Wind Models vs Reality in the Caribbean

[RyanC]

Since we've arrived in the Caribbean in November the wind models have been off by quite a large margin compared to what we are really experiencing. First I was attributing to local anomalies while sailing in the BVI (i.e. winds accelerating through islands and valleys) though now we've been well away from land and there still is a huge discrepancy.

Sailing in the BVI in December winds were regularly forecasted to be 20-25knts though they were closer to 25-30 with gusts well over 40. Moving further east and south to Anguilla, St Martin, St Barth's and now Antigua we've had passages forecasted 20-25, but again seeing sustained winds in the mid 30's with gusts well over 40.

Just two days ago on our last passage from St Barth's to Antigua the forecast was showing winds 15-20, but we sailed the entire leg only seeing winds drop below 20 maybe twice for a few moments and back up to 27-33knts.

I'm using a variety of sources including passageweather.com, sailfow.com, various Gribs etc.

So are there more reliable weather sources I need to be looking at? Or is wind forecasting in the Caribbean just that difficult?

[MarkJ]

I always add on 5 knots to Grib files/Passage Weather when its above 15 knots. And drop 5 knots below 15.

Worse, is that the NOAA marine forcepasts have been way too low, and too generalised, as well. When they say its 15-20 knots thats one hell of a difference to 25 to 30.... About quadrouple in wind force from 15 to 30.

The Christmas Winds were early, lasted a long time and were very strong for 2 1/2 months. I thing everyone should have gotten their act together and better predicted wind strengths.

By the way, you will notice th epilot charts underestimate too.

[boatman61]

They've been out off whack all over for over a year.. Biscay-Adriatic-Med-Atlantic... as they say in Australia...
When you go out there... expect the winds and waves to be up to 50% stronger and larger than forecast.

[Jim Cate]

G'Day all,

Various sources use different protocols, but here in Oz the f/c strengths are for 10 minute averages. This is a measurement that is sorta hard to work out in situ, and I believe that most of us yotties tend to weight their wind estimates nearer to the gust strength than the average. I reckon that this is just human nature!

Again in Oz, the daily broadcasts all begin with the statement "wind strength are averages, and maximum gusts may be up to 40% stronger" (or words to that effect). They also state that maximum wave heights may be twice the predicted heights. So, we hear these warnings every day, but still folks complain about inaccuracies when they experience winds that they view as "stronger than the bloody BOM said".

I don't know what is going on in the areas the OP is talking about, but have to wonder if a similar anomaly is present.

Cheers,'

Jim

[Dockhead]

In the English Channel and Bay of Biscay, the GFS models consistently make the wind at one Beaufort force less than what it turns out to be in reality. Which is great -- it means it's actually quite accurate, but just needs interpretation.

So if the forecast is 15-20, you can be sure it will be a F6, not a F5, and some reefing will be inevitable.

I think once you get used to it, the GFS modelling is amazingly good, bearing in mind the limitations of forecasting much ahead in unstable weather.

[MarkJ]

Yes, i know what you mean by gusts, but these are trade wind areas. There are gusts but not like non trade wind areas.

The last three months the forecasting has bee off. I know as i am in an open roadstead anchorage and the swim platform has been wet for weeks, so its not blowing 15 knows with a 2 minute gust at 30. Its blowing the cream out of the custard and someone's gotta be to blame. I'm a TAX PAYER!

[MarkJ]

Originally Posted by Dockhead

So if the forecast is 15-20, you can be sure it will be a F6, not a F5, and some reefing will be inevitable.

.

Well these boofhead scientists should get it right. How can we believe them on global warming when they cant even work out the bloody wind speed?

[conachair]

Originally Posted by MarkJ

Well these boofhead scientists should get it right. How can we believe them on ** when they cant even work out the bloody wind speed?

Dig deeper

http://weather.mailasail.com/Franks-...ib-Notes-Users

[Dockhead]

Originally Posted by conachair

Dig deeper

A Warning about Using GRIB Files / Franks-Weather | The Weather Window

"In particular, it is important to remember that any model can, at best represent weather on a scale of about 5 x the grid length. For the GFS, this is about 100 to 150 NM. As a result, winds speeds over the open sea will be under-forecast by about one Beaufort force or 20%"

Bingo. Exactly my experience.

[conachair]

Originally Posted by Dockhead

"In particular, it is important to remember that any model can, at best represent weather on a scale of about 5 x the grid length. For the GFS, this is about 100 to 150 NM. As a result, winds speeds over the open sea will be under-forecast by about one Beaufort force or 20%"

Bingo. Exactly my experience.

Frank is weather God

For UK this is different and better than anything you've read so far, IMHO

http://www.amazon.co.uk/Reeds-Weathe...rank+Singleton

Originally Posted by Dockhead

"In particular, it is important to remember that any model can, at best represent weather on a scale of about 5 x the grid length. For the GFS, this is about 100 to 150 NM. As a result, winds speeds over the open sea will be under-forecast by about one Beaufort force or 20%"

Bingo. Exactly my experience.

Sent from my SGP312 using Tapatalk

[RyanC]

"In particular, it is important to remember that any model can, at best represent weather on a scale of about 5 x the grid length. For the GFS, this is about 100 to 150 NM. As a result, winds speeds over the open sea will be under-forecast by about one Beaufort force or 20%"

If it's that simple how come they just don't account for that in the models?

Once I get a better connection (HotHotHotSpot sucks here in Antigua) I'll definitely look at Franks weather site, looks like a great resource, thanks conachair.

But at the moment we are enjoying one of the first calm nights in a long time, it's quite nice.

[boatman61]

It was not the speeds that bothered me coming through the Med.. that I can adapt to... but when you set off early to ride a forecast NW7 for 24hrs and its turns out a SW8 on the nose instead.. that really pisses me off..
Passage weather was crap late Oct early Nov in the central Med.. did not get a damn thing right

[MarkJ]

Nice link on the Grib file error, but as Ryan just said, why dont they factor it in.

And NOAA, they should have it in, anyway. Shouldn't they?

[conachair]

Originally Posted by boatman61

It was not the speeds that bothered me coming through the Med.. that I can adapt to... but when you set off early to ride a forecast NW7 for 24hrs and its turns out a SW8 on the nose instead.. that really pisses me off..
Passage weather was crap late Oct early Nov in the central Med.. did not get a damn thing right

You should ask for your money back

[beverley]

Originally Posted by boatman61

It was not the speeds that bothered me coming through the Med.. that I can adapt to... but when you set off early to ride a forecast NW7 for 24hrs and its turns out a SW8 on the nose instead.. that really pisses me off..
Passage weather was crap late Oct early Nov in the central Med.. did not get a damn thing right

It was not much better in the Irish Sea over the summer. Wind directions and forces were off so much we gave up. Day one of my charter forecast SW force 3 to 4. After leaving harbour and an hour out we had Easterly force 8 to 9.

It made for an interesting day.....