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5
UNDERSTANDING FORECASTS
Checking the weather forecast before putting
to sea is one of our obligations under SOLAS
regulations. It is also advisable to cross
reference a number of weather sources.
‘I think a lot of people are too reliant on
spot forecasts.’ says meteorologist and sailor,
Chris Tibbs. ‘In part that comes from the
availability of some good apps and websites,
which are useful but there is a tendency to say, for example, that
Portland Bill is going to be 20 knots and leave it at that, but
unless we look at the big picture then we don’t know the
accuracy of that forecast.
‘To get the feel of the weather, a straight forecast doesn’t tell
us the variables we are describing. Today on the Isle of Wight we
have a lot of cloud and showers because there are the remains
of a front coming though. That means we are going to see lots of
variability in the forecast with rain, sun and different wind
strengths. Spot forecasts don’t reveal details, just a broad view.
‘We need the big picture and the pattern of the weather.
A week or so out from a journey, I would always look at the
synoptic charts from a few different sources to see the pattern
of the weather and look at whether there are fronts moving in
and that sort of thing, checking their reliability against each
other. Then I would be looking at the Shipping Forecast to give
a general pattern and broad trends, then finally I would look at
the inshore forecast to get a view of what the coastal conditions
are likely to be. For many that might seem like a basic checklist
and for myself, I would also be looking at GRIB files too. But it
is something people don’t really do enough, especially for a
relativity short day sail. Having a broad overview of what
weather is doing on a macro level is the only way a spot forecast
becomes useful, otherwise you are relying on incomplete data.’
‘Forecasts for 48 hours ahead are very accurate but you can
never be 100 per cent sure,’ says Nicola Maxey of the UK’s Met
Office. ‘Rain is particularly difficult as it is a bit like looking into a
bowl of boiling water and trying to guess where the next bubble
is going to come up.’
REMEMBER: Build a picture with your forecasting, and use
all the resources available. Modern forecasts may be more
accurate but that is only useful if you have the full picture.
6
LOCAL WEATHER EFFECTS
‘The biggest misunderstanding I see from
people when looking at the weather is that
they don’t appreciate the effect of localised
weather features,’ says Tibbs. ‘Everyone
knows that there is going to be an increase
in wind round a headland but people still
get caught. If you look at a headland, the
windspeed increase is around 10 knots and
can be as much as 15 knots. If you think about that, you would
happily leave a marina for a sail in 15 knots of breeze, but you
might not in 30 knots.
‘When I am forecasting for a trip I always split it into two parts,
the actual broad weather pattern, and potential land features and
thermal effects. During a UK summer if the land is going to heat
up and the gradient wind is in the right direction, blowing
onshore, then the sea breeze is going to have a massive effect.’
REMEMBER: Always include local weather effects in your
passage planning and to build your own forecast. A strong sea
breeze might be the difference between setting off or not.
EXPERT ON BOARD
Comparing weather
forecasts from different models,
like here on http://www.windguru.cz,
will help differentiate a reliable
forecast from an uncertain one
Local wind effects can be
bigger than you imagine
and should feature in
passage planning
