September’s Two Faces

Summer 2015 has proved persistently changeable for  most of the UK, with high pressure rarely asserting much influence over us for more than a day or two at a time (stay tuned for a detailed analysis later this month). Yet now, as we embark upon what some consider to be the first month of autumn, we are seeing a marked improvement in the UK’s weather prospects, as a strong area of high pressure very slowly moves across from the North Atlantic.

As of noon today, this high is still to the west of the UK (see Figure 1), with an area of low pressure to the east bringing enough instability to the atmosphere to allow numerous showers to develop across central and eastern parts of the UK. It’s also turning very cool for the time of year from the north.

Figure 1: GFS Surface pressure chart for 12:00 GMT Monday 1st September 2015. Image Courtesy of Netweather.

The battle between these two weather systems is expected to continue all week, but the high is expected to gradually gain control. By the start of next week, it’s likely to be sat right over the UK (see Figure 2), bringing fine, dry weather to most places, with temperatures recovering after a cool first week of the month.

Figure 2: GFS sea level pressure chart for 1200 GMT Monday 7th September 2015. Image courtesy of Netweather.

 

It can be argued that this is very typical for the time of year; think about how often the British public find themselves commenting on fine weather arriving just after the kids have gone back to school. I have heard it said that on average, September has the highest atmospheric pressure of any month in the UK, and there is an ancient proverb, “Fair on September 1st for the month” which suggests that September has long displayed such a tendency toward fine weather.

You might think, then, that September could be considered the most reliable month for fine weather when booking an outdoor break in the UK. Yet while there appears to be a good chance that high pressure will bring a lengthy fine spell at some point in the month, September has also been known to bring the first major Atlantic storms of the autumn season, and these don’t always turn up after the equinox (around 21st September).

The majority of these autumn storms are a result of increasingly cold air developing within the Arctic regions (as the received sunlight amounts fall rapidly through the month) and spreading south. Where that cold air meets warm air from southern parts of the North Atlantic, their interaction gives rise to large storm systems which often pass close to the UK and sometimes make a direct hit. Spells of wind and rain bring a typically autumnal feel to the weather, and as a storm system moves away from the UK, it can turn quite chilly as what remains of that air from the Arctic makes itself known.

Figure 3: Reanalysis chart for 12:00 GMT Monday 12th September 2011, showing surface pressure (white lines with numbers in millibars) and 500 mb heights (shaded contours – dark blues are usually associated with deep low pressure). Image courtesy of Meteociel.

The remaining storms – usually just a few each autumn – are a consequence of tropical cyclones moving north from the subtropical Atlantic and transitioning into powerful non-tropical storms (often referred to as extratropical storms, or ex-hurricanes). These can pack a particularly nasty punch, as was apparent in September 2011, when ex-hurricane Katia skirted the northwest of Scotland (see Figure 3) and brought a spell of winds gusting to near hurricane force (74 mph) to much of the UK. The strongest recorded lowland gust reached 81 mph at Capel Curig in Wales, while the summit of Cairn Gorm reported a 98 mph gust at the peak of the storm.
While this is among the stronger storms to have affected the UK in September, it’s not uncommon for at least one storm to bring wind gusts exceeding 50 mph for a time across many areas, which can prove hazardous – for example, leafy branches being torn from trees.

 

So there you have it – the two dramatically different faces of the UK’s typical September weather. In light of which, the increasingly fine, settled outlook for the coming 7 to 10 days gives reason to suspect that the second half of the month may see the return of stormier weather. Even September 2006, which stands as the warmest on record and sported an exceptional amount of fine, warm weather, saw a couple of spells of wet and windy weather crossing the whole country. So if you possibly can, I advise making the absolute most of the weather this coming weekend, even if it means a little bit of travelling!

Thunderstorms Ease Across The South East

 

Thunderstorms Ease Across Kent & Sussex.  Severe or Extreme Thunderstorms Expected Across Western Europe Today.

A turbulent mix of heavy downpours, general thundery rain and thunderstorms will continue to move away from Kent and parts of Sussex in the next few hours, to be replaced by more general heavy rain, with occasional intermittent torrential bursts.

Festivities are under way at the Notting Hill festival today and those frequenting the event will need to dodge the heavy rain, thick cloud and localised downpours.  An unsettled affair that’s for sure.

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Image courtesy of Estofex.org

The heavy thunderstorms are expected to increase in severity and coverage across large parts of Western Europe today with the European Storm Agency Estofex already issuing a rare level 3 alert for extremely severe weather for parts of Northern Spain and the Pyrenees.

Heavy rain and showers will continue across large parts England, Scotland and Wales through the rest of today, however drier and somewhat brighter conditions are expected across the South West through the early afternoon.

Lively Moments

 

Storms To Continue Across Kent & Sussex

Heavy showers and thunderstorms are expected to continue to track North overnight and through the early hours of Monday morning with Kent and Sussex most likely to be worst affected.

The storms should begin to ease off  early to late Monday morning being replaced by more dynamic frontal rainfall.

Main  threats include heavy downpours, localised surface water flooding, small hail and frequent lightning.