anticipating the next two days would be good days to get out, it was right. Typically they show systems moving in and out further in advance, they just don't have them pinpointed.

Now, you never can fully depend on them because there are times they won't even be close. That's the reason to keep looking and use more than one source.

I've been on the water most of my life and found marine weather reporting poor, but slightly better than 20 years ago. It seems to be better on the Atlantic side. Wind makes waves and learning to read weather maps over time will make you a better forecaster than NOAA. I assume it's all computer forecasts from instruments because none of the local guys apparently open the window.

For 7K trawlers cruising in coastal waters the local over air TV will have all the info required to decide weather to go, or drop that second anchor.

So far we have had good luck with Stormsurf see post #15. But, we do check several and take a good look out the window, trusting eyes and ears most of all.

Given that some coastal reporting areas cover hundreds of square miles and well over 50 miles in length, immediate vicinity accuracy can indeed be less than perfect. Buoy data for coastal cruising has been very helpful for us to assess interrelationships between waves and currents. Rain forecasting is immaterial, not so fog predictions.

When I lived in Michigan near Lake St Claire, we used to joke, they would take down the small craft warning on the weekend, so they could wash the flag.

Location in the country can be a factor too.

There are spots that tend to have slightly more settled weather, especiallh depending on time of year and whether a el nino or la nina year.

But all in all, expecting forecasts to hold within 5 knots (for many with under 50 footers, 5 knots from 10 to 15 or 15 to 20 is huge) for more than 5 days or so is not all that common in my experience.....

...and for many years my job or hobby has had me looking at the weeks forecast.....while improved.....still iffy for that "weekly" forecast to be "right on"..... close, but sometimes close still changes your plans or comfort if you go.

The NOAA weather forecasting models are typically poorer in accuracy than their European and “private” weather models. The Canadian weather models are also poor. The reasons are complex and have been reviewed by Cliff Mass, below:

“U.S. Numerical Weather Is Still Behind and Not Catching Up: What is Wrong and How Can It Be Fixed?”

http://cliffmass.blogspot.ca/2017/10/us-numerical-weather-is-still-behind.html

Jim

When I hear the forecast for wave heights of 2' to 4' I just add them together to get 6' and it may be accurate.