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Old 25-09-2014, 09:34   #1
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What good are gribs for real conditions?

I am planning to jump the 300nm over the sea of cortez from Puerto los Cabos to Puerto Vallarta and I had a look at forecast conditions from multiple sources yesterday.

All the "grib pictures" from passsageweather.com, buoyweather.com, sailflow.com, NOAA surface charts, etc. showed easy conditions.

In fact, an enormous cell with 35kt winds, spanning the whole distance of the passage was out there. You would not have known it without color coded satellite imagery.

Furthermore, even on passage with a SSB + modem, or weatherfax, or Iridium + satellite image service, I might see a black and white depiction of a big cell if I requested the image at just the right time, but with out color coding, would I have indications of intensity or movement?


My conclusion is that near real time color coded satellite imagery is the bottom line, and all the above internet weather sources are pretty worthless for anything but macro views of whats going on or about to happen. And in fact are dangerously misleading when it comes to predicting passage conditions.

Or am I missing something?
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Old 25-09-2014, 10:13   #2
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Re: What good are gribs for real conditions?

Did you check out the CAPE? The wind GRIB files only show the general average wind speed and direction. Local winds due to vertical instabiity are not included and can not be included.

You need the CAPE values to make predictions about the potential of vertical movement and with that the likely hood of squalls.

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The above screen shot from Weather 4D shows the CAPE just before noon today. The value is over 2700J/kg. That is extreme and I would not go out even with zero underlying wind. Any CAPE over 1000 is questionable, over 2000 you want to be extremely careful.

Weather 4D shows the CAPE index, if selected. Some other GRIB viewers do not have that option. I find it extremely important for passages, especially how much to reef for the night.

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In the above screen shot I added a route from the north to the south through the Sea of Cortez with a graph in the top right showing the CAPE along the route as you travel.

The vertical green line on the left is the beginning of the trip the one on the right is at the end of the trip. The values are for an average speed of 5 knots.

I find Weather4D provides a great safety feature.
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Old 25-09-2014, 10:14   #3
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Re: What good are gribs for real conditions?

Gribs are only as good as the weather models that feed them. They are a lot better than they were 20 years ago, but they aren't perfect and they don't spend a lot of effort tracking that area. The WFAX is supposed to have humans checking things like the satellite imagery, but they must have been asleep yesterday. However, you can't depend on imagery to forecast what is going to happen in 24-48 hours.

We did the same trip and got slammed by an unpredicted 35k from the SE. We ended up in Mazatlan with water in the fuel tank and a cracked bulkhead.

In any case, I wouldn't dream of the leaving now given that TS Rachel is wandering your way--unless you feel unsafe in Cabo.
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Old 25-09-2014, 11:19   #4
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Re: What good are gribs for real conditions?

Quote:
Originally Posted by roetter View Post
I find Weather4D provides a great safety feature.
I love Weather4D and use it a lot (the Pro version). But I did find it was a hard nut to crack. You need to figure out which models to use and how to get them (tidal info from TidalTech needs a subscription, for instance), and the interface is not always intuitive. How long did it take me to find out you have to press the middle of the screen to switch on/off the various data you want to see.....

The manuals are getting better but still it is a bit of a hard graft figuring it out. Once you do, it really is very good.

I love the way that you can import your route, insert your departure time and then see how the weather, tide, waves and CAPE will be around your boat over the course of the passage.

You can't rely on it too much - you need other information such as the 500hPa charts - just like previous posters have said, the underlying models can be fairly basic.

But all in all, it has served us very well. When we crossed the Bay of Biscay, the depressions hit us down to the minute when they were predicted, which I was impressed by.


Onno
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Old 25-09-2014, 12:05   #5
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Re: What good are gribs for real conditions?

As long as you know what you are looking at and what the limitations of the tool are, all sources are good.

Get wefax, get gribs, get radar. But first and foremost get sound understanding of what the atmosphere is doing, when and how.

There are no "bad" sources but there are plenty of bad ways to use any source.

I found that seeing what is out there is just the beginning. Then one must "guess" what comes next, and that's where weataher forecasting and routing fun begins.

b.
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Old 25-09-2014, 12:11   #6
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Re: What good are gribs for real conditions?

Gribs are based on the GFS (global forecasting system) model. As such, they cover very large areas and do not forecast localized conditions. 300nm is not a long way on a global scale. I have found gribs to be pretty good over long distances, although they do, in my experience, tend to get the wind direction pretty much right but under-forecast wind speed by 5 to 10 knots. If you're only going 300 miles you're better off using local weather forecasts. The main advantage of gribs is that they are tiny files that can be downloaded relatively cheaply on a sat phone. They are not meant for hopping across a 300 mile gap.
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Old 25-09-2014, 12:35   #7
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Re: What good are gribs for real conditions?

Roetter,

You're the man.

Having "kJ/kg of vertical stability estimates" for an intended route would seem to be the missing tool I was looking for. As you say "underlying wind estimates" (gribs) showing benign would not be a green light. In fact, that could get you into trouble.

I'll spend some time getting to know this site.

Thanks!
Journeyman
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Old 25-09-2014, 13:04   #8
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Re: What good are gribs for real conditions?

just fyi . . . . the freeware zygrib also can show Cape, as well as a whole bunch of other grib data.
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