Very wet end to September. September Review.

After a very dry month with only 14mm of rain Wednesday saw a wide rain band cross the area producing 14.3mm that brought the monthly total to 29.7mm, which was only 47% of the 36-year average and the driest since the record year of 2003 and 2009 when only 11.1mm was recorded. The southerly wind was very gusty during daylight hours with a maximum of 29mph.

Not surprisingly, due to thick cloud, the maximum temperature was depressed with a high of only 14.7C being 4C below the 36-year average.

The opposite occurred overnight as the cloud meant a mild night with the thermometer not falling below 9.2C being 2C above the October average.

There was hazy sunshine after dawn on Thursday followed by brighter periods and brief glimpses of sunshine.

September 2020 Review

Change in the month. Change in the weather

Although there were light rain showers at the beginning of September, amounting to 3.0mm, the first half of September was predominantly warm and dry. This was in direct contrast to the very wet second half of August.

An anticyclone developed in the eastern Atlantic that for a week brought light westerly winds. As the high pressure moved eastwards over the Continent, the warm, dry weather was brought on 1500-metre wide band of warm, dry air on a southerly and south-easterly breeze. During this dry, warm period, maximum temperatures were often above average.

During warm autumnal days the range of temperatures between day and night, referred to as Diurnal, can be considerable. An example occurred during the twenty-four hours on the 13th/14th with extremes of 24.3C and 6.9C producing a diurnal temperature difference of 17.4C. By way of contrast the diurnal temperature for the 7th/8th was just 2.6C with a maximum of 18.2C and a minimum of 15.6C. Incidentally, the range for the maximum diurnal has increased from an average 17.5C in the 1980’s to 19.0C in the last year or two.

Summer returned briefly from the 13th to the 16th as the thermometer soared to maximum of 27.1C, which was 8.4C above the 36-year average. This was almost a record but not quite as on 5th September 2004 the thermometer crept 0.2C higher.

Mid-September is the peak of the Atlantic hurricane system, which ends on 31st October. By 16th September already 19 storms had been named with just 2 named storms to go after Paulette, which became a hurricane, then Rene and Sally. Only twenty-one letters are used to name storms with the letters QUXYZ not used, as there is a lack of names beginning with those letters. If more than 21 storms form, the letters of the Greek alphabet are used. This has only happened once before and that occurred in 2005. By the 18th further tropical storms had developed that meant that the names of Alpha and Beta were used.

A storm is named when there are sustained winds of 39mph and a hurricane when wind speeds reach 74mph. It is important to the UK that these storms are monitored as they can be caught up in the Jet Stream, and although weakened as they cross the cooler water of the north Atlantic, could produce very stormy weather in the UK.

Whilst the Atlantic tropical storms were developing we enjoyed 18 days without rainfall and mainly modest winds whilst Hurricane Sally dumped 457mm of rainfall on the US Gulf Coast causing catastrophic flash flooding, which was the total rainfall we received in the first six months of this year. This dry spell of 18 consecutive dry days was the longest dry period since mid-May when that month also produced 18 dry days.

At the time when this unusual number of Tropical Storms developed in the Atlantic, a ‘’Medicane”, so named as a hurricane in the Mediterranean, developed and moved northwards over the Greek Islands. This was an unusual weather phenomenon and a new meteorological event for me.

During mid-September the wind came from a persistent north-easterly direction as the high pressure eased away over the North Sea, that still brought us dry and sunny weather if a little cooler. The strongest gust of wind this month, measured at 37mph on the 25th from the northwest.

Our variable weather, always changeable, produced an unusual set of conditions from the 16th to the 21st. Due to our geographical position, our weather can come from all points of the compass but during these six days the computer trace from the anemometer showed an almost unbroken line as the wind came steadily from the northeast.

September was a very dry month up to the 29th as total rainfall amounted to just 15.4mm whilst through evaporation from ground sources and plant life the equivalent rainfall of 60mm was lost to the atmosphere.

The month ended with a very wet day that produced 14.3mm of rainfall and boosted the month’s rainfall to a total of 29.7mm. This relatively dry month was the driest since the record set in 2003 and 2009 when just 11.1mm was recorded. The wettest September occurred in 2006 when more than twice the average fell with 131.9mm.

The mean temperature of 13.9C for September was almost exactly average compared to the last 36 years. However, that statistic hides the fact that the daytime average was 0.8C higher than the average whilst a number of cool nights meant the average minimum was 0.9C below.

An analysis of the average September temperatures shows quite clearly an upward trend. My data for the 1980s and 1990s produced an average of 13.2C but after 1997 there was a clear but modest increase to 13.8C that in the last few years lifted to 13.85C

It was a sunny month with solar energy the highest since this instrument was installed in 2009. We enjoyed a total of 174 glorious hours of sunshine.