Updated: 2 days ago
In this article we talk about weather forecasts and how to understand them
Sources of Weather Information
I'm only going to address inshore and coastal weather sources. I will talk about ocean-going weather sources provided via satellite or long range radio in another article. For the purposes of this article, I am talking about services available in the UK, but the principles around understanding a forecast are the same everywhere.
I hope we all agree that going to sea without a weather forecast is beyond stupid? In fact, so much information is readily available that ignoring it would be reckless.
Most sailors now use apps on their smartphones for accessing weather forecasts. As long as you have access to mobile data or wifi, obtaining a forecast is easy. Of course, after you leave land, your mobile (or cell) will quickly lose its ability to connect, unless you have a router and satellite on board.
I use the following apps on my iphone;
I tend to find that using Windy in conjunction with Predict Wind allows me to consider 'big picture' in tandem with realtime and for me, the dynamic nature of the way the Windy app works, helps me to understand what's happening and potentially identify how the forecast might change.
The Boatie App, available in the UK, is simpler, but uniquely to these three apps, it does provide updates of the actual Met Office Shipping Forecast and Inshore Waters Forecast.
In the UK, we also have other sources for weather forecasts, broadcast periodically in various forms. These include TV and radio (although only some radio stations provide sufficient detail for sailors). In particular, In particular, BBC Radio 4 broadcasts shipping forecasts on a regular basis at different times of the day.
Most marinas and harbours post weather for the coming +24 or +48 hours on their door or noticeboard.
You can also gain access to periodic weather forecasts via your VHF. The UK Coastguard usually broadcasts announcements on CH16 and then broadcasts the current forecast on their designated working/weather channel/s.
The best place to find broadcast information for different areas of the coastline is to look at your Reeds Almanac for the area in which you are sailing. The almanac contains a comprehensive table of coastguard stations, their broadcast times and forecast types and also details which channel they broadcast on for which area.
If you have Navtex, this can also provide you weather within about 250 miles of a coastal station and this option can be great for longer offshore or coastal passages.
Understanding a weather forecast
The first thing to appreciate is that a forecast is just that. It's not necessarily reality! Things can change. That said, modern weather forecasts in the northern hemisphere are usually fairly accurate up to +48 or even +72hrs ahead.
If you are listening to a forecast, detail is important. Most broadcasts are delivered at a reasonable pace, but sometimes they are rushed or radio reception can be poor, so make sure you are ready at your chart table with pencil and paper in hand and have no distractions.
Know your Sea Area
If you are sailing in unfamiliar waters, make sure you know the boundaries of your sea area.
Remember, whilst you are listening to a forecast on VHF, you may not be able to hear other traffic. Of course, you could set a dual watch on your VHF, but then it's likely you will miss parts of the forecast! Best let the deck monitor traffic via your handheld VHF and tell them to keep a good lookout.
Let's look at the +24 hour Inshore Forecast for the inshore sea area, Selsey Bill to Lyme Regis on Thursday 08 April 2021. There is then an outlook for the following day which I note as +48hrs.
It is presented on Boatie as follows;
Get the times right
You can see that the forecast is from 0600 on Thursday 08 April 2021 until 0600 on Friday 09 April 2021.
If you are listening to a forecast on the VHF it is critical that you delineate between the time the forecast is broadcast and the time that the forecast is issued and also, to what time period the forecast relates.
You see, a forecast might be broadcast at 1000hrs on Thursday, but be a repeat of a forecast issued at 0530 hrs that day for a time period from 0600hrs on that day. The detail matters, especially if you are expecting 'gales imminently'. This means gale force winds are expected within 6 hours of the forecast. But if you are listening to the 1000hrs repeat of a 0600hrs forecast, you might think you have 6 hours to get home when really, you have no more than 3!
What's the weather?
OK, so now let's look at what's forecast which is valid from 0600hrs on Thursday, 08 April 2021, for 24 hours.
WIND: West 2 or 3, backing South West 4 or 5, decreasing 2 at times later.
These numbers are referencing the Beaufort scale. If you want to see our video on how to convert Beaufort Scale to wind speed in knots, click here.
This means we can expect 5 - 10kts of true wind from the West. We can then expect it to back to the South West (which means move to the South West, anti-clockwise) and increase in speed to a F4 - F5 (approximately 15 - 20 kts). You might expect gusts of up to 25 - 28kts.
The wind will then decrease to F2 at times later (approx. 5kts).
SEA STATE: Smooth or Slight
As you'd expect from light airs at the beginning of the forecast, the sea state is forecast to be smooth, increasing to slight later, probably after the F4 arrives. A smooth sea state is 0.1 - 0.5 metres. Slight is 0.5 - 1.25 metre wave height.
Wear your jacket! It also suggests slightly unsettled weather. It could be gusty.
Good means visibility is more than 5 miles. On a small yacht, that's pretty much to the horizon in a flat sea. It also means no fog - which is always good!
That's great. We have a forecast and let's assume that it doesn't look like anything for our boat and our crew can't handle. What else can we get from this forecast?
Bringing Information Together
Once on passage, we should be comparing our forecast with reality. To do this, we can monitor our barometer to measure and monitor changes in barometric pressure. If the barometer starts moving, we can expect a change in weather and if it is moving at speed, we know to expect more wind. Perhaps a lot more.
Keep an eye on cloud cover. If high level cloud starts to cover more of the sky and/or cloud cover starts to thicken and come lower, you may have a depression coming your way. Check this evidence against the forecast. Turn your back to the wind and in the Northern Hemisphere, the centre of the low pressure should be to your left and slightly forward. This is called Buys Ballot's Law.
Are the waves larger than expected? Is this because of the overfalls you are sailing through, the fast-flowing tide or is there a larger swell than expected? A large swell can suggest more wind is on the way.
Also, keep an eye out for rain showers and tall, isolated clouds that might suggest rain showers and localised squalls. In certain weather conditions, I've experienced 5 - 10kts of wind interspersed by violent squalls. It's quite common near the Doldrums or along the East Coast of Uruguay, for example, in October.
In 2019, I spent an interesting 24 hours sailing along the coast of Uruguay in 10kt winds interspersed by huge 75 - 80kts squalls. That was sporty.
See below video of one of our close competitors on that race, at that time, just a few miles behind us.
THE BIG PICTURE
And finally, we bring it all together. Remember that the wind is backing and increasing from the West to the South West. We have rain showers and wind is going to increase slightly. If we were to look at our isometric charts for the same period, we would see that the edge of a warm front passes just north of us, whilst we are on the very edge of a High Pressure system giving us light Westerlies.
Buys Ballot's Law will confirm this is the reality when we are sailing. See our post on this if you haven't already.
The following +24hrs forecasts a veer to the North East and an increase in winds to F5 - F6 later in in the East, near Selsey Bill (near Portsmouth on the South Coast)..
Later means 12 - 24 hours after the forecast time.
The North Easterly in the Eastern part of the inshore forecast area is likely to be as a result of the low pressure system sitting between Scotland and Norway. So if that system doesn't do as expected, we might see the weather change as a result.
Here are the UK Shipping Forecast Terms, for your information;
Imminent - within 6 hours of issue of forecast
Soon - 6 - 12 hours
Later - 12 - 24 hours
Good - More than 5 miles
Moderate - 2 - 5 miles
Poor - 1100 yards* to 2 miles
Fog - Less than 1100 yards*
*A yard is equal to 3 feet, or 0.91m
Steady - Barometer change less than 0.1 mb in last 3 hrs
Rising or falling slowly - Change 0.1 to 1.5 mb in last 3 hrs
Rising or falling - Change 1.6 to 3.5 mb in last 3 hrs
Rising or falling quickly - Change 3.6 to 6 mb in last 3 hrs
Now rising or falling - A change within the last 3 hrs
Speed of Movement of a system
Slowly - Up to 15 knots
Steadily - 15 - 25 knots
Rather Quickly - 25 - 35 knots
Rapidly - 35 - 45 knots
Very Rapidly - Over 45 knots
The Douglas Sea Scale
0 0 metres (0 ft) Calm (glassy)
1 0 to 0.1 metres (0.00 to 0.33 ft) Calm (rippled)
2 0.1 to 0.5 metres (3.9 in to 1 ft 7.7 in) Smooth (wavelets)
3 0.5 to 1.25 metres (1 ft 8 in to 4 ft 1 in) Slight
4 1.25 to 2.5 metres (4 ft 1 in to 8 ft 2 in) Moderate
5 2.5 to 4 metres (8 ft 2 in to 13 ft 1 in) Rough
6 4 to 6 metres (13 to 20 ft) Very rough
7 6 to 9 metres (20 to 30 ft) High
8 9 to 14 metres (30 to 46 ft) Very high
9 Over 14 metres (46 ft) Phenomenal
I hope this gives you a good indication of how to obtain and understand a weather forecast. Of course, you would update your forecast before you leave and whilst on passage and plan for alternate ports of refuge if the situation were to change.