Weather Guru Bob McDavitt – Podcast

Wild weather - Lightning Strike south of Mooloolaba Pond anchorage December 2008. Photographed by Lindsay Turvey

Wild weather - Lightning Strike south of Mooloolaba Pond anchorage December 2008. Photographed by Lindsay Turvey.

Bob McDavitt is a weather guru who has helped captains, skippers and navigators get the best out of the weather systems available to them. 

In this Podcast interview we chat about how he became a meteorologist and went on to support yachtsmen around the globe with his forecasts and predictions of weather at sea. Bob also gives us some handy links to free software for downloading and interpreting weather data at sea through HF Radio.

Bob has a refreshing ability to make the diverse and complex weather around us easy to unnderstand.

Remember to like and share if you found this useful. 


LINDSAY: Today I’m interviewing New Zealand’s world-famous weather guru, Bob McDavitt. Bob is a New Zealand meteorologist who’s helped boatloads of sailors make the best of whatever cards the weather gods have dealt them.

Welcome to the interview Bob.

BOB: Good day to you there Lindsay. Yes, thank you.

LINDSAY: You’re a busy man even though you’ve retired, I know that you’re still doing a bit of work on the side there. We really appreciate you giving us your time and sharing your wisdom, and perhaps a few little-known facts about the world of meteorologists. When did you first start becoming interested in weather forecasting.

BOB: Well in weather itself - when I was at school in 1968 in Wellington. We had a big storm which was cyclone Gazelle crossing the North Island and it deepened very quickly just near Cook Strait. Sunk the Wahine. There was a loss of life - 53 dead in that one storm. The roofing iron was blowing all around Wellington. That sort of blew the weather into me I suppose - an interest in the weather. When I was studying at the Victoria University in Wellington I thought, “What should I do for my career?” Then I thought about weather forecasting. Basically, because in terms of the spectrum of chaos you have those parts of science which study things like earthquakes and volcanoes - seismology that work on very chaotic systems and there’s not that many scientists interested who actually get a job out of that. On the other end of the chaos spectrum those people who produce the tide tables, looking at the lunar forces on earth - there’s only one of them in the whole of New Zealand.

I was looking for something in between and weather forecasting has a particular type of chaos about it. Weather is a mix of pattern and chaos. There’s always the reward in investigating the data.

LINDSAY: So, you studied at Victoria University in Wellington, now Wellington is perceived to be the windiest city in New Zealand and it’s a good location for a meteorologist university that’s for sure.

My next few questions are going to be about educating those who don’t appreciate what happens behind the scenes prior to a forecast being published. I’d like to start by saying, what did have to do to actually qualify as a meteorologist?

BOB: Yes, these days a good degree in mathematics or maybe physics is a prerequisite requirement for becoming a meteorologist. To be a good meteorologist you need to have enough mathematics to be able to work out the omega equation. This is an equation which looks at the mathematics of the forces that are linked to the old thing that…You know how when you tap a barometer - an aneroid barometer, if the needle flicks downwards that means the pressure has been falling, and if the needle flicks upwards the pressure has been rising. A change in pressure is related to the surrounding weather, and that equation which studies that is called the omega equation. It’s quite complicated. There’s integrates from the surface of the earth right up to the top of the atmosphere and in three directions, it also has part of it which changes with time, so it becomes a forecast equation. You need that much mathematics.

You also need a twinkle in your eye. A detective's ability to be able to ferret through data and find the information. It’s not really a job that is applicable to anybody or anybody who has just come from university either, it gets wilted down. As you look at the number of people with the mathematics or whatever background being produced every year from universities, out of all of them only about 10% become meteorologists. There’s only 100 meteorologists working in New Zealand.

LINDSAY: Quite an elitist group then and only the best of the best mathematicians goes in that direction. That’s what you’re saying?

BOB: Yeah. It’s not just maths it’s also the ability to become a bit of a detective. To have a twinkle in your eye, to ferret through data, separate the goodness from the chart - the wheat from the chart. It’s a great job for that sort of people that are interested in that kind of thing. For many other people, it becomes a mind logging job and also it can become a job of boredom if you just continually doing the same thing over and over again.

LINDSAY: It’s not the sort of thing that anybody can do, that’s for sure. It’s a certain type of person that actually goes down that path and becomes a meteorologist. I guess you’ve got to be a little bit thick skinned too because you often hear people saying negative things about forecasters.

BOB: Any job where you’re trying to vouch an idea on the future, it opens itself to derision straight away. Of course, naturally. We look at weather as being a mixture of pattern and chaos, and we look at the pattern and from that we can provide you with the latitude of least risk. Or that least tentative - to turn the forecasts themselves in terms of what to avoid. So, here are the warnings - we expect gales here, there and there, and then. We draw boundaries around the areas of nasty weather and send them out to everybody. That way we try and help people to avoid the nastiness. Between those areas of nastiness there’s a large area - the rest of space which is not so nasty, and where the weather is kind. Because we only have a certain number of hours in the day to produce the forecast we don’t concentrate too much on those areas. We concentrate on telling people to avoid the nasty. That makes us therefore very much the messenger who gets blamed when the nasty arrives.

LINDSAY: There’s a certain amount of responsibility on the individual to actually look out the window themselves and read the clouds, and check their barometer and see how that matches up with the forecast. A lot of people don’t take on that responsibility I suspect.

BOB: I suppose people make short cuts. Take short cuts. The forecast itself is just basically based on the pattern of the…the observed pattern of the atmosphere. Perhaps extrapolated into a future by computers. But in itself is not the real world. It’s an idea. The first thing you should do when you get an idea is to check to see if it gels with your real world. When the forecast was written they had no idea what your real world was. The only person who knows what your real world is, is you. No one else. Therefore, the barometers…You might have something which helps to relate where you are with the weather around you, is your communication system to the environment. You can check and see just how the forecast itself with all of its details combines with what you’ve got, just by looking at the barometer. One little thing.

LINDSAY: Where and how is the information you use to forecast gathered and coordinated these days? What sort of tools do you use to gather that information?

BOB: Well over the ocean we have some voluntary observing ships - with EOS a whole fleet of them, of different types of ships. Some fishing, some container ships, some Navy vessels all out there all the time and reporting what they have got once every three hours. All of the land reports are nowadays, and more and more so the robotic reports I suppose you’d call them. The automatic weather stations. All that information is gathered together. First of all, the data has to be…The instruments themselves have to be calibrated so they are reading correctly. Then the way the data is sent around in terms of codes has to be standardised so that we’re talking the same language around the whole planet. This is all coordinated by the world meteorological organisation - WMO, which is based in Geneva. We all follow the standards set by WMO.

Every nation has its own weather forecasting centre which is usually run by the government. In New Zealand, it’s run by Met Service which is a company which is state owned. I guess it’s senior government. The data is gathered and coordinated at that weather centre.

Good news for people like me who forecast from home is that a lot of the stuff is on the internet. The tools that I use are basically the computer program - prognoses, the global weather computer models. There are several of those, but the top ones would be the European Community for Medium Range Weather Forecasting - ECMWF, which is abbreviated to EC, and the GFS - the Global Forecast System which is produced by Noah as a government department from the United States. There’s the Meteo France Model, and several other models which I’ve got some access to.

Over in the United Kingdom the UKMO model - United Kingdom Meteorological Organisation model is one of the top ones in terms of accuracy, but it’s the hardest one to get in terms of availability. It’s normally only sold to the main weather presenters who do the presentations on TV like BBC and so-forth.

We’ve got all these computer models, we also have the observations in real time - not just from the weather stations but also from satellites and radar. All of that combined together, and there’s a few extra websites there like one particular satellite which is also keeping an eye on lightning data. That’s in real time. You can see weather lightning is occurring anywhere on the planet. And also, some sophisticated satellites which show you where there are fires around the planet on the surface. Quite interesting to live in this time when there’s so much data to look at.

LINDSAY: Does that make your job easier or harder having all those bits of information coming in.

BOB: We’re getting to the stage where we’re writing bots - little programs that will look at the data for us and wake us up when something unusual is starting to happen. Plus, also if you’re running a special event well then you might actually put some special weather data around the course of the event just to look at the weather on a finer detail than just for ordinary regional forecasting.

LINDSAY: That sort of leads me into my next question. I know that you’ve done weather routing for yachtsman. When did you start forecasting specifically for yachtsman?

BOB: Well I was a marine forecaster earlier into my career anyway at one stage, and also an aviation forecaster. What was happening back in 1987 was that New Zealand challenge decided to put in as a syndicate to have an entry for the competition in the America's Cup over in Perth. They had a meteorologist who was looking after them but then he thought he wouldn’t actually hang around for the final part of the racing. They put the ear to the ground and asked people. When I was working in Kelvin in those days which is in Wellington - the weather forecast centre and said, “Anybody interested in sacrificing three to six months to do some forecasting”… and be seconded to, in those days it was called New Zealand Challenge. They got us down to about three of us. In the final interviews, I think I won the day for myself by telling people that I was pretty versatile. I was doing aviation forecasting at the time and within a month I’ll be able to offer something exactly the same amount of accuracy as we get from issuing a terminal aerodrome forecast, for the landing of an aeroplane on an international airport. Within five degrees per hour. The direction would be within five degrees during the whole day, per hour. The international standard is ten degrees of direction. Now remember there’s 360 degrees to a circle so that’s pretty fine. The speed had to be within five knots of the actual speed. In other words, the route means square speed error would be less than five knots.

I had great fun over in Perth designing systems to try and beat the locals in the weather forecasting. By January ’88 - the finals of that race, we certainly had the meteorology there but we didn’t have a boat that could beat Dennis Connors Stars and Stripes.

LINDSAY: That was the start of forecasting for yachties. Now it’s about eight to ten-day passage for many cruising yachts coming to New Zealand. The accuracy of the forecast reduces as the days go by obviously. I was wondering if you can give our listeners and readers some general indication of the percentage of trust they should reasonably expect in terms of the age of a forecast for both stable and unstable conditions during that passage to New Zealand. If they were at Fiji and they’ve got a forecast and then they don’t hear anything more for the rest of the passage. What sort of accuracy can they put on that information that they got before they left?

BOB: I like to call it the way that the computer models mimic the real world. The degree of mimicry is of course inversely proportional to the time between the forecast and the time of the map that you’re looking at. If you’re looking at what’s the forecast is for tomorrow it has very good accuracy. If you’re looking the next day it’s less accuracy by maybe a third because there’s over three days. If you’re going out to four days, again it’s down to a quarter because there are four days in the forecast. After about five to seven days you shouldn’t really expect your computer forecast to actually be a good mimic of what will actually happen in the real world. Remember the weather is a mixture of pattern and chaos. The computers can’t take into account for chaos that’s actually occurring on planet earth all the time. Even if you get a computer that gives you a perfect mimic of the real world at time zero, you’re going to have that decay. After five to seven days you can expect something completely different. Which is no good at all for an eight to ten-day passage.

What you have to do is try and work with the pattern of the weather. Realise that fronts are not going to be six hours apart. Once you have a front which leading edge of a new air mass on a weather map, it was going to trundle across you and then of into the distance and it’s such a marked feature on a weather map that it takes isobars about a day or two to draw around it. Therefore, once you’ve had a front you can expect the next one won’t be until maybe…Depends on the time of the year but it could be five days away, it could be seven days away. Sometimes it’s as quick as three days away. If you can time your passage so that you encounter that front - and there’s going to be one every week anyway so you’re going to have at least one, if you can count it at about mid voyage where you’ve got plenty of room to run and do other things, it will sure avoid that front. Maybe perhaps sail with it, or against it, or across it, whatever. Then you should be able to expect once that’s front has gone through, that the next three days, the next half part of the passage should be good enough for arrival in country.

LINDSAY: That’s kind of what we did when we left New Zealand on our yacht Blue Heron. We had a whole series of fronts going through Whangarei. It was a terrible winter. We left a little bit late in the season but when we did leave we left the town basin while there was a nasty front going through and had a great sail all the way to Fiji. Which was fantastic. We got a good mix of wind directions but they were all less than the day that we left. Quite a good strategy leaving.

BOB: Therein lies the answer. The strategy of departure - when you leave, is the way to try and pick the best weather for getting to New Zealand from the Tropics or vice versa. There are lots of tools on the internet and gives a good view of both the EC data and the GFS data, and it goes out 10 to 14 days in the future. Remember that rule of mimicry. It gives you the expected weather pattern according to two computer models, so you can compare and contrast each of the models. You can decide for yourself, is this the middle of a series of fronts coming across, or is it the end one. Which is the one to go after. To help you decide that you can zip up to the upper air where the weather is smoother. You can tell more quickly where the highs and the lows are. Where the ridges and the troughs are. With a little bit of training, that’s the way to go to try and do a cruising forecast.

LINDSAY: I know when I was trying to get an interview with you earlier you were busy helping a fleet of yachts head north for the winter. I was just curious, what services are you still offering in your retirement for yachtsman?

BOB: Yes, it’s amazing. I’ve just given you the clues to how to work out the best strategy when to leave, and yet there’s still yachts people who go into some kind of an analysis paralysis when they are given tools on the internet to then decide, to help them to go sailing and cruising. I thought I’d offer myself up now that I’ve retired from Met Service about five years ago, to be available to help with assistance with the weather forecasting for planning purposes, for departure and for a voyage forecast upon departure, which will give them way points to where they head to, and also a service where they can email me or text me their position during the voyage itself. I can reply with little instructions as to what to do next. So as to avoid the bad stuff and stay with the good stuff.

I charge $10 for every five minutes of my time and certainly been given enough requests for it to do enough work to keep me going, so that’s good.

LINDSAY: Okay. A very valuable service and it certainly avoids that, like you said, analysis paralysis that quite a few of us have gone through trying to work out. A great service. Whereabouts can people find that?

BOB: Basically, they can go to my website and have a look there. It’s got my email address. The website is The email is You’ll see on the website there’s an example forecast plus also you can sign up for my weekly blog - weather blog, which basically has a quick highlight for the week ahead. Doing the ordinary forecasting thing of telling you what to avoid in the coming week. I also do some services for adventurer yachties. There’s one at present who’s attempting to sail around Antarctica all by herself. Unassisted. Alone. From Australia to Australia. At present, she’s about 10 days away from finishing in Australia, but there’s a bit storm Australia this afternoon her time. She’s have to I suppose release the drogue. We’ve been commenting she hove-too to last night. She doesn’t like hoving-too to much with this vessel because it doesn’t hove-too that well. We’re expecting waves over seven significant metres. It’s definitely a drogue situation.

LINDSAY: That’s very good that you’re still helping us yachties get out there and giving us up to date information, and that’s a simple email through. Is that how you reply, just in word, or is it grib files as well? That sort of thing?

BOB: Yeah text. I might give a way point, a point to head to, heading in direction. On my website, you can see one of the text tables that I produce which shows you for every few hours or so what to expect each hour. You don’t have to have as much data as that. You might just need to know a particular turning points or waypoints for this particular trip which would be most appropriate.

LINDSAY: For those that are on tight budgets, you’ve already touched on it pretty much - what tools are out there and what they can do to sort of plan their trip by themselves without asking for your services. If you had to really narrow it down, what are the things that matter the most to help them make good predictions without getting into this analysis paralysis. What sort of things should they focus on the most?

BOB: It depends I suppose also how much time a yachty has to spend on terms of listening to the radio. You can download the data freely on the radio. You’re getting all the marine warnings updated two, maybe three times a day, but I can imagine if you’re just sitting next to the radio trying to write down the warnings, it doesn’t give you much time to do real sailing. In terms of efficiency and the most important tool you’ve got, is the barometer. In itself it’s measuring changes in the air pressure around you. Therefore, it’s your first indicator as to an incoming front. It should be giving you data about that incoming front well before you see any clouds changing or anything like that. The combination of what the barometer’s doing and what the clouds are doing is going to give you at least a six-hour lead time on any strong winds or any wind changing. That’s probably the most important thing you need going sailing. To know how to read the barometer and associated with how it works with clouds before the actual wind changes.

In terms of working out the date of departure, well then go to that website and just have a look. It’s a good viewing tool of what to decide upon for departure. Again, the barometer is quite useful. You could write down a little log of target pressures for what you expect the barometer to be doing as you go north. Then even if you don’t have any access to any internet or anything like that from the time you depart, at least you’ve still got that log of expected target pressures and you’ve got your barometer itself which will tell you how near or far away the computer and mimicry of the real world is.

LINDSAY: So, you can adjust the speed of the front just by observing your barometer. You can work out whether it’s coming early, whether it’s coming a little bit later than what they had forecast.

BOB: You can change the sequence. You can see how the sequence is changing. Also, once you’ve moved into the period maybe three or four, five days into the trip itself where you’ll find that the forecast that you got at the beginning of the passage is no longer relevant, you may as well just toss it out the window. Your barometer is still going to tell you what’s happening for the next six to twelve hours in terms of expected wind speed and direction. That’s if you know how to read your barometer. It’s definitely the single most useful tool for a cruising yachtsman without any other ways of measuring the weather.

LINDSAY: You just reminded me of a couple of things that we used to do when were out cruising. That’s using some sort of recording device on the single side vane so that you could actually come back to it later and if you were busy doing sail changes or something like that. You could record the weather forecast and then come back and listen to it in your own time, or go over the bits that weren’t very clear. That was quite handy.

When we were sailing between Hawaii and Victoria, Canada we got stuck in the North Pacific high - a big ridge of high pressure sprung off and we got stuck in that. We had seven days without any wind at all, so we had plenty of time and I recorded a mean sea level analysis in Morse Code. I decoded this Morse Code into numbers - groups of five, and then decoded that again into a picture of what was happening with the isobars around the Pacific Ocean and we understood why we didn’t have any wind. It didn’t make any difference but at least we understood, and we had plenty of time to do it. Yeah it was a lot of fun doing that actually. It took me all day.

BOB: Yeah, the fleet code is still being produced by the Fiji Met Service. Combined with a program which you can download from Mike Harris Pangolin he’s based in Tasmania, but you can write a computer program which will convert the morse code itself back into a map. You can download the actual code if you have access to email on your yacht maybe by Pactor modem. All of this is written up in a recent blog that I did a couple of weeks ago [Bob Blog 25 June 2017]. If you go to my blog page and look through the archives a couple of weeks ago, it tells you all about fleet code. The ability to be able to draw it as a map from a yacht just with short wave radio has been around for the last 15 years or so. It’s quite a useful tool because if you don’t have any access to any weather maps and you have to listen to the actual marine forecast on the radio, that could be a bit too long. If you can download the email of the latest fleet map and then plug it into the PhysPlot I think is the name of the program, which turns it into a map, then bang - there you are. There are all the convergent. There is the South Pacific convergence zone and there’s a few isobars, and where they get together it’s windy. That’s just what you wanted.

LINDSAY: I really love that you dedicate so much time and effort to helping people safely passage to and from New Zealand, through a stretch of water that’s perceived to be too scary for some to even attempt. Is it really that bad, that stretch of water between New Zealand and Fiji from the islands?

BOB: It has its challenges. In terms of stormy-ness, I don’t think it’s the worst patch on the planet. You’ll find Cape Horn is definitely more windy and wavy and has the bigger number of fatalities and mishaps. Sailing around the South Pacific you’ve got to get to know the locals that you find on the weather map, because they’re slightly different from anywhere else in the world, that South Pacific convergence zone is a moveable beast. It’s like a dragon - keeps snaking around. It’s not like the inter-tropical convergent zone which stays put in the same place. The South Pacific convergence zone can be quite squally.

Then we have these mobile anticyclones which tends to travel from west to east across the South Pacific. It’s about the only part of the world where the anticyclones are mobile. I suppose, Australia as well because it’s the same part of the world really. Everywhere else and every other ocean, the anticyclones tend to be semi located. They don’t really move around much. Some of the highs can be quite big and that creates…Whenever the pressure is over 1030 I have a saying that the weather is going to get dirty. As soon as you have to create more room on the weather map to make a bigger and bigger high - 1030, 1035. Just push all these isobars on the periphery closer together as a consequence - you create a zone of squashed isobars miles and miles away from the actual high centre which is the cause. That zone of squashed isobars is normally in the tropics and it creates…Could be only strong winds but because the waves are being bumped one on top of the other like so, they have very short wavelengths and it becomes very rough, very quickly. Not so good.

You got to know the different types of weather in the South Pacific if you want to go cruising in the South Pacific. In terms of it being the worst in the planet - nah, nah it’s not. It’s just got these local challenges, that’s all.

LINDSAY: You just reminded me that you wrote a publication a while back which has got a photograph that I took of Ramtha in a storm called the June Bomb in 1994. That’s a fantastic publication. A lot of what you were saying just now is in that book. Is that book still available?

BOB: It’s still available. You can get it from Boat Books or go to my website. I think there’s a link to a place where you can buy it online from Boat Books. It’s called the Met Pack. It basically describes the weather in the South Pacific.

LINDSAY: Mariners Met Pack South West Pacific.

BOB: I’m meant to be updating it in my retirement. Tim Ridge - the proprietor of Boat Books has been on to me a couple of times saying, “When are you updating it Bob?” and I think, “Well, when I get some time.”

LINDSAY: A good winter job perhaps.

BOB: Yeah, it’s maybe one of my plans for the future.

LINDSAY: Well that leads me onto my next question actually as we get down to the end of them here, what are your plans for the future?

BOB: I’ve got to get that book out and Tim has also asked me to write another book which is more for the coastguard for coastal weather forecasting in fact. So that we can get that as an adjunct into people who are doing their local boat masters course - which is good.

After I’ve got all that under way, I think it’d be time for me to move into the twilight years and do nothing for awhile. I’ll be trying to find an exit strategy.

LINDSAY: Is there anything more that you can offer our listeners to help them get started, gain momentum and realise their hopes and dreams of living a life on the sea, from a meteorologist's perspective?

BOB: Probably you will not be surprised at all for me to say to that, that everybody should get their own barometer. Make sure you know how to read one. There are various times of the day when you shouldn’t be reading them, how to log the observations and in my book there’s a whole chapter there which explains how to convert the reading of a barometer into a forecast. Perhaps you should track down that book as well. If you do those things you’re going to have a lovely life on the sea cruising around the South Pacific.

LINDSAY: Well thank you very much Bob. I really appreciate the time you’ve given us to share your knowledge and wisdom. There’s a lot of years of forecasting there that we’ve just touched on slightly. Thank you very much for writing those publications as well and the other one, the coastal one for around New Zealand. I imagine that there’ll be a few overseas yachts coming down in about four years time when the defence of the America's Cup is on. New Zealand will be buzzing then. If you’re still in semi-retirement they can look you up on is it?

BOB: Yeah. that’s correct.

LINDSAY: Okay. Hopefully you’ll still be busy giving advice.

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