Wiseweather September 2010 forecast & August monthly retrospective

Summary: CET temperature 14.3 or 0.6C above average*; rainfall 100% and sunshine 110%

*The average September CET 1971-2010 is 13.7C, last September 14.2 C, 2008 13.5C. 

Issued: 30th August 2000BST 

August was yet another month that saw a decisive split north/south; however whilst the north generally saw a fairly average month overall  the south, SE and east was both generally dull and wet. Mean temperatures were about 0.6C degrees below the CET overall but parts of the north west and south east were close to average.  

After a fairly dry but dull start for many overall until the 10th, it then became rather unsettled in the west & south and stayed that way with a couple of very unsettled wet periods though it remained relatively drier in the north. East Anglia saw as much as 300% of the average August rainfall in places, the South East was similarly afflicted. Sunshine was well below average everywhere, except in NW Scotland & N Ireland where it was nearer average (final sunshine and rainfall figures yet to be ratified).

Soil temperatures were near average generally whilst sea temperatures remained rather above average around most parts of the UK, though still near or slightly below average in the Irish Sea and around the SW approaches.

Looking now in more detail at August. August 4th saw thunderstorms develop widely across the south & SE in particular as a cold front with triple point associated low pressure moved east across the area. There was some heavy rain with flooding reported near Haywards Heath & Wivelsfield; Uckfield town centre was particularly badly affected with up to 48mm said to have fallen in 1 hour unofficially. August 12th was very wet in the Manchester area too with 51mm of rain reported at Woodford in the 24 hours to 1800. Localised flash flooding was reported. The next day, August 13th saw a slow moving occluded front moving south, as thunderstorms and localised flash floods caused disruption in parts of Kent, with Deal the worst affected area. One resident said that at the peak of the flooding, the water was waist high with 25mm estimated to have fallen in an hour in a few places. August 14th also saw low pressure continuing to sit across England with an associated slow moving  wraparound occlusion in place giving a very showery day with plenty of thunderstorms about. The wettest place was Manston in Kent with 35mm in 24 hours to 18Z. The heavy rainfall caused local flash flooding in eastern parts of Kent. Kent Fire and Rescue Service were called to the Westwood Cross centre in Thanet after six inches (15cm) of water flooded the shopping area with many shops suffering water damage.

There then followed a few days respite before another very unsettled period from the 20th to the 26th. August 20th/21st was a very active period across the country as a warm, very moist tropical airflow brought deep low pressure across the far north west close to NW Scotland and slow moving waves ran across the country further south especially across Wales and central parts. In the strong W-SW'ly gradient in the far NW close to the centre of the low pressure it was very windy with a gust of 70mph reported at Altnaharra, Sutherland, around 0200 GMT on the 21st, the strongest of the month. The warm moist air gave copious amounts of rain on upslopes too, Shap in the Lake District reported 51mm & Capel Curig (N Wales) reported 49mm in the 24 hour period to 18Z. However, the cloud broke in the east of England during the day allowing temperatures to lift and Weybourne (Norfolk) reported 26.4C, somewhat remarkably the highest temperature of the month- below 80F. As the cloud filled in again temperatures did not fall substantially in the moist tropical flow and many locations in the south and east didn't fall below about 19C. Weybourne's minima was 19.5C. Earlier in the day a pulse of the heavy rain across SW Wales became thundery due to its instability at medium levels and gave flash flooding in the Swansea area with the fire brigade responding to at least thirty emergency calls. Up to 30mm is thought to have fallen in an hour in places. On August 22nd/23rd the unsettled period continued as a developing low pressure area with some very warm & moist tropical air wrapped into it swung up across the SW and then out into the southern North Sea by noon on the 23rd. It gave substantial rainfall across parts of the south with around 40-50mm falling in parts of the Isle of Wight & Hampshire overnight on the 22nd/23rd, Thorney Island (Hants) reported 41mm. It was quite breezy in places too with Langdon Bay nr Dover (Kent) reporting a gust of 52mph. August 25th was another very wet day in the south as low pressure stalled in the SW approaches and its warm front pushed very moist semi tropical based air into the south and south west. Some parts of Cornwall saw as much as 40-50mm in places early in the day, Scilly reported 43mm in the 24 hours to 18Z and unofficially Carnkie near Wendron reported 60mm. In Devon fire crews were called to pump out flooded homes in Dartmouth. South Wales was very wet too, the Mumbles (Swansea) saw 60mm fall and at Tyn y Waun at the top of the Rhondda Valley, 63mm of rain fell in a 24 hour period with local flooding reported in Swansea. It was a cold day in Wales too, Velindre (near Brecon) reported its coldest August day since 1972 with a maximum temperature of only 13.5C. Further very heavy rain moved into the far south coastal counties in the late afternoon and was especially heavy in parts of Hampshire with 20-30mm falling in a few hours with further incidents of  local flooding in East Sussex and Kent, where the fire service was called out to deal with floods at several locations in Herne Bay and Ramsgate. Colder air pushed south though as the low pressure moved away into the North Sea and August 26th saw a cold night in parts of Scotland with Altnaharra (Sutherland) falling to -1.3C, the lowest temperature officially recorded in the UK this August. It was locally very wet in the east too with Wattisham (Suffolk) reporting 48mm in the 24 hours to 1800Z. As the month closed it was a cool bank holiday for many and a few places even started frosty on the grass too with Pershore (Gloucs) and Woodford (Cheshire) falling as low as -2C at 5cm.

Forecast summary for September 2010. 

The first week, September 1st-7th looks like initially staying dry and fairly settled for most eastern & southern parts, bringing more sunshine than recently for most as high pressure dominates the weather for a few days. Chilly with some fog, mist & low cloud for many inland by night though. As low pressure moves  west of Ireland a stronger S’ly flow developing though and the north and west unsettled again by the 4th onwards with a threat of quite heavy showery rain at times, this affecting central areas too by the 5th with gales in the far NW. It may well become very wet in the west & northwest at times in fact with localised flooding. Some reasonably warm days in the south & east at times at first but nearer average temperatures elsewhere. Over the weekend with low pressure likely to be dominating to the west, it looks like remaining unsettled with showers or showery rain with localised thunder slow moving across the west and central areas but the SE & east may well stay dry and rather warm in a maritime or continentally based tropical airmass. 

Temperatures overall rather above average though some chilly nights in prone areas but closer to average in the west, rainfall rather above average in the west but near or below average elsewhere though there may be heavy localised downpours and sunshine generally above average in the east and near average in the west.

By week two. September 8th-14th, there remains some uncertainty still about how unsettled it will be. High pressure may well still be fairly close to the east, bringing a rather SE’ly flow though the threat of rather more in the way of showery rain certainly remains here but at least some sunny weather at times and maybe some mist and fog by night. As an upper low becomes slow moving across the west though the north and west are very likely to remain unsettled for at least a few days and his probably includes all of Wales and the SW of England too where it may be well be quite wet at times with some thundery downpours. In between, in the more central parts, it does not currently look exceptionally wet but the threat certainly exists. By midweek drying up as an upper ridge moves across but then another area of low pressure moving into the NW and bringing further unsettled weather in the north though the SE may well stay drier and brighter by day with mist by night and ground frosts too. The week ending as a cooler NW’ly flow develops across all parts with showers by day and localised night ground frosts.

Temperature near average in the east overall probably but rather below in the west and north west, rainfall near average in the west and central parts but below in the east, though complicated by the risk of localised heavy falls, sunshine generally near average but perhaps above average in the east and SE and below average locally in parts of the west.   

During week three, September 15th-21st it seems as if the cool NW’ly flow will likely dominate across most parts for a few days at least still with showers in the west by day though generally drier in the east and SE but staying generally rather cool everywhere; however by the 16th/17th bring brighter drier weather to all areas with night frost and fog patches more widespread inland.

Temperatures generally below average overall for most parts with some cool nights especially though perhaps not too far from average in the east, rainfall generally below average though locally average in the west and sunshine near average or a little above average in the east. 
 

Whilst there are no clear indications about the end of September as yet from the 22nd onwards, it will probably become mainly unsettled however as the high pressure sinks away south east or maybe north east and low pressure dominates to the west & SW. Whilst the east & SE (and perhaps the NE too) stays drier, brighter longest, rain and showers will likely affect much of the west. Some patchy fog by night inland at first for a few days in the east but it will be too unsettled in the main for this to be very widespread with a SE’ly then a S’ly flow developing with a lot of low cloud about though especially in the west.

Temperature near average so probably not too chilly anywhere though in this period with a few rather warm days in fact in the SE and east again and mild nights; a lot of low cloud about though especially in the west. Sunshine overall below average except maybe average in the SE and East Anglia and rainfall generally near or rather above average.

Sea temperatures staying near or slightly above average for most parts, especially around the east coasts but perhaps slightly below average in the SW still. Soil temperatures near average for most parts with soil moisture also near or slightly above average for most areas. 

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