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Old 28-11-2007, 13:41   #1
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grib files question

I was reading a blog about someone preparing to buy and circumnavigate a Mahe 36 and this is from there.




""Kevin's also been receiving daily downloads of grib (weather) files. He was looking tonight for the Cat 5 storm (Hurricane Felix) approaching the Yucatan pennisula, except...it wasn't on the grib files for today. They were giving nothing more than 30 knot winds in that area, whereas the weather reports are predicting 165 mph (140kts) soon. It would appear from what we can find out that the area affected is quite small at the moment and seems to be below the scale of the grib forecasts. Interesting lesson to take from the sofa rather than from the cockpit I think!""

Is this common?
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Old 28-11-2007, 14:07   #2
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It is common for tropical depressions / hurricanes to be under-reported in a GRIB file. The typical grid used in a grib is 2 degrees, which can be as large as 120 NM. A tight depression can "hide" between the datapoints. I usually see some evidence of the depression in the surface wind barbs, and sometimes in the pressure contours (in a grib), but the max windspeed reported in the grib is almost always going to be less than the actual windspeed.

You can increase the grib resolution to 1, or even 1/2 degree, but this is at the cost of a 4x or 16x increase in filesize. This is one reason to also get the wfax chart, or text forecast, as a supplement to the gribs. I would also trust the storm track predictions of the wfax and text forcasts, more than the grib predictions.

I find it useful to regularly download gribs and wfax to my home computer, when I am not sailing. This way I gain experience interpreting a wide range of data. Of course you have to actually study it, which I don't always do.
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Old 28-11-2007, 15:54   #3
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Thank you Paul.

The more I learn beforehand..........................
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Old 02-12-2007, 22:44   #4
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Whenever we looked at grib files, we always saw a disclaimer that said grib files don't apply in the presence of a tropical depressions or hurricane.

The weather models used for creating grib files are inaccurate when there is a hurricane or cyclone in the area. The weather models that predict the behavior of a hurricane are totally different than the weather models used in predicting the movement of routine highs and lows.

If there is a hurricane or significant tropical depression in the area, don't look to the grib files. You should be looking at a weather fax.
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Old 07-12-2007, 07:58   #5
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Quote:
Originally Posted by maxingout View Post
Whenever we looked at grib files, we always saw a disclaimer that said grib files don't apply in the presence of a tropical depressions or hurricane.

The weather models used for creating grib files are inaccurate when there is a hurricane or cyclone in the area. The weather models that predict the behavior of a hurricane are totally different than the weather models used in predicting the movement of routine highs and lows.

If there is a hurricane or significant tropical depression in the area, don't look to the grib files. You should be looking at a weather fax.
Ahhh! I have been illuminated...thanks for the great info. I often wondered the reason for the disparity between the two. Seriously,
now I know, good thing I joined the forum. Thanks,
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Old 07-12-2007, 13:17   #6
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[quote=maxingout;115403

If there is a hurricane or significant tropical depression in the area, don't look to the grib files. You should be looking at a weather fax.[/quote]


That reminds me that I have been pretty good at interpreting my own weather from weather maps over the years. When I started sailing I listened to the local weather man on the TV and found his idea for MY sailing did not match. I started doing my own predictions from his maps (and others) and found that for MY sailing where I was sailing was much better.
I am thinking weather maps and Pilot charts for wind would work for me pretty well.
Hmmmm.
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Old 08-12-2007, 09:10   #7
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I call grib files by another name - GRIB FANTASIES.

I find grib files helpful, but often they are a source of optimism rather than actual wind speed and direction. For the faint of heart, they can be a real help. When we were in the Cape Verde Islands, there were yachts anchored waiting for the grib files to show seven days of perfect wind before they set sail across the Atlantic. Optimistic gribs gave them the courage they needed to set sail. Unfortunately, when you take your grib files out seven days,you probably are looking at grib fantasies.

There's a reason that weather faxes are updated frequently. It's impossible to predict the weather accurately out farther than a couple of days.

Weather faxes keep me honest and I download them several times daily when I am offshore. They keep me from falling into the clutches of the grib fantasies.
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Old 12-12-2007, 05:52   #8
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The issue with grib models, as I understand it, is that the models make some assumptions about the rate of change of a number of parameters (continuous first and second derivatives if you vaguely recall calculus). The very apparent problem with large depressions including hurricanes is only the most apparent reflection of the shortfall. I believe there are similar problems anywhere there is a front, since a weather front represents a discontinuity in the pressure gradient.

The upshot, in my somewhat limited experience, is that gribs are fine for strategic planning of a crossing but need to be taken with a grain of salt for tactical (wind prediction) decision-making. I get more value from weather fax.
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