November 25, 2011

Pacific Passage Weather Thoughts


Evan here. I thought I'd pass on some thoughts about weather across the Pacific. Hopefully these hints and ideas will prove useful to future sailors crossing in the following years.

Getting a good grasp of South Pacific weather was really important to us having a successful time. Most weather texts do a poor job of S. Hemisphere weather. Read (& re-read until you are sick of it) Jim Corenman's “South Pacific Weather” 1994 letter – found at the Latitude 38 site.

s/v Soggy Paws website has a good listing of a number of these weather documents.

Read and understand how the cycles of highs and lows that track W to E affect the weather in the tropics. For example it wasn't until F. Polynesia that I realized how a big High will reinforce (strengthen) the SE trades. Read anything else you can get on the S Pacific weather systems. I think our understanding of weather systems was probably better than most cruisers and we never saw more than 25 knots of sustained wind on a passage (more in short lived squalls, but you can't predict them and they really don't affect your comfort).

Have a strategy for crossing the ITCZ at right angles to the equator – not at an angle which extends your time in this area of confused seas and thunderstorms. It is important to recognize the further west you go before crossing the equator, the closer to the wind you will have to sail to get to the Marquesas in typical SE trades. Even crossing at around 128 deg. as we did, we still had BIG beam seas that were uncomfortable for several days. Those crossing around 132-4 had winds well forward of the beam when they headed south.

Concentrate on the big picture as well as local conditions. Every few days it's a good idea to get a large area, coarse GRIB at say 2x2 deg resolution and covers a week or so.

Weather Resources underway 

GRIB files are used by everybody – but they have lots of limitations. Understand the limitations and you will use them better. They are prepared by computers with NO human intervention. Unless you are looking at multiple models, you might miss something that a particular model is not showing. They don't show fronts, and the associated strong winds though they usually get the frontal wind shift. If you see a strong wind shift with nearby low winds beware – that is probably a front with strong winds!

Before you leave cheap and good internet coverage – check out the various fax options from NOAA and find the correct ones for your area. Their schedule covers a lot of areas so I printed it out and used a highlighter on the few ones that I liked. Bookmark the NOAA fax, Fiji weather maps etc. so when you are using expensive internet in F. Polynesia you can go straight to the correct page.

So also use weatherfaxes and online weather maps when you have internet access. The ones I found most useful were the E and Central Pacific 24,48,72 hr surface faxes from NOAA Honolulu, and close to Australia, the 1,2,3,4 day MSLP forecasts from Australia. NOAA GOES IR satellite pics showed the ITCZ pretty well, but you can't really pick a good area to cross; it changes so fast and moves so much daily The NZ Metservice color fax series is useful for those passaging to NZ.

I didn't ever get good reception from NZ faxes but for those passaging to NZ would probably do best to pick these up, even though they only go to 72 hrs prognosis. The Aus met office online also has a 10 day pressure series that is good to see how fast H and L's are travelling across the continent. The systems to watch (mainly) seem to be S of Australia because they hit the Tasman sea and then turn northward a bit.

I really like the NADI fleet code (send “fleet.nadi”) issued by the Fiji met office. It's only a current surface analysis but you can usually pick out the SPCZ on it. It is usually shown as a trough though. You will need an auxiliary program like Physplot to turn this text file into a weather chart. If you are online you can get the Fiji current chart in better detail from the Fiji met office website. Pick the “new chart”s - it's in colour and is clearer than the B&W versions. NADI also offers text email forecasts for Fiji, Tonga, Cooks and Samoa – but we only got the Fiji versions so I can't comment on the accuracy for other areas. The Fiji forecasts were OK but they only cover one area (Fiji) which is ~300 x 300 miles; too big for a single forecast.

Do get faxes for multiple days in a row before and on a passage. You will see how the systems are moving, and how they are likely to affect you. And don't forget, winds circulate CCW around a H in the S. Hemisphere!

Religiously get Bob McDavitt's weekly weathergram (online via his blog or email via saildocs “nz.wgrm”) every Sunday night NZ time. He can spot big picture stuff for the week ahead very accurately.

The F.Polynesia email forecast (send “fr.poly”) is a bit repetitive (showers and squalls every day), and it is in French so you will need a French – English weather terms dictionary to translate it, but if it says 25+ knots pay attention because conditions are usually ugly when the winds get that strong. Hide in a decent anchorage if you can. You need to have the F. Poly sea areas JPEG chart to understand what area “A25” means.

Australia also has a number of marine forecast documents available through saildocs.

Web sites I use for weather:

http://weather.noaa.gov/fax/hawaii.shtml Honolulu weather faxes – forget the wind/wave charts; they are too general and have weather arrows for only 5 degree squares. Pick the surface charts and 24, 48, and 72 hour forecasts. The colour ones are easier to understand

http://www.bom.gov.au/australia/charts/viewer/index.shtml Australia surface analysis – and forecast maps for the future (click the “Play” buttoms at top)

http://www.bom.gov.au/australia/charts/4day_col.shtml The 4 day map for low bandwidth connections

http://www.met.gov.fj/ Fiji met office

http://www.met.gov.fj/sat-map.html Fiji weather maps – you want the “new” surface maps in colour



http://www.meteo.pf/ French Polynesia met office - in French

NZ weather charts for a few days prognosis

Trade Wind Sailing - and thoughts about routes

The trade winds are generally SE, but they can be S or E or beyond, depending on what the recent H or L that has passed is doing to them. So if you are crossing the Pacific from E to W, you want trades that have more E than S in them for the most comfortable rides. For monohull owners, that also means you should think of biasing your boats cargo, fuel and water loads to the port side of the boat to reduce heeling (as much as you can anyway) in the southern hemisphere. For cat owners, we all seemed to agree beam seas are the most uncomfortable (and noisy as the waves slap the stbd inboard hull). It's important to understand on which passage you might want really E biased trades. The Marquesas to the Tuamotoas or Bora Bora to Raratonga for instance are more SW courses, so anything you can get in an ESE or E trade wind is more comfortable.

Cold Fronts

Generally warm fronts don't seem to appear on weather charts, and only cold fronts or occluded fronts are shown. Cold fronts will usually bring a dramatic wind shift to the NE, N, then NW, and finally SW before shifting back into the normal SE quadrant. If it's a vigorous cold front expect nasty conditions in it's vicinity. Naturally W quadrant winds are headwinds, and are to be avoided so avoid cold fronts when you can. If one is going to hit you for certain, head N in advance of the front. Cold fronts get weaker closer to the equator. As the wind clocks into the W you can then bear off back to your rhumb line and not have to beat.

Warm fronts are not as nasty, and generally turn into occluded (mixed warm and cold air masses) in the tropics. Don't ignore them but you probably won't see them as much more than some rain.

Sailing to Australia route choices

The obvious route is Vanuatu – New Caledonia (Noumea) – Brisbane. The less obvious one that I would suggest and recommend is Vanuatu – Chesterfield Reef – Bundaberg. Here's why
  • Vanuatu to Noumea is very strongly SSW course so in typical SE trades you might end up beating. Ugh.
  • Noumea to Brisbane is about 700 n.m. and you are more likely to have a cold front hit you on the way because the cold fronts are coming along every 7 days and you are sailing into their direction of movement, increasing the closing rate. It's also a longer passage to try to get a decent window
  • Vanuatu to Chesterfield is about 500 and Chesterfield to Bundie is about 450 miles; much easier to get a decent window. The sailing angles with the trades are easier too
  • Chesterfield is absolutely lovely
  • Chesterfield to Bundie is further north, so cold fronts are weaker if you do hit one. Get to Bundie and then coastal hop to Brisbane, through the beautiful Fraser Island and Great Sandy Strait.
Class dismissed – oh are there any questions?

2 comments:

clarke said...

Hmm, I think it's DIanes birthday soon! Happy Birthday

xoxox
Clarke and Kathleen

Evan Gatehouse said...

Thanks, Clarke and Kathleen:)