A low pressure system sits to the NW of the UK

Understanding what weather can do to the tides

A little understanding of what the weather can do to the tides does help explain what occurred last night. It was not simply a case of a ‘very high tide’ – the weather conditions played a significant part in causing the flooding. Storm Brendam being the principal culprit this time…

For those that know the area, the show field in Strontian and the Kilcamb fields were both under a little water. The Corran Ferry was closed around 7 pm due to high winds and an incoming tide that put the water onto the road outside the Inn At Ardgour. The A861 was impassable due to flooding at Inverscaddle Bay (and probably other areas as well) in Loch Linnhe. And the Crannog Restaurant in Fort William was also flooded.

The Crannog Restaurant in Fort William at the peak of the tide last night.
Image from The Scottish Sun online.

Yet the tide height forecast was only 4.3m at Salen!

In the four years that I’ve lived here, I’ve seen many tides forecast above 4.6m and the highest at over 4.8m.

So how did last nights weather make the tide height at least a meter higher than forecast?

Understanding how atmospheric pressure affects water levels

Do you remember the weather forecasts when they still showed the isobars?

Met Office Surface Pressure Chart for Monday 14th January 2020.
A surface pressure chart, or synoptic chart, courtesy of the Met Office. The isobars (the black lines) join areas of equal pressure and help indicate where the centre of the low-pressure system currently resides.

This is still one of my favourite ways of monitoring weather patterns around the UK in order to broadly see what is coming in and to help plan multi-day trips days in advance.

These charts show the atmospheric pressure and the isobars join areas of equal pressure together in the same way that contour lines on maps do for height above sea label.

You will hear talk of ‘high pressure’ or ‘low-pressure systems’ coming in and how they affect the weather. For now, let’s say that high pressure brings stable, often good, weather and low pressure unstable and foul conditions.

Last night at high tide the pressure was 971mb. This coming Tuesday it is forecast to be up to 1040mb and still rising. ‘Average’ air pressure at sea level is around 1013mb (Encyclopaedia Britannica in-line), so 971mb is pretty low. The lowest recorded in the UK was around a pretty extreme 920mb!

XC Weather forecast for Monday 13th January 2020.
The XC Weather forecast for last night. Low pressure (971mb), high winds (41 mph, over 60 earlier in the day) from the SW.

A high-pressure system exerts more downwards 

Consider that a low-pressure system consisting of ‘lighter air’ exerts less downward force on the water. With less ‘weight’ of air pushing the water downwards, there is a ‘rebound’ effect and the tides will be higher than forecast.

Around one meter higher for every 30mb drop in pressure…

So with last nights pressure being around 40mb less than an average, we could expect it to be about 1.2 meters higher than the forecast height.

Enough to take it over the top of the slipway in Strontian and into the parking area.

Sunart Jetty under water and starting to flood the car park area.
Strontian Jetty and the Green Shed 1.5 hours before high water last night. Already higher than I have seen it in four years1

What effect does the wind have?

This is where Strontian got off a bit lighter than Fort William. The winds were blowing up from the south for over 24 hours with gusts over 60mph on the forecast.

As Loch Sunart runs broadly east-west, it was relatively sheltered from these winds (let’s save orographic winds for another day). Fort William sits at the northern end of Loch Linnhe which runs roughly north-south.

With such strong winds pushing water northwards and a rising tide (peak tide was after 20:15) flooding towards Fort William, the sheer volume of water was higher than normal. As the Loch funnels through the Corran Narrows and again towards Fort William, the effects would have been even further emphasised and quite catastrophic in places.

Looking back low at the data from last night, it is not surprising that the tide was over two meters higher than forecast in Fort William. And all due to atmospheric weather conditions being caused by Storm Brendan located north-west of Ireland.

Windy graphical representation of the  weather patterns on Monday 13th January 2020.
A few hours before high water, this image from Windy shows the location of the low-pressure system NW of Ireland. It also shows quite large dark purple patches – or winds in excess of 50 mph!

I hope that those affected by last night’s extreme weather made it home (even if that meant a detour through Glenfinnan and Glenuig and coming home to a power cut). And I really hope those affected by the flooding can pick up the pieces and resume their lives again.

The tide heights are dropping, the atmospheric pressure is starting to rise again and the wind speeds abating slightly. That all means that we are unlikely to see a repeat of these conditions again today. Or this week.

Seaweed in the snowfield carpark on Tuesday 14th January 2020.
Seaweed in the show field carpark this morning and the new Strontian swimming pool in the show field itself!
2 thoughts on “Understanding what weather can do to the tides
  • Sally Faith

    Fascinating, Karl. Glad we went out mid Dec rather than mid Jan!

    • Me to! Although I usually get my first trips in during the second week of January under crisp, clear blue skies and perfectly calm water... But not this year.