September Gales Pack a Punch

Although it's been rather unsettled at times in August, it has seen nothing like the unsettled weather than can set in by mid September across parts of the UK. As the Atlantic jet starts to develop, warm sea temperatures contribute to the development of warm air masses coming up from the south at the surface and as the upper air cools, it allows more vigourous low pressure systems to develop and then the usually stronger jet rapidly transports them across the UK with their attendant frontal systems, rain and strong winds.

Whilst we have heard a lot about Hurricanes Dean and Felix this season in the Caribbean, one of the problems that can sometimes occur in September and October is that tropical moisture can get wrapped up in our own low pressure  systems and this can cause some exceptionally unsettled weather, even quite early in on the season, if conditions are right.

For example, by mid September in 2005 the remnants of tropical storms ‘Maria’ and ‘Nate’ were queuing up to bring some very wet weather to NW Scotland and the Western Isles (also later going on to affect western Scandinavia). The following image shows the synoptic analysis at 00Z on the 13th: http://www.met.fu-berlin.de/de/wetter/maps/Analyse_20050913.gif

The dynamics of the atmosphere dictate that the warmer the air the more moisture it can hold and when very warm moist air reaches the hills of the western coasts of Scotland it is forced to ascend and cool, resulting in the moisture having to rapidly condense out (cooler air is not able to contain as much moisture), so falling as very heavy rain. This heavy rain moved into the west of Scotland by late on the 12th, and then continued pretty well unabated throughout the following day. Broadford (Skye) reported 128mm (over 5 inches) in the 24 hours to 10pm on the 12th. Aultbea (Wester Ross) reported 82mm in a 36 hour period.

As a result of the rain there was some flooding which affected parts of Skye around Portree and Broadford. On the mainland the steam train to Mallaig had to abandon its journey due to severe flooding at Glenfinnan (Highland), where the surrounding rivers had swollen and burst their banks. It was reported that the flood waters were as deep as a car window for much of the day.

In fact, the depressions containing ex-tropical storms “Maria” and “Nate” caused further problems as they continued east-north-east across the Norwegian Sea. When moving across Southern Norway, the towering mountains forced the still very moist warm air to ascend, so increasing the amount of precipitation falling yet further, as the air cooled rapidly. As a result, in Fana, south of Bergen, nine people were injured and 1 killed after a mudslide destroyed several houses in the area. People were immediately evacuated and a state of disaster declared. Overnight rainfall totals on the 13th/14th brought over 100mm to the mountains and 50mm generally across a large part of Southern Norway in fact.

Such weather is not as rare as might be expected during September. For instance, in 1999, the remnants of hurricane Floyd gave some very disturbed weather around the 22nd which gave some severe thunderstorms and heavy rain in places as well inducing a tornado that affected Pagham (W Sussex) early on the 23rd, causing damage up to 3km inland.

More memorably for many, back in 1961, the period from the 16th-17th saw the remnants of Hurricane Debbie cross NW Ireland, which brought severe gales to both Ireland and into western and northern parts of Scotland. As Debbie moved rapidly north-east, (see its track just here http://www.hurricane.com/hurricane-season/hurricane-DEBBIE-1961-865.php) unusually it maintained its true tropical characteristics until just before it hit Ireland. It was still a true hurricane just south-west of Ireland with 70kt (80mph) core winds at 0600Z on the 16th. There was a lot of coastal erosion caused in southwest Ireland as a result of the storm and the coast took decades to recover. The remnants of Debbie then caused heavy damage across the west of the UK and directly caused eleven deaths in Ireland.