COLUMN: Weather Wise

September floods: stranded vehicles near Sharow, Ripon.  Picture  ' Adrian Murray (12092642AM)
September floods: stranded vehicles near Sharow, Ripon. Picture ' Adrian Murray (12092642AM)

Gordon Currie examines the extraordinary weather of September and takes a look at prospects for the week ahead:

Red berries, last roses, the blue

Of the sky,

The wires a-whistle, to the

Swallow goodbye!

Through colours of autumn the

Change will grow stronger,

For Time’s gentle hand will

Linger no longer,

But the pathway to Winter

Weaves one golden strand,

With these riches of Nature,

The gifts of the land.

There was a feeling of inevitability when we reached the final day of September this year in bidding farewell almost thankfully to the summer half of the annual season.

Somehow it seemed by the grace of God the last day of the month had brought normality back to our weather, the exhilarating virtues of a genuine westerly wind, the saviour of our local climate in every twelve months.

Wonderful ‘streets’ of stratocumulus cloud rolled across the Vale of York with their chequer-worked patterns of sunshine and shadow across the early autumn fields.

Characteristically, they piled high across the up-slopes of the North Yorkshire Moors, looking quite ominous to the casual observer but there was no rain falling from these thickened grey rolls creating condensation within the clouds due to ascending currents towards Sutton Bank.

This was the typical facade to a normal September departure which seemed to unashamedly conceal the weather events of a truly extraordinary month.

We have been reminded yet again about the old piece of weather lore: “September dries up ditches or breaks down bridges,” a puzzling wisdom to many people but completely understandable when you consider the particular stage of the year, when weather conditions can either be influenced by lingering warm summer anticyclones that have truly outstayed their welcome following weeks or even months of drought, or alternatively, the cyclonic influence takes over, greatly assisted in some years (this one being a good illustration) by tropical storms that have wandered from their tropical sources into the mid-level latitudes of the North Atlantic.

In the case of September 2012 the emphasis was certainly on rainfall and broken bridges. Total rainfall amounted to 120mm (4.8in) making it the wettest September since 2000 with exactly the same rainfall.

These months constitute the second wettest Septembers on my records, beaten only by 1976, with 127 mm (5.08in).

Paradoxically, last month produced 16 completely rainless days – the highest number by a good margin in comparison with the summertime counterparts, June, July and August.

Over the past 12 years, the driest Septembers were 2004 with 13.3mm (0.53in) and 2007, 22.4mm (0.89in).

Reviewing the temperature levels, it was a slightly cooler month than average with a mean figure of 55.8F(13.2C) against the long term average, 57F (14C) which was the figure recorded in September last year, in spite of a heatwave in the final three days of the month.

The warmest Septembers on my records are 2006, 62.5F (16.9C) and 1949, 61.2F (16.2C). It is interesting to note that in 2006, September had 17 days with maxima of 70F (21C) or higher up till the 25th day of the month, peaking at 81F (27C) on the 21st.

Nine of the past 12 Septembers have been warmer than average. At the other end of the scale, the coldest September which started a very cold prelude to an early winter was in 1952, 49.5F (9.7C).

Looking at some of the month’s weather features this year, September began with nine consecutive dry days of westerly winds, featuring high pressure established to the south-west, an ideal situation for our conditions in the Vale of York.

Temperature maxima of 73 - 74F (22-23C) occurred on the 2nd, 3rd, 7th, 8th, 9th, and 10th, with two exceptionally mild nights, minima 63F (17C) on the 1st-2nd, and 9th-10th.

From the 10th onwards, temperatures began the seasonal decline with conditions becoming more unsettled as depressions and fronts moved across from the Atlantic, followed by sweeps of colder, polar air coming down from high latitudes on west to north-westerly winds.

However, the main event of the month was the development of a depression to the west of Portugal on the 21st - 22nd, which became seriously complicated by the intrusion of a tropical storm into the upper atmosphere.

The effects of this caused a “two-tier” situation, probably with the surface low pressure being steered by the upper level tropical core.

The surface depression moved northwards over England to the Scottish border, at which point it was halted, then reversed in its movement southwards again.

It is very rare for a depression of such depth (978 millibars) to change direction, which suggests some abnormalities occurring in the upper flow on the lines I have described. The consequence of these developments produced record rainfalls of 8.5mm.(0.42in) in the 24 hours up till 9am on the 24th, 65.5mm (2.61in) on 25th, and 21.2 mm (0.84in) on the 26th.

A glance at our historical records of September’s rainfalls relates that one of the wettest months ever recorded in the history of British weather was September 1918. In that month, the whole of England and Wales had more than twice the normal for the month.

Over a considerable part of this area, the fall exceeded three times the normal, while at Garforth, near Leeds, it exceeded four times the normal, which is around 60mm(2.40in) in that area.

It was estimated that in this one wet month, the average amount of water which fell was sufficient – if it could have been stored – to supply the entire population of the British Isles in 1918 with an allowance of 36 gallons per day, the amount used in London, per head, at that time, for 20 years henceforward!

PROSPECTS FOR THE WEEK AHEAD

North Atlantic cyclonic activity is likely to become more energetic again following a quieter period in the current week. Frontal systems will move across the country though the coming week but they will weaken towards the southern parts of the British Isles, with high pressure fairly resistant across the south of England.

A changeable week, with brief spells of rain or showers, generally lighter across the Ripon and Vale of York areas, with longer dry spells and more sunshine. Strong westerly winds at times, with day temperatures near normal, 55 – 57 F (13 – 14 C). One or two cold nights with slight air frosts possible, 32F (0C).