Archives for April 2010

It's twister season out in the US Midwest, with the long, straight roads of northern Texas, Oklahoma and surrounding states once again the annual hunting ground of hundreds of storm chasers and weather enthusiasts from around the globe.

Be they professional or amateur, they're all pursuing some truly awesome and rare thunderstorms of a very specific variety: Supercells.

The colossus of the Cumulonimbus family, supercells are characterised by a persistently-rotating updraft (called a mesocyclone) that ensures the longevity of the storm cell; influences it's ground track and fuels it's propensity to deliver a variety of severe weather.

Sadly, the devastating nature of such extreme weather on lives, homes and businesses was exemplified over the weekend in Mississipi, as a supercell storm spawned a very powerful tornado - estimated at nearly a mile wide - which tracked across Choctaw, Yazoo and Holmes Counties, killing ten people, including three children. Elsewhere, further tornados struck across neighbouring Louisiana, Arkansas and Alabama.

But what causes such violent storms - and are we at risk of them here in the British Isles?

Well, thunderstorms are commonplace worldwide, but severe ones - in the strict meteorological sense - are not. Approximately 2,000 thunderstorms are occuring around the world at any one time, with some 100,000 recorded globally every year. However, only 10% of these can be classified as severe. And of these, supercells are much rarer still.

To create any thunderstorm, some common factors must exist: deep instability in the atmosphere; moisture at low levels; plus a 'trigger' mechanism to force otherwise innocuous cumulus clouds to evolve skywards into giant storm cells. For example, this forcing might arise from the passage of a cold front; or when low-level winds converge and air is squeezed upwards; or from the lifting generated where air rises over high ground.

But for the atmosphere to cook-up supercell storms, a vital 'special' ingredient is also required: strong vertical wind shear.

Rather than replicate here some very good web examples illustrating this process, it's worth taking a peek at this website, which graphically shows how the critical influence of vertical shear can create a supercell storm, versus a 'normal' thunderstorm.

A colourful lexicon of meteorological and storm-chaser terminology has been developed to dissect this King of Storms into identifiable features. If you're after Overshooting Tops; Beaver's Tails, Wall Clouds or Flanking Lines, they're all here... See the graphics above and below, courtesy of the excellent website hosted by the US National Severe Storms Laboratory...

I've been under supercells in the USA and even for the most ardent storm aficionado, they can prove decidedly nerve-wracking experiences.

About ten years ago - on holiday just outside Nashville, Tennessee - I saw a tornado warning issued on local TV and I ventured out to video the weather, taking shelter under the eaves of a domestic garage.

The ink-black, turbulent cloudbase beneath a massive supercell delivered torrential rain; wild, gusty winds from the storm's outflow and frequent flashes of lightning. In the comparative clutter of this suburban environment - with many trees and houses blocking any clear, expansive view of the horizon - I appreciated just how difficult it would be to spot an approaching tornado in such circumstances during daylight, let alone after dark. It was frankly a claustrophobic experience.

The American system of tornado warnings has been developed over many decades, combining local doppler (NEXRAD) radars coupled to state-of-the-art meteorological analysis; a network of trained severe weather spotters across each state; an immediate media response to broadcast warnings and a generally high level of public safety awareness.

A video of a tornadic supercell in Poland, August 2008. Three people were killed and several injured as severe storms lashed the country.

What about closer to home? Supercells occur across many parts of Europe and in 2008 and 2009, caused some deadly weather in parts of France, the Netherlands, Germany and Poland. They are also recorded in the British Isles, but are exceptionally rare here. Nonethless, it's worth noting that the unique dynamics of these storms - with their rotating updraft - were first described not in the USA, but in England, by noted meteorologists Keith Browning and Frank Ludlam. Their landmark study (available here in .pdf format) used radar to observe the characteristics of a spectacular hailstorm that tracked above Wokingham on 9 July 1959.

Last summer, I remember a couple of times - while on duty at the weatherdesk - watching supercells on radar and satellite forming in various parts of Europe, including just across the Channel over France and the Netherlands. I'm not aware of any that formed across the British Isles in 2009, but what will 2010 yield? We shall see...

Have you experienced any European or British supercells? Share your experiences on the blog and feel free to send me any photos (instructions are here) - I'll show the best ones below.

Yes, they're back.

You've quite possible noticed how this morning's otherwise azure skies are criss-crossed by the high altitude contrails of airliners, as the ash-borne paralysis of UK airspace was relaxed overnight in a phased approach.

I've had a number of photographs sent to me this morning from Points West viewers, commenting on the sudden presence of the contrails.

"They're like pick-up sticks... the first indication that we're back in business," comments Steve Tucker, who took the photo below - in Yeovil, Somerset - at 8.30am today.

Isn't it noteworthy how something so otherwise familiar in our modern skies can suddenly seem so very significant at a moment like this? They're a skyborne reminder of how we take modern air travel for granted - and how our world struggles when the power of nature brings it all to a sudden halt.

Fingers-crossed we'll see a good deal of stranded passengers brought home in the coming days. They'll find some very decent weather here on their return and by the weekend, it'll start getting noticeably milder as a southwest flow starts to establish.

Indeed next week has the potential to turn decidedly warm, even very warm, in the south and southeast especially.

Quick Links:

• Latest Met Office updates and advisories
• BBC News live news feed on the ash cloud

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I awoke yesterday (Saturday) good and early to provide online weather nowcasting for the BBC's coverage of the Chinese Grand Prix qualifying.

On venturing (somewhat bleary-eyed) outside later about 9am - and greeted by the expected blue, cloudless skies - I was astonished at the state of my car. A right mess. And so was every other one parked nearby. Take a look at the photos I snapped below:

A very conspicuous film of sandy-grey dust covered every inch of my vehicle; the coating streaked effectively clear here-and-there, as if cleansed by dribbles of water poured from a watering can. 

Roaming further afield to some local shops a short while later, it was evident how the phenomenon had been repeated on every car parked outdoors overnight, across a swathe of Bradley Stoke.

But no rain had fallen overnight. And no prevailing southerly flow existed aloft, so no chance of Saharan sand deposition - as often happens here into the warmer months. 

You've doubtless now guessed the source: yup, it's that volcanic ash, now making it's presence felt down here at ground level across southern England.

The finer dust has progressively fallen lower through the atmosphere. Overnight, it settled - albeit very finely - onto cars and other surfaces. The uniform coating was subsequently disrupted and streaked by the formation of dew before dawn; this later evaporated away after sunrise to cause a myriad of spots and drip-like streaks across car bonnets and windscreens.

A spectacular sunset over Westbury, Wiltshire, where a veil of volcanic ash hung aloft at around 6000ft by Friday evening (Photo:Dwayne Alexander) 

The current and forecast extent of the ash cloud - and the altitudes affected for aviation - can be viewed through this link to the Met Office.

So, have you experienced the same where you live? It's been witnessed in other areas of the country already. We're feeding reliable reports of the dust deposition back to the Met Office, so do share your observations here on the blog...

It's more bad news for air travellers this morning as Britain's airspace continues to be restricted - alongside similar widespread flight cancellations across much of NW Europe.

Ironically, it's the very same high pressure cell destined to yield some glorious spring weather for us this weekend that keeps the volcanic ash problem persisting for aviation up above 20,000ft.

As I explained in yesterday's blog, the winds up aloft - circulating clockwise around the area of high pressure north of Ireland - will continue feeding ash southwards from Iceland, over a fair swathe of NW Europe and the British Isles. It's not going to readily budge over the weekend and the net result, at least in the near-term, is the extension of flight restrictions across the UK until at least 1am tomorrow (Saturday).

And it's by no means the end of this matter: as of 7am this morning (Fri.), the Eyjafjallajoekull volcano was still throwing more ash up to around 18,000ft, but it's only doing so periodically, resulting in a non-continuous plume. However, until we see a change in the prevailing upper air weather pattern, the feed of further ash to high altitude and effectively south-southeastwards will continue, at least on-and-off.

There's certainly a hint of changes to our weather by the end of the weekend and into next week - especially across northern parts of the British Isles - but we'll have to await further assessment by the Met Office to understand the implications for air travel, be they positive or otherwise.

Meanwhile, the ash cloud is likely to offer some vibrant sunsets and after yesterday's blog, I've received some photographs that offer a hint of some extra-colourful scenes witnessed across the West Country and further afield yesterday evening and indeed this morning. Have a look (below) - there's another reproduced on Mark Cumming's blog (Mark presents BBC Radio Gloucestershire's Breakfast Show).

(ABOVE) Thursday's colourful sunset, as seen from Tog Hill, near Bath (Photo: Steve Slade)

This specacular yellow sunset greeted residents at Gildersome, Leeds (W. Yorks), last night (Photo: Corrina Noble)

The sun looks a distinctive pink tone in this shot taken looking offshore from Portishead, North Somerset (Photo: Jon Botting)

Dawn as seen from Minehead, Somerset, this morning... a hint of more colour than normal, perhaps?(Photo: Si White)

Another view of the sunset off Portishead, with a vibrant glow above the distant cloud deck (Photo: Sydney Poots)

The lights of Bristol shine beneath vivid hues in this sunset view from hills east of the city (Photo: Graham Smith)

You've probably awoken today to hear the news of a volcanic eruption in the Eyjafjallajoekull area of southern Iceland - the second such event there in less than a month.

The volcano erupted beneath a glacier and was still actively spewing-out vast quantities of ash by midnight last night (Wednesday). Spectacular stuff indeed.

The impact here across the British Isles has become profoundly apparent this morning, with numerous commercial flights cancelled as the ash cloud - up at between 20,000 to 35,000ft - potentially threatens the safety of airliners.

I was very interested listening to retired airline pilot Captain Eric Moody chatting to Steve LeFevre on BBC Radio Bristol's Breakfast Show this morning.

Eric was flying a British Airways Boeing 747, Flight 009, at night near Java in June 1982, when - at a cruising altitude of 36,000ft - his aircraft unexpectedly penetrated a vast cloud of volcanic ash thrown-up by the erupting Mount Galunggung. Ingestion of the highly abrasive ash resulted in the sequential failure of all four engines.

A major tragedy was narrowly averted when the crew's persistence eventually paid-off, managing to re-light the damaged engines after a powerless descent seemingly towards inevitable ditching in the dark Indian Ocean. Their close escape emphasised the dangers posed by such volcanic clouds to aircraft and so believe me, if your flight has been cancelled today, it's been for very sound reasons!

The wider weather situation today is not great news for flights in and out of the country, as the ash cloud is expected to spread further southwards with each advancing hour, including across many parts of Britain.

We've got an area of high pressure located to the NW of Ireland, around which the clockwise circulation - including the winds high aloft - will tend to guide the ash cloud closer to us, dispersing slowly as it does so.

Down here at ground level in the West Country, any meaningful effects will be essentially nil. You're very unlikely to see visible dust, such as brought to us occasionally on warm plumes from the Sahara, as I explained in a blog from last year.

However, one potential outcome could become apparent during our sunsets and sunrises over the next couple of days.

The ash cloud, spread in aerosol form high aloft, can often result in very vivid colours at dawn and sundown. Bright reds, oranges, violets and pinks can adorn the sky in such situations, as exemplified by many contemporary reports from across the British Isles after the devastating August 1883 eruption of Krakatoa.

Obviously, this time it's a phenomenon on nowhere near the same pan-global scale (and the Krakatoa event resulted in worldwide temperatures falling by up to 1.2C the following year). Nonetheless, it's certainly worth watching for any changes in our skies towards sunset and indeed through tomorrow and into the weekend, too. If you spot anything by way of ultra-colourful sunsets or sunrises, do send the photos through to us and I'll put them on the blog - you'll find instructions for sending these here.

Shanghai, 16-18 April 2010

(This forecast will be regularly updated. This entry: Sunday 18 April 2010, 06:00hrs BST)

Quick Links:

• BBC Sport: Formula One
• BBC Weather: Shanghai forecast

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SUMMARY: Practice 1, 2, 3: Dry Qualifying: Dry Race: Rain expected 

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NOWCAST/FORECAST UPDATE: Sunday 06:00hrs BST: OK - here's the final forecast and it's essentially edging back more definitively to the thrust of our forecasts leading-up to today: wet running. It's much as we'd expected from the Met Office's Global Model, which held firm and was far more bullish on this, whilst some other models (notably the GFS) continued inter-run 'flipping' right to the last and wanted to lessen the rain considerably. As noted a couple of days ago, this one was always going to go to the wire, forecast-wise!

Rain is now falling quite extensively west of Shanghai and - albeit dry right now at the circuit - current trajectories should see wet weather affecting it within the next hour or so with moderate rainfall rates and the odd heavier burst possible. It's then all about the showery nature of things: will we see intermittently dry or drier spells during the event? As they'll be doing on the pitwall - keep an eye on the local rainfall radar at https://www.nmc.gov.cn/publish/radar/shanghai.htm

Enjoy the race!

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Compared to steamy, showery Sepang a fortnight ago, it's a very different weather scenario forecast in Shanghai for the teams, drivers, spectators - and Eddie Jordan's shirts, assuming he arrives there after flight delays courtesy of the Icelandic volcanic ash cloud.

Because one very noteworthy aspect - and a nod towards the conditions likely to prevail at various European races - will be the temperature.

After a decidely cool start on Friday morning, the race weekend will see these gradually rise by Sunday. Nonetheless, with a range from possibly as low as 12-14C on Friday, peaking around 20-22C by Sunday, it's a notably cooler prospect challenging the teams compared to Bahrain, Australia and Malaysia.

Indeed, it's shaping-up to be a little below average for this time of year in Shanghai (and worth noting the annual mean is 16C), but certainly presenting a much more comfortable working environment in the pitlane! Whether it'll make tyre performance more comfortable for some teams is another matter altogether... and it's fascinating stuff. It's a car-v-car dynamic we've yet to see, despite the rather cool but largely unrepresentative running experienced during winter testing.

During April, some 90-95mm of rain typically falls across the district of Jiading, where the circuit is located to the NW of Shanghai centre. The climate is classically sub-tropical maritime monsoon, but dry weather will prevail during most periods of track action.

Note I said 'most'...

....because Race Day has a risk of some wet weather.

Broad model agreement, with very good continuity, offers dry weather across all the practice sessions and qualifying too (McLaren and Ferrari radar-watchers will breath a collective sigh of relief!).

Sunday, by contrast, has been subject to all manner of forecasting swings to-and-fro and will be a wholly different challenge. It's likely to be affected by some rain at times, although the finer hour-by-hour detail for the afternoon remains elusive. The latest forecasts collectively emphasise a threat of showers at any stage of the race. However, it's by no means a 'done deal', if you are after a wet spectacle! I expect the heaviest rain to arrive after the event finishes.

The GFS (Global Forecast System) model, produced by the USA's NOAA, offers a snapshot of the inclement weather expected by Sunday early afternoon across eastern China. The oranges and greens represent rainfall, with a front close to Shanghai (arrowed, red) clearing slowly east.

One model (the USA's GFS) has tended to flip-flop the rainfall progression with sufficient variation to offer a very wet race on some previous runs, versus a more intermittently wet scenario on others. It continues to do this with almost every run of the supercomputer, seemingly struggling to handle the position and phasing of rainfall. The China Meteorological Administration's medium-range mesoscale modelling; UK Met Office's Global Model; Canada's global model and forecast products from Taiwan and Korea all offer broad support for a threat of showers appearing during the day, but with varying degrees of forecast precipitation rate/accumulation and subtle differences in the areal spread and timing of wet weather. Some bring it by the morning; others only by late afternoon. What a nuisance...

The broad forecast theme is thus:

FRIDAY: I wouldn't be surprised if there's some generally low visibility around beneath a weak inversion by daybreak. Shanghai will lie beneath the ridge axis of an area of high pressure, drawing cool air off the Yellow Sea and some low-level moisture beneath dry, cold air aloft. As the day develops, it will remain dry with hazy sunsine. Comparatively cool ambient temperatures will be the most noteworthy feature however (and feeling that way too, with low relative humidity); perhaps up to around 16C given sufficient insolation by mid-afternoon (and probably closer to 12C for Practice 1). Track temperature will clearly be rather low, too - perhaps 25+C but certainly way, way below the values experienced at previous race weekends this season. Breezy at times, in the E-SE flow.

SATURDAY: The inversion is still fairly pronounced by dawn, so rather a murky, hazy start is likely again. With the ridge axis just starting to topple eastwards and a more southerly component to the prevailing flow, this - combined with higher (circa 14C) Theta-W air - suggests not as chilly a morning. Otherwise, it's a continuation of the dry story this time around - so no repeat of Sepang's memorable qualifying 'banana skins'! Pitwall radar screens should be redundant on Saturday afternoon. Hazy sunshine should typify the scene at Shanghai; some high and medium-level veils of cloud (increasingly so later in the afternoon) and warmer at around 20C air temperature. Track temperature should readily recover to 25+C or so in these conditions. Wind predominantly SE and breezy at times.

SUNDAY:

Showers will start to make eastward progress into parts of eastern China throughout the morning, turning to more persistent rain at times.

Model runs from various forecast organisations tend (even at this rather late stage) to disagree on the rainfall distribution, accumulation and timing of eastward clearance. This rather poor continuity, both inter-run and between each model, has not been unusual. Nonethless, the combined ensemble collectively affords only moderate precision for the precipitation aspect of Sunday's forecast (at best) and I do stress this! Crucially, for example, one model offers a dry window effectively throughout the race. A real forecast headache, for sure. It'll go to the wire in terms of pinning this one down with higher confidence.

On balance though, the race does look likely to be rain affected at some stage, but just how wet (or not) might the prospects be? The teams will be very conscious of how much of the data assimilated through the dry practice and qualifying sessions could prove negated, come Sunday. If the entire race runs dry (which is not wholly impossible), they'll be highly relieved. All eyes to those radars again, come Sunday...

A few brighter spells are possible but the largely overcast conditions will depress temperatures, with the circuit seeing 18-20C ambient. Aided by higher dewpoint air, higher partial thickness aloft and a warmer southeasterly flow, it won't feel chilly - even with any leaden skies. I doubt track temperatures will reach much above 25-28C beneath the fairly extensive cloud cover.

Rather breezy again; winds from the south / SE.

More will follow!!

Nestled amongst the frozen veg, meat and chips in our freezer is a little sealed bag containing a handful of hailstones.

They're mostly about pea-sized, but nonetheless by far the largest I've seen falling here in Bradley Stoke for around a year or so. They arrived courtesy of a very lively thunderstorm yesterday (Friday) afternoon, which lit up the rainfall radar with very bright returns (see image, lower right) at about 5.45pm.

The skies had turned leaden across north Bristol as the impressive base of this Cumulonimbus rolled overhead, from which the distinctive white smears of falling hail were very visible. I explained how to spot these in an earlier blog.

And then down it all came... a spectacular and prolonged spell of hail, clattering on the roofs and quickly settling across pavements and lawns.

This particular storm cell turned thundery right above my area and the Met Office's ATDNet lightning detection system - from which we can obtain very accurate time and location mapping for each cloud-to-ground flash across Europe - showed strikes around the M4/M5 Almondsbury interchange, only a mile or so from my location. It was the first thunder I'd heard in quite a number of months. Last year, we experienced surprisingly few days of electrical storms, at least here in this part of South Gloucestershire.

During my morning shift yesterday on tv and local radio, I'd been mentioning the strong likelihood of some areas across the West Country seeing thunderstorms with hail during the afternoon. As a storm aficionado, I was delighted to have the most intense one showing anywhere on the radar deliver the goods right above my home!

I wasn't quite so delighted at the slow decay of the showers through the evening, however... we'd originally forecast them to dissipate more readily by early to mid evening, but a convergence zone kept them stubbornly persistant for some districts well past 9pm. Such is the nature of providing the breakfast time forecasts - in developmental situations such as yesterday, the local detail by evening is often impossible to accurately judge some 12 hours ahead...

We're likely to see some further heavy showers - potentially turning thundery in places - throughout the course of today (Saturday) too. Low pressure is tracking east along the English Channel, offering an unsettled flavour to the weather across much of England and especially for southernmost counties, such as Dorset and Hampshire, across to the southeast, where the heaviest downpours are forecast through this afternoon. Some parts of the Home Counties, for example, can expect rainfall to exceed 25mm during the rest of the day.

Oh - and worth stressing that tomorrow (Sunday) looks a good deal quieter all-round! A ridge of high pressure will largely keep the showers surpressed across the West Country. Some further rain is expected by mid-week, but thereafter - as we approach next weekend - I'm hopeful we'll enjoy a spell of some mild, dry and settled weather. 

UPDATE, MONDAY 5 APRIL:

Many thanks to Nikki Bradley, who has sent me this great shot taken of hail completely coating Braemar Road in Filton, NW Bristol, from the same storm.