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Weather routing software

J

jamottep

New member
Good day,
I've started to spend more time with SailGrib. I was wondering how folks use routing software.

Here's one way to go, taking into consideration a file size limit of 50k; this is more an uneducated guess than anything:

1. Get weather charts (analysis, 24/48/72 forecasts): this is to get a sense of the big picture, what ever that means (i.e. how do I interpret these to get a sense of where I go next? I don't know.).

2. Get a low resolution (2 degrees) GRIB file for the whole area to HI for the next 12 days (less as I get closer) with a 12 hrs step, including wind and pressure (44ko). Run a route from where I am to Hanalei.

3. Get a high res (0.5 degrees) GRIB file for the next 3 days' forward area with a 3 hrs step, incl. wind (44ko). Run a route to the approximate waypoint given by previous route.

4. Run a great circle route.

Compare all 3 above and understand differences.

It seems one can get lost in this and forget to sail ...
 
I don't use routing software, though if I had Expedition I probably would. It's a bit like knowing how to do arithmetic, you need to know how to add and subtract before getting a calculator to do the math for you.

If you're not sure how to interpet the surface weather fax information then it would be time well-spent to understand what that the drawings are telling you before diving off into using that information in conjunction with routing software to come up with alternative routes.

Once you figure out the surface level data, the 500 mb charts are really useful as well - though they are more difficult to understand as they are quite different than the surface level diagrams.

The key to a race to Hawaii is the Pacific High. You want to know where you are in relation to the High and where the High is forecast to shift to. The signifcant hurdle to cross during the race is the ridge that (typically) extends from the High to the southeast, often down to to the southern end of Baja Califiornia. How you position yourself north-south relative to the High dictates how much breeze you will be sailing in, and what your wind angle will be. Where you cross the ridge dictates your position into the tradewinds. Where you enter the ridge is determined by your course on your departure from San Francisco. In short, you want to have a good idea of what you're plan is, as you will begin implementing it the moment you clear Pt. Bonita/Mile Rock. Any changes in plan after that time are going to create extra miles sailed which in turn costs time.

The most important piece of equipment you can carry on the race, besides a GPS for position info, is a barometer. Look at the barometer, it is giving you real data exactly where you are - if the barometer is rising the High is getting closer to you, if it's falling then the High is moving away. Stan Honey likes to use the 1020 mb isobar as a reference line, if you in less pressure you probably have more wind but you sailed more miles to get there, if you have more pressure you have less wind and sailed fewer miles.

I do not use forecasts out beyond 96 hours for two reasons - I can not move the boat fast enough to play weather systems, and the data beyond 72 hours is getting into fantasy land. On a crossing I'll look at the analysis + 3 day forecasts WFax twice daily (morning and evening), and make decisions accordingly. The GFS model GRIB data is an interesting correlary to the WFax, and can help pick out details that aren't obvious in the WFax drawings. Another useful dataset is the High Seas text forecast.

Based on what you've written, I'd suggest that worrying about routing software is unnecessary.

- rob/beetle
 
Skip's step-by-step rundown on the SHTP is virtually perfect, as far as I'm concerned! Brought back a whole lot of memories, too....
Almost makes me wish I had decided to do the race this year...oh well,,,,
I send my best to all competitors and the RC.
 
tiger beetle said:
If you're not sure how to interpet the surface weather fax information then it would be time well-spent to understand what that the drawings are telling you before diving off into using that information in conjunction with routing software to come up with alternative routes.
Click to expand...

I had hoped that Lee Chesneau's workshop was going to help ... but it didn't work for me. It was not exactly a workshop and it was a little all over the place.

I find myself in a position where I'm advised against using a routing software but also struggle to get enough knowledge to make informed decisions.

It sounds like this is all going to come down to: steer straight to HI ...
 
jamottep said:
. . . It sounds like this is all going to come down to: steer straight to HI ...
Click to expand...

Well, for a displacement cruising boat like mine, I think that's right, but with one very big caveat: Steer straight to the finish line ONLY AFTER crossing the ridge of the SE lobe of the EPAC High. Figuring out where to make that crossing seems to be the outcome-determining strategic decision of the race. After that, if you're in a sport boat, make decisions whether to sail more miles for better wind. In a displacement boat, it's the rhumb line from that point on.

That's my take-away from Skip's document and from other SHTP and Pac Cup presentations. There's a video of a Stan Honey presentation on the Pac Cup website, for example, that goes over all of this. My conclusions from all this could be wildly wrong, and if someone with actual experience weighs in to say so, that would be welcome.

Also, I am told that in 2016 some weather/race guru came to the skipper's meeting the day before the race to give an in-depth analysis of the then-current forecast. I'm hoping that will happen again this year. Does anybody know if or who?
 
Last edited:
jamottep said:
It sounds like this is all going to come down to: steer straight to HI ...
Click to expand...

Read through Skip's write-up, that is really good information.

There are a number of write-ups out on the web that walk through how to read/understand a weather fax drawing; those should help, but they won't be specific to the case of interest - the bit of ocean between San Francisco and Hawaii.

What I've done prior to race start is collect the weather faxes morning and evening (e.g., before and after work), the same data I'd be looking at on the boat. I would start this two weeks prior to race start. What I'm looking for is not necessarily where the High is right now, but in which direction it is generally moving - I want to know, at race start, if the High is moving away from the coast or towards the coast. If it's moving away the isobars should spread out between the coast and the High and it will be relatively light air along the coast. If the High is shifting towards the coast and compressing the isobars then it can be extra windy down the coast towards the ridge.

You might try starting such a collection now, reviewing the data will force you into reading enough to understand what is on the drawing, and you've got access to a variety of "how to read weatherfax" information available online.

available from NWS Pt Reyes station: http://tgftp.nws.noaa.gov/fax/ptreyes.shtml

0000Z data (there is an equivalent set for 1200Z
analysis: http://tgftp.nws.noaa.gov/fax/PYBA01.gif
24 hr forecast: http://tgftp.nws.noaa.gov/fax/PPBE00.gif
48 hr forecast: http://tgftp.nws.noaa.gov/fax/PWBI98.gif
96 hr forecast: http://tgftp.nws.noaa.gov/fax/PWBM99.gif

What I do is print out the four charts, punch them for a 3 ring binder, look at them to see what the forecasters think is going to happen. In the evening I get the next set of charts (whichever is most current), print them out, then compare the newer analysis to the older forecast to see how good (or not) the forecast was, then compare the 3 new forecasts to the older forecasts. When I started to look carefully at what information is on the charts and I couldn't figure something out, that's when research on the web proved helpful (though we didn't have the web when I did my first race over - I used several books to help out).

The nice part about doing this before the race is you have online access to lots of meteorological data you're not going to have on the boat - so you can do some digging to look for what happens around a cold front, what are the hurricane paths into Hawaii, what are the ship reports telling you, what's happening with the jet stream way up high and how does that impact the High's movement?

So yes, you can point the bow straight at Hanalei and you will get there. If the High is way north that might even be the best choice (least miles sailed in breeze no worse than is available down south). It's more likely that sagging south a bit to curve under the High will get you to Hanalei slightly quicker than if you sailed straight at the finish line.

But don't forget to have fun along the way!

- rob/beetle
 
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