Thursday, May 19, 2016

Weather Routing Discussion - Day 2

For those interested in Ken's response to Bill's question about how best to head towards Bermuda, here is Ken's daily update and some back and forth:

Good morning BIll and crew -

Wind speeds over the next day or so will be fairly light as a weak frontal boundary to your north becomes less organized while settling very slowly south. Wind direction should remain generally in the W for much of the day, but will be variable at times, and a few showers will be in the area, perhaps a thundershower or two at some point. The remnants of the front will likely kind of ooze past your position later in the day which will lead to the wind direction ending up ENE by evening, but very light, probably 6 knots or less.

Very light ENE winds will veer to ESE later tonight. Tomorrow as you move east of 75W, ESE winds may veer a bit more toward SE in the afternoon, speeds still less than 10 knots through the day. Through tomorrow night winds will veer toward SSE with little change in wind speed, then on Saturday winds will veer to SW and increase to 13-17 knots in the afternoon. Scattered showers are likely in the afternoon as well. SW winds will increase to 15-19 knots through Saturday night with more widespread showers, and on similar conditions are likely through Sunday. Seas will build to 4-6 feet through the second half of the weekend.

SW winds will ease a bit through Sunday night and Monday and showers will continue to affect the route. For the approach to Bermuda SW winds will increase to 14-18 knots later Monday night and Tuesday, and showers will continue to occur.

The rhumb line route still looks best. Light headwinds will require quite a bit of motoring through the next 24 hours or so, but by later tomorrow as winds gain a more southerly component, sailing will become possible. Making big route alterations will only increase the distance needed to travel, and with light winds no matter what route is taken through the next 24 hours, little gain will be seen by deviating from the route. In fact, going as far north as 32N/71W would likely lead to less favorable conditions later in the weekend and early next week with lighter winds then . Best to endure motoring through the light headwinds through tomorrow morning with the promise of better winds after that. 

Also best to be prepared for some wet weather from later in the weekend through arrival at Bermuda.

Call or email with any questions.


Hi Ken. I think that Expedition is routing us north because of currents seen in an RTOFS grib file. There seems to be 2 eddies - one CCW from 29N/76W to 30 30N/74 30W, and one around 32N/ 72 30 W and it is routing us just along the southern extent of them. It seems to be indicating that gains of some positive current plus the absence of adverse current is worth going about 70 nm further, while arriving 5 hours earlier.

Do you see those eddies by any chance? I wonder if I should believe Expeditions data interpolation.



For a little background, Expedition is navigation and weather routing software used by the top ocean racers and developed here in New Zealand by Nick White.  We first started using it in 2006 after being beaten by a boat running weather routing software in our first Marion-Bermuda race.  It takes in weather information in the form of Grib files (Gridded Binary) which are space efficient ways of getting accurate up to date weather data over limited satellite or HF radio internet connections.  We download files for both wind and waves, as well as gulf-stream (current) data.  Bill's reference to the RTOFS file is a surface current grib file that uses satellite spectromony to measure actual surface currents, then runs a computer simulation to predict what the surface currents will be doing.  It always shows very detailed eddies and back eddies around the gulf stream, but it is hard to know if those are real or just interpolation of measured data.  There are other tools for looking at gulf stream data such as thermal images but those are more data intensive so we don't use them onboard.  Expedition also has detailed performance information on VofJ in the form of custom "polars" which I have developed based on IMS VPP prediction, then fine tuned and refined over the last 8-9 years based on actual measured performance data within expedition.  The "polars" tell expedition how fast to expect the boat to go at any point of sail in any wind speed.  We even have different polars for racing, cruising, and motor-sailing to simulate us to putting the engine on when going upwind or in light air.  Expedition is suggesting going about 70nm off rhumb line to chase better wind angle and favorable current which could pay off by 5 hours if the data is correct.  Ken tends to look more globally and is suggesting keeping to the straight line, but may not have looked at the detailed current data which is always hard to know how accurate it might be.  While the computer models for wind prediction are VERY good these days.  The current predictions are a bit more hit and miss.  Will be curious to see which one Bill follows.

-- Gram

p.s.  Let me know in the comments if anyone is interested in the weather discussions and my running commentary....won't waste my time if not, but happy to keep everyone apprised here if any are interested.


Sandy said...

I used to be a live aboard cruiser so I appreciate and enjoy the weather commentary. but I realize I'm only one person on the globe following the journey so obviously don't do it just for me. Fair winds, folks.

Sierra said...

I think it's interesting. A bit over my head at times, but interesting.