December 13, 2012

Weather - a very brief tutorial

The following is an email I sent to friends on s/v Totem who have been cruising Papua New Guinea and just arrived in Indonesia today. During their time in PNG I have been weather guessing for them since I have a fast internet connection aboard and they had basically only Grib files via sailmail.




I sent this email 22 November.  What I describe became Super Typhoon Bopha hitting the Phillipines 4 December.  Frankly I was lucky to notice this event so early in its development but all the signs were there.


When I write a forecast to them I usually jump back and forth between weather websites and then gather my thoughts and write. This happens to be an unusual email because I was writing as I was looking.  Very stream of conciousness. And sorry for the shorthand - when sending emails via sailmail (where every character counts) I try to be concise.




Wx : to your NE there is a wee CLOSED low that pops up next Friday on ECMWF model. GFS has it further east (more N of Soloms).  It is persistent so probably not a model artifact. When I see closed lows so close to the EQ I pay attention.

Whoops. Looking at GFS precip it's really moist and spiraling.  Serious L? Cyclone? ECMWF 500 mB has it right around surface L (reinforcing), advancing to N of big island PNG next Sunday (2 Dec).  So I've got a closed Low, close to EQ, warm water, spiralling, etc.  This is one to pay attention to folks I think.  More attention over the weekend. Spidey sense is tingling on this one. 

OK. Checked GFS wind gribs (something I sometimes do last).
[eventually] Tropical storm force winds at least, located at 5N-10N 150-140E heading WNW so not a threat to you. Pull a wide area sparse GRIB starting +96 hrs out if you can with sailmail and you'll see it clearly. Storm swell will start arrive maybe next Wed. 14m seas at core of storm! 1st time I've seen Passagewx just print a number on map instead of using colour legend.





Weather is an important topic when you're cruising on a sailboat, especially if you are far from regular forecasts or in an area where data is sparse.  I'm a better than average weather guesser for a cruising sailor.  Not as good as a professional weather router, but a good amateur.  When crossing the Pacific we never got hit by bad weather - was it coincidence or luck?  I don't think so. It was picking the weather carefully, sailing tactically when required (not just sailing the rhumb line), and being conservative.


Here are a few thoughts that might make you a better weather guesser too:

  • Do be patient. The patient sailor gets the best weather
  • Don't be a sheep. Follow your own instincts when it's time to depart.
  • If the Grib files are showing 5-10 knots more likely it's 5 knots or flat calm
  • If the Grib files are showing 25-30, it will often be blowing 30+.  Avoid these conditions
  • Do gather data from as many sources as you can.  When forecasting for Totem I would look at Fiji weather maps, Australian weather maps, NZ weather maps, Gribs (2 models of surface winds, 500 mB charts, precipication), Bob McDavitt's weekly summary, and other sites if I had the time. At sea I rely on Gribs, weatherfaxes, and radio reports from other sailors up to 500 miles away to form a picture in my head of the situation.
  • Don't believe Grib files if they show a sudden change in wind direction without change in wind strength along a line.  That's a front but the wind strength jump doesn't show up well on Gribs.
  • Cold Fronts or Lows are not usually your friend.  When you see them on a weather map be prepared for a violent wind change in direction and increase in strength
  • Highs can be bad too - if you are cruising in the trade winds of the South Pacific. As a lower latitude high moves under you the counterclockwise circulation over the top of the high will bring reinforced trades. Often these are too strong for comfortable sailing. Wait for a day or two for the associated swell to die down before leaving on a passage.
  • Do Believe in patterns.  After you look at the weather in a region for about 3 weeks you start to see the patterns of the progression of weather around the globe as it moves from W to E (large areas). Watch how the Highs and Lows often move in a particular. The atmosphere is a swirling cauldron of gas fueled by solar radiation. Think of the swirls you get when you drag a spoon sideways through milky coffee and you get a nice model for system movements.
  • Do read about weather. You have a lot to learn.  Read lots of books about marine weather.  I have 2 paper types and 1 electronic book.
  • Do read about 500 mB charts if you're in temperate latitudes . They can often predict surface low formations better than surface charts will.
  • Do understand the ITCZ and SPCZ if you're cruising the tropics and why it's a good area to avoid  when they meet or if the intensity is high.
OK - now go out and get reading and be a better weather guesser for yourself and your friends

    - Evan

5 comments:

mike pedersen said...

enjoyed your post, as i'm in the early stages of a 3-5 year plan to get the family on a boat and looking to learn as much as i can before then. one thing that i was wondering if you could enlighten me on is marine internet. from this last post it sounds like you have a high speed satellite solution. i've looked at a few options and they seem in general to be somewhat cost prohibitive. any insight would be greatly appreciated. thanks...

- mike

Behan Gifford said...

Hey Evan, that as such a help back in November. We simply couldn't see the big picture- literally- on the little gribs we download. And the text forecasts... well, PNG is kind of a black hole! The percolating system drove our weather decisions for a while there.

So Mike, while you're right to assume that a fancy superspeedy internet connection is needed for something like this- it's out of reach for most cruisers, as you discovered! But the Ceilydh crew is in Australia where you can get broadband internet service for not-too-awful monthly pricing. You just need to be in range of a tower, which describes a reasonable amount of the E coast of Oz. It's been such a huge help while we are relatively disconnected to have Evan's highly informed eyes on stuff and sending it our way, but you won't get this on any of the usual cruising internet service budgets.

Diane, Evan and Maia said...

Hi Mike,

Just to add to Behan's comments - we used 3G internet modems in Mexico, Fiji, and now Australia.

In French Polynesia there were a few hideously expensive wifi providers in popular cruiser's anchorages. Aitutaki, Cook Islands we used an internet cafe at hideous cost for a slow connection and in Vanuatu and Tonga we used cafes with so-so speeds but lowish prices.

At sea we use sailmail to receive Gribs and decode weatherfaxes and receieve text forecasts. No high speed internet at sea for us either.

One tip I learned from another cruiser was to get a wide area but data sparse grib every week or so if you can't get weather maps. Helps to see the all important big picture if you're only getting local area gribs.

- Evan

Diane, Evan and Maia said...

Hi Mike,

Just to add to Behan's comments - we used 3G internet modems in Mexico, Fiji, and now Australia.

In French Polynesia there were a few hideously expensive wifi providers in popular cruiser's anchorages. Aitutaki, Cook Islands we used an internet cafe at hideous cost for a slow connection and in Vanuatu and Tonga we used cafes with so-so speeds but lowish prices.

At sea we use sailmail to receive Gribs and decode weatherfaxes and receieve text forecasts. No high speed internet at sea for us either.

One tip I learned from another cruiser was to get a wide area but data sparse grib every week or so if you can't get weather maps. Helps to see the all important big picture if you're only getting local area gribs.

- Evan

Dale said...

Good read, thanks for sharing.