Weather Watch: what to expect this September

Keen-eyed residents have been spotting some rainbow-washed skies as we transition into a new season. September will bring changing weather conditions - as well as migrating bird species and a harvest supermoon. Credit: Clockwise from top left: Kim Morton, Romano da Costa, Tina Hull, and Richard Bougeard

The summer holidays have drawn to a close, winter coats and umbrellas are being dug out the back of wardrobes, and longer nights are creeping in - September has definitely arrived.

But what weather conditions can we expect this month and what other environmental changes will we see? When is the official start of autumn? What is a supermoon and when can I see it?

Read on for the answer to all your meteorological musings this month.

Autumn and the Equinox

September officially marks the beginning of autumn, though the precise date depends on your definition.

Meteorological autumn begins on 1 September. This start date allows the season of autumn to coincide with our calendar months, making it easier for meteorologists to track and compare weather trends.

Astronomical autumn is determined by the Earth's axis and orbit around the sun. It begins on the day of the autumn equinox, which falls between 22 and 24 September depending on the year.

On this day, the sun shines directly over the Earth's equator and day and night are nearly equal in length.

The word equinox comes from the Latin words "aequus"- meaning "equal" - and "nox" meaning "night". Credit: Chris Page

September weather trends

In September, we'll notice cooler temperatures due to significantly shorter daylight hours, with the longer period after sunset also sending nighttime temperatures downwards.

In the Channel Islands, the average daytime maximum in September is around 20°C - comparable to June temperatures - with colder spells expected towards the end of the month.

However, as a 'shoulder month' - marking the transition from summer to autumn - it is not uncommon for hot summer weather to extend into September. In fact, in 2023, the hottest day of the year was on the 9 September - reaching 29.5°C in Guernsey and 31.2°C in Jersey.

Equally we can start to see colder spells affecting the islands, with this more likely to occur towards the end of the month.

The UK Met Office defines September as the beginning of the storm season as we see a greater chance of low-pressure systems and associated strong winds and rainfall.

These can sometimes occur due to ex-hurricanes that have crossed the Atlantic and begin to affect the British Isles.

Less sunshine and stronger winds than during the summer months are expected, with conditions beginning to get drier into September and drier still in October.

September 2023 was the hottest on record in Jersey Credit: Jersey Met

How warm will it feel overall?

So will you need to start packing your umbrella this month? Our weather varies year on year, but the following data can give an idea of what conditions we expect to see:

The average temperature in September:

The average rainfall in September:

The average amount of sunshine in September:

Wildlife watch

Wrynecks are rare visitors, but can be spotted passing through the islands in September. Credit: Romano da Costa

We are coming to the end of the flowering season. It is mostly later flowering plants - such as bramble, ragwort, knapweeds thistle ivy and heathers - still holding on.

September is also migration time, and we can expect a wide and varied selection of bird species passing through the islands.

Regular travellers include the yellow wagtail, common sandpiper, swallow, and lesser black-backed gulls.

Budding twitchers should keep their eyes peeled for some rarer species - from the dotterel and black tern to the wood sandpiper and Caspian gull.

Additionally, most of the world's critically endangered Balearic Shearwaters move into Channel Island waters to feed and moult.

If you are lucky, you could spot the rare dotterel passing through in September. Credit: Romano da Costa

Marine life

As the days shorten and sea temperatures drop, some seaweeds will mimic trees on land by changing colour, shedding foliage or dying back.

Animals at the mercy of the tides and winds - such as barrel jellyfish, Portugese-man-of-war and by-the-wind-sailors - will wash up as westerly winds pick up.

Mermaids' purses, which are eggcases from rays and dogfish, also appear along the shoreline as the glue that sticks them to rocks during spring and summer comes loose.

Whelks spend the summer building up reserves in preparation for spawning in the colder months when they attach their eggs to rocks, lobster pots or even seaweed.

Familiar commercial species like lobsters will feed less frequently as the temperature drops, and seasonal fishy visitors including bluefin tuna and bonito will head off.

It is common to see 'by-the-wind sailors' wash up during the autumn months. Credit: Tracey Williams

September skies

This month will mark the arrival of the Harvest Moon, which will also be a supermoon and feature a partial eclipse.

The Harvest Moon is the name given to the full moon closest to the autumnal equinox which falls on 18 September.

Supermoons happen because the distance between the moon and the Earth changes over time. A supermoon occurs at perigee: the point in the moon's orbit where it is closest to Earth so it may appear bigger and brighter. Supermoons in 2024 are 19 August, 18 September, 17 October and 15 November.

This year's Harvest Moon is unusual as it also coincides with a partial lunar eclipse, where the Earth's shadow will cover a portion of the moon's surface.

Channel Islands weather forecast
In 2018, residents witnessed a large red Harvest Moon hovering over the island. Credit: Richard Bougeard

Key dates for the diary

  • 1 September: the start of meteorological autumn

  • 18 September: Harvest Moon, supermoon and partial eclipse

  • 22 September: the equinox, and start of astronomical autumn

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