Reliance on GRIB Files

[Dockhead]

Originally Posted by DumnMad

What do you mean here by 30ft breaking ??
Kelaerin appeared to be in deep water.
In 60kts and open ocean I've seen 30ft waves but only the top 10ft or so appeared to be breaking.

Compare this with a shoreline where you see quite small ocean waves rise up to a breaking height of over 10ft. e.g. Google Greymouth bar.

The top 10 feet falling off a 30 foot wave is quite enough to destroy your boat. These people were rolled. What else do we need to know?

I was knocked down (thank God not rolled) by a wave like that and don't ever want to experience it again. True that was in the shallow, funnel-shaped North Sea, where bad sea states happen quite easily, but bad sea states and dangerous breaking seas exist in deep water, too. In this case, if you look at the wefax chart, two systems were pushing together, which means two wave systems were interacting with each other. That is another classic scenario for dangerous, breaking seas.

People get hung up on wind speed, and that is a mistake. High wind speed is harmless without conditions for breaking seas. In the Solent, people go out all the time in 50+ knots of wind, because unless the tide is running against the wind, there cannot be a dangerous sea state with so little fetch, and they like to practice using their storm sails. As you said, 60 knots in deep water is often completely harmless, making a big long period rolling swell which is not a problem even for small boats. But get conditions for breaking seas, and even 30 knots of wind can create survival conditions -- in the Gulf Stream, for example, with the wind blowing against the Stream, or when two systems are pushing against each other, as in the Kealerin case.

[Dockhead]

Originally Posted by Dsanduril

One thing I frequently see with GRIBs is a too-narrow focus. You're going 400 miles and you grab a GRIB to cover that area. When just outside, in the shadows lurks...

If on any length of a passage you really need an ocean-scale picture so you can see what might be gaining on you. You can get that with GRIBs (we tend to use a 1 degree GRIB for local conditions and a more spaced GRIB for ocean level). With WeFAX you're essentially forced to look at ocean level. That's the only size they transmit. You can get similar information from a GRIB, but you have to actually set it up that way and many people don't.

I think this is an excellent point, actually a key point.


GRIB files, particularly those showing just a small area, show only the predicted end RESULT. They don't show the underlying weather processes and so you don't have any grasp of the likelihood that something might turn out differently.

Besides the likelihood of different wind speed and direction, there is also the issue of SEA STATE, which we were just discussing. Sea state, not wind speed, is what creates danger, and sea state is not a direct or simple result of wind speed.

The Kealerin case might actually be a really good example of this very point -- see the two systems running into each other?

The GRIB wind arrow is showing 40 knots of wind in one place, but Kealerin might not have seen even that much in the GRIB wind file. If they had looked at the wefax, however, they would have seen a much more ominous picture.

[Dockhead]

Originally Posted by Paul L

. .. For longer term forecasts the WeFax aren't available...

Indeed, but wefax charts show the underlying PROCESSES, rather than just the predicted end result, so if interpreted with at least a little knowledge, can give a feel for what is going on and what might happen later, and particularly, what might be the risks of conditions being different from what the GRIB wind files predict.

I don't claim to have this knowledge, but I am studying and trying to gain it.

[Dockhead]

Originally Posted by Paul L

Too much info is not helpful. When we are waiting to decide when.to leave we download multiple GRIB models and forecasts. They invariably are close for the first few days. If they diverge significantly in the later days then I have little trust in either forecast.
Offshore I get only one model.downloaded and make my decisions based on it and what I can.see out the port. Having conflicting info while trying to make decisions in trying offshore conditions is not helpful. Take the base info and, as captain, make your decision and move on. No matter what you see forecast you will still deal with whatever weather you get. The only weather conditions you absolutely get to pick is the conditions when you start the passage.

I was on a passage once with a confusing weather scenario after the first 3 days. One of the boats on the same passage had info from the 3 different shoreside weather routers. Each had a different waypoint for where they should head to minimize issues with an.oncoming weather system. Two of the waypoints meant adding many miles and hours of motoring away from the rhumbline. Rather than getting comfort from the additional expert info from shoreside routers, the skipper ended up with dealing with a lot of unneeded stress, lost sleep and running low on fuel.

Another excellent point.

Some people say it's "lazy" to rely on one source, but it's not that simple. It is worse than useless to have multiple sources of information if you don't have any way of choosing between them. Using three weather routers is insane.

Making decisions based on weather is a whole process which can't work if you don't have some way of efficiently getting at your BEST GUESS at what things will be like, and here the simple GRIB wind files are the easiest -- which means not just satisfying lazy people, but most EFFICIENT to use when you're making complex decisions about routing and just need some BEST GUESS even if you know it may not be all that accurate.

For this reason, I gave up long ago trying to choose between the different computer models and just use GFS for the ocean (different ones for places with a lot of land around like the Baltic).

But I ALWAYS read the official maritime forecast for the day and the next, which with amazing consistency catches out errors in the GRIBs, and I am trying to understand other weather information -- not competing computer models -- in order to put the predicted conditions into context.


As others have said, we cannot know with great certainty what the weather will be like more than a few days ahead, and it's important to know what we don't know and can't know, and be ready for it. And there is a certain risk that on a long passage in difficult latitudes, we will encounter survival conditions, no matter how well we have analyzed the so-called "weather window" we sailed into.

[conachair]

Originally Posted by Dockhead

Indeed, but wefax charts show the underlying PROCESSES, rather than just the predicted end result, so if interpreted with at least a little knowledge, can give a feel for what is going on and what might happen later, and particularly, what might be the risks of conditions being different from what the GRIB wind files predict.

I don't claim to have this knowledge, but I am studying and trying to gain it.

Don't get any of that..... What process are you talking about that appear of synoptics but aren't visible on gribs? You can get a 2deg wind/pressure 5 day 24h grib of the atlantic for below 100Kb which I actually find easier to watch how the sytems are developing with the mouse wheel than flipping through synoptics, having both are great of course, but no idea what "processes" there are, both are "end result" predictions for a specific time. Both show the tracks of the pressure systems up to that point. Just get a big enough grib, doesn't have to be high res.
Biggish download for RMS on a ham set though, which is where both grib & Wfax are such a good combination.

[Dockhead]

Originally Posted by conachair

Don't get any of that..... What process are you talking about that appear of synoptics but aren't visible on gribs? You can get a 2deg wind/pressure 5 day 24h grib of the atlantic for below 100Kb which I actually find easier to watch how the sytems are developing with the mouse wheel than flipping through synoptics, having both are great of course, but no idea what "processes" there are, both are "end result" predictions for a specific time. Both show the tracks of the pressure systems up to that point. Just get a big enough grib, doesn't have to be high res.
Biggish download for RMS on a ham set though, which is where both grib & Wfax are such a good combination.

I suppose if you are using large enough scale GRIBS (someone else's point above) you can recognize systems by pressure and isobars, and it may be that with enough knowledge, or anyway more knowledge than I have, this is all the information you need. But the GRIB files don't identify cold fronts, warm fronts or show the shape of those systems, which do NOT follow the isobars. Maybe you can discern these things from the GRIB files; I cannot. There is also human interpretation in the synoptic charts, which is not present in the GRIB files.

[conachair]

Originally Posted by Dockhead

But the GRIB files don't identify cold fronts, warm fronts or show the shape of those systems, which do NOT follow the isobars. Maybe you can discern these things from the GRIB files; I cannot. There is also human interpretation in the synoptic charts, which is not present in the GRIB files.

You can see the fronts with just a little practice, overlay a grib with rain and gusts visible over a synoptic and they are quite easy to see. Wfax doesn't show very well just how nasty a front is forecast which grib wind gusts/rain give a better indication of, also just how much the wind will veer. Not sure how accurate the wave data is, but it's there if you want it. Very educational, IMHO, looking at gribs on top of wfax charts.

And remember there is a mass of data available in grib files which is in a wfax chart image, CAPE, wind gust, rain, - If anything it's a bandwidth issue at sea, just getting the data which is where Wfax is great, free and just need a cheap receiver. And with GFS gribs you're just getting just the one model, where the NWS are looking at many. Still easy to lay a grib over a 60H MSLP to check if they are in agreement though.

[conachair]

Having a bit more of a play, the grib wave data might actually be useful. This is a nice resource, probabilistic wind data, this for 6am tomorrow wind over 25Kt>

NAEFS Probabilistic Wind Speed Guidance



And MSLP with grib and waves coloured , same time >



But end of the day what do you want? IMHO grib over synoptic gives a very good way to look at the data and hopefully try to not get hammered, if they both look similar then chances are it will be close. If they are different then one is wrong.... but which one.....or both?

[Kenomac]

Originally Posted by Dockhead

The choice of this or that boat won't always make that much difference. As Paul L said, a 30 foot breaking wave will roll and devastate any cruising boat, and the wind speed is never the issue -- it's the sea state.

Your much-posted video did not show any kind of extreme conditions -- it's always hard to judge wave height from a 2D video, but the horizon is visible in that video, so the seas were not higher than your eye height, and they were not breaking. Those conditions are not threatening even to day sailers. The wind speed is irrelevant.

You are certainly right that you can't be sure about weather forecasts, certainly not long term ones -- something we should all keep in mind -- but the choice of boat is not a panacea. You need tactics, you need proper gear (Jordan Series drogue for me), you need skill, you need stamina, and after all, you actually need some luck. Nothing wrong with a good, stout, seaworthy boat (especially a LARGE seaworthy boat -- size counts a lot), but that's just one part of the equation, which by itself will not get you out of every situation.

My take on the Kelaerin disaster is that the boat had nothing to do with it -- they made a few mistakes which any of us could have made, including possibly using inadequate weather information -- but mostly they were just really unlucky. The boat held up fine -- was not holed, not dismasted, just took a bunch of water down the inadequately secured companionway, and then their pumps clogged, like ALL bilge pumps will do in that situation. Do you have a proper dewatering pump on board? Do you always secure the companionway? Is your washboard latch strong enough to withstanding a direct hit by 5 tonnes of water? We all make mistakes on this level, every day, at least -- I certainly do. There but for the grace of God . . .

Originally Posted by Kenomac

There’s also another angle to explore which will contribute to the same outcome on other boats when the owner relies too heavily on weather reporting.

One also needs a boat which can handle more extreme weather conditions, and as much as many on this forum will argue against, the fact is.... not all boats are created equal.

What I’m trying to say, is you can’t rely 100% on weather forecasts and need to be prepared and have a boat that’s well-suited for the unexpected.

Maybe this visual will help.

[belizesailor]

Originally Posted by conachair

Don't get any of that..... What process are you talking about that appear of synoptics but aren't visible on gribs? You can get a 2deg wind/pressure 5 day 24h grib of the atlantic for below 100Kb which I actually find easier to watch how the sytems are developing with the mouse wheel than flipping through synoptics, having both are great of course, but no idea what "processes" there are, both are "end result" predictions for a specific time. Both show the tracks of the pressure systems up to that point. Just get a big enough grib, doesn't have to be high res.
Biggish download for RMS on a ham set though, which is where both grib & Wfax are such a good combination.

An important different between GRIBs and synoptic charts which pertains to the "process" is that synoptic charts show weather features like ridges, trofs, tropical waves, etc which are not shown on GRIBs. To me, this is very helpful in understanding the dynamics of the overall systemic weather patterns and how they might evolve.

For example, Im doing weather routing for a friend now. Im using many different products/tools/models, but mostly Im comparing GRIBs to synoptic charts. With the synoptic charts I can see the forecast position of tropical waves and this damn persistent high pressure ridge N of the Carib basin that just wont dissipate. GRIBs show me at a higher resolution the anticipated affect of these features.

As posted earlier, I think the most prudent methodology, as a correlary to Bowditches oft quoted axiom, is to use all means at your disposal to evaluate the weather...and today we have a bewildering array of options.

[belizesailor]

A few related notes on things I find useful most recently:

"Windy" is an awesome app for weather data visualization. In addition to just visualizing basic parameters like wind/seas, there are a couple of features Ive been using a lot lately (we just finished out cruising season and Im now ashore doing weather routing for others): Multiple Model Comparison: with one click you can compare the output of several models. I use this as one data point to assess forecasts confidence: if they all generally agree, then those conditions are pretty likely, if not, maybe not. High Altitude Winds: a really cool and useful feature is that by using a simple slider bar you can choose the altitude of wind forecasts. I found this really useful recently to visualize the process associated with an unusual weather situation: Forecast CAPE index was crazy high in an area (over 5,000 in one spot! Thats biblical weather range...like F5 tornados!), but no severe weather was forecast. I didnt understand how that could be, until I used this feature to explore upper level winds...they were honking and creating wind shear which prevented any tropical weather development. Standard GRIB datasets/viewers wont do this for you.

ECMWF. This is a privately development model which can be accessed via Windy. Ive only used it in a limited area so far (Panamá), but have found it to be quite good...typically much more accurate in this area than GFS for example.

ASCAT. Actual wind obsevations via satellite. I use these to compare actuals to forecast to gauge forecast confidence. Link:

https://www.ospo.noaa.gov/Products/a...cat/winds.html

Atlantic Discussion. A text based product that discusses current significant weather features, giving additional detail to charted features and insight into how the pros expect these to develop. Link:

https://www.nhc.noaa.gov/text/MIATWDAT.shtml

[belizesailor]

Originally Posted by conachair

You can see the fronts with just a little practice, overlay a grib with rain and gusts visible over a synoptic and they are quite easy to see. Wfax doesn't show very well just how nasty a front is forecast which grib wind gusts/rain give a better indication of, also just how much the wind will veer. Not sure how accurate the wave data is, but it's there if you want it. Very educational, IMHO, looking at gribs on top of wfax charts.

And remember there is a mass of data available in grib files which is in a wfax chart image, CAPE, wind gust, rain, - If anything it's a bandwidth issue at sea, just getting the data which is where Wfax is great, free and just need a cheap receiver. And with GFS gribs you're just getting just the one model, where the NWS are looking at many. Still easy to lay a grib over a 60H MSLP to check if they are in agreement though.

Thats pretty slick. Thats all done w OPENCpn right?

[MartinR]

I think GRIB or WFAX will both give reliable info for the next 24 hours, but even then, if you sail in the weather kitchens of the moderate latitudes, weather can develop very quickly.
More than 3 days, and the uncertainty of the models is very high. For long passages, there is not much you can do to avoid the weather, except preparing yourself and the boat.

Even on a big ship, where we can reliably do 15 knots and use weather routing, we get caught many times. The major weather systems are just to big to avoid.

For short passages I find the GRIB data very useful for planning. As Belizesailor writes "Windy" displays lots of useful data. Also, depending on the model, down to local variations.

[hpeer]

But this particular boat was on a roughly 21 day route. IMHO GRIB times are pretty good 48 hours out and of some use at 72 hours. But once you are a few days out of port you are looking at weather that is going to happen pretty close to where you are. You might be able to make some improvement in your situation, but not huge.

How much does it really matter in the real world? Not saying to ignore it, but isnit it really a pretty bit too littl too late?

Does this make any sense to you guys?

[Dsanduril]

Originally Posted by hpeer

....How much does it really matter in the real world? Not saying to ignore it, but isnit it really a pretty bit too littl too late? ...

On the last passage of this length (actually 20 days) that we did we changed course based on weather review about half a dozen times and sailed about 800 miles further than rhumb line. In the end that meant we never saw wind forward of about 60 degrees or over about 30 knots. An after the passage review of actual weather showed that it could have been a lot worse (the worst would have been hard on the nose into about 40-45 knots) if we hadn't spent time bobbing and weaving around systems.

In the bad old days I would have said yes, once you get two or three days out using a good window you got whatever you got after that. But with communications, data, and forecasting as they are today getting an update every day that gives you two or three days notice of upcoming weather is not impossible, and thus it does become possible to materially alter your circumstances.

Certainly not infallible, so you need to be prepared for when the weather information just isn't right or the system is just too big, but a worthwhile effort nonetheless so that you reduce the chance you'll use those preparations (and unfortunately that may lead to complacency, which is part of the reason for this discussion).