As I write this, the weather outside has a distinctly autumnal feel. Rain showers are scuttling through on a brisk wind, the sun battling to have a say in between.
Yet this is one of the less windy days of late… and much less wild than is expected tomorrow.
Just four days after ‘Storm Ellen’ hammered Ireland with wind gusts that peaked over 100 mph on part of the southern coast, ‘Storm Francis’ is set to bring some unseasonably rough weather to the southern half of the British Isles. As this tweet by Simon Lee details, this storm’s minimum sea-level pressure (SLP, lower SLP = stronger storm) will be about 4 standard deviations below the long-term average, falling in the lowest 0.1% of historical observations!
Such rare events are difficult to foresee and plan for without the right tools to hand. Via suspended work, power cuts and travel disruption to name a few, they can inflict a significant economic cost across many industrial sectors.
A Disruptive Wind Forecast
The winds unleashed by Storm Francis are expected to peak tomorrow afternoon through to early Wed 26th Aug. The strongest gusts are forecast to be around 50-60 mph for inland Wales and Southwest England, near 70 mph for western coasts. Meanwhile, 40-50 mph is expected for remaining parts of England south of a line approx. Manchester eastward, including the Greater London area.
This Photo by Michael Garnett shows a 100 ft crane that collapsed during the St. Jude storm of 27th-28th October 2013. Licensed under CC BY-NC.
At any time of year, gusts to 40 mph or more are very disruptive to outdoor construction work. For health and safety reasons, it’s generally recommended that all such activity is suspended during such weather. Mobile crane activity may be stopped by gusts as low as 25 mph. Fixed cranes, depending on the weight and shape of the load, may have limits at 30-35mph. Working at height increases further susceptibility to this impact.
These short-notice delays can cost construction companies considerably, especially if these uncommon, but occurring weather events are not accounted for during the tendering/planning phase of projects. Successive events can have a multiplier effect, causing delays and even penalties, if severe & prolonged.
Threat from Vulnerable Trees
Where present, deciduous trees in full leaf are an addition complication when windstorms hit in late summer or early autumn. This part of the year, they are at their most susceptible to wind damage, increasing the likelihood of natural flying debris – twigs, branches – and tree falls. These pose dual risks – of injury and of delays to transport.
Placing Storm Francis into Context
Storm Francis is looking capable of bringing wind gusts of at least 40 mph to London, potentially 50 mph.
To demonstrate how unusual this is, I’ve looked at some wind gust statistics for the city.
It’s clear that wind gusts to even the lowest of these thresholds are uncommon in August. In fact, even through September, the percentage mainly stays in the 10 to 20% range. Which is to say, wind gusts to at least 40 mph typically occur once every 5 to 10 years.
Gusts to 50 mph are even scarcer. This has only been observed on than half the dates in Aug-Sep, and on those days, the percentage is rarely more than 10%.
Even in the more traditionally wild month of October, it’s generally a once in 5 to once in 10 years event. So, if that threshold is met tomorrow, it will be a truly exceptional occurrence! Yet, one that MetSwift’s quality assured historical weather data demonstrates is far from unprecedented.
Will Things Wind Down Soon?
Given the historical frequency of strong wind events, it’s reasonable to expect that quieter weather will return to the UK before long. Weather forecast models are hinting at a less windy spell within the first half of September.
However, looking on a broader scale, I have seen signs that this may not characterise the UK’s weather for September as a whole. This will be detailed in my next blog piece, which will explore the autumn weather prospects for Europe.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by David Baird is licensed under CC BY-SA