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It's twister season out in the US Midwest, with the long, straight roads of northern Texas, Oklahoma and surrounding statesÌýonce againÌýthe annualÌýhunting ground ofÌýhundreds of storm chasers and weather enthusiastsÌýfrom around the globe.

Be they professional orÌýamateur, they're all pursuing some truly awesome and rareÌýthunderstorms of a very specific variety: Supercells.

The colossus of theÌýCumulonimbus family, supercells are characterised by a persistently-rotating updraft (called a ) thatÌýensures the longevity of the storm cell; influences it's ground trackÌýand fuelsÌýit's propensity to deliver a variety of severe weather.

Sadly, the devastating nature ofÌýsuch extremeÌýweather on lives, homes andÌýbusinesses , as a supercell storm spawned a very powerful tornado - estimated at nearly a mile wide -Ìýwhich tracked acrossÌýChoctaw, Yazoo and HolmesÌýCounties, killingÌýten people, includingÌýthree children. Elsewhere, further tornados struck across neighbouring Louisiana, Arkansas and Alabama.

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

Well, thunderstorms are commonplace worldwide, but severe ones - in the strict meteorological sense - are not. Approximately 2,000 thunderstormsÌýare occuringÌýaround the world at any one time, with some 100,000 recorded globally every year.ÌýHowever, onlyÌý10% of these can be classified asÌýsevere. And of these, supercells are much rarer still.

To , 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 mightÌýarise from the passage of a cold front; or when low-level winds converge and air isÌýsqueezed upwards; or from the lifting generatedÌýwhere air rises over high ground.

But for the atmosphere to cook-up supercell storms,Ìýa vital 'special'Ìýingredient is also required:Ìýstrong .

Rather than replicate hereÌýsome very good web examplesÌýillustrating this process, , 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 andÌýbelow, courtesy of theÌýexcellent website hosted by theÌý...

I've been under supercells in the USA andÌýeven for the most ardent storm aficionado, they can prove decidedlyÌýnerve-wracking experiences.

About ten years ago - on holiday just outside Nashville, Tennessee -ÌýIÌýsaw a tornado warning issued on local TV and I ventured out to videoÌýthe weather, taking shelterÌýunder the eaves ofÌýa domestic garage.

The ink-black, turbulentÌýcloudbase 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 environmentÌý- with many trees and houses blockingÌýany clear, expansive view of the horizon - I appreciated just how difficult it would be to spot an approachingÌýtornado in such circumstances during daylight, let alone after dark.Ìý It was frankly aÌýclaustrophobic experience.

The has been developed over many decades, combining local coupled toÌýstate-of-the-artÌýmeteorological analysis; a network of trained ; 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 asÌýsevere storms lashed the country.
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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. , 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, butÌýin England,Ìýby notedÌýmeteorologists Keith Browning and Frank Ludlam. Their landmark study ()Ìýused radar to observe the characteristics ofÌýa spectacular hailstorm that tracked above Wokingham on 9 July 1959.

Last summer, I remember a couple ofÌýtimes - while on duty at the weatherdesk - watching supercells on radar and satellite formingÌýin 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 areÌýhere)Ìý- I'llÌýshow the best ones below.

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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 .

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 firstÌýindication that we're back in business," comments Steve Tucker, who took the photo below -Ìýin Yeovil, Somerset - at 8.30am today.

Isn't itÌýnoteworthy how something so otherwise familiar in our modern skies can suddenly seem so very significant at a moment like this?ÌýThey're aÌýskyborne reminder of how we take modern air travel for granted - and how our worldÌýstruggles whenÌýthe power of nature brings it all to a sudden halt.

Fingers-crossed we'll see a good deal of in the coming days. They'll find some very decent weather here on their returnÌýand by the weekend, it'll start gettingÌýnoticeably milder as a southwest flow starts to establish.

Indeed next week has the potential to turn decidedly warm, even very warm, inÌýtheÌýsouth and southeastÌýespecially.

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I awoke yesterday (Saturday) good and earlyÌýto provide online weatherÌýnowcasting for the ³ÉÈË¿ìÊÖ's coverage of the Chinese Grand Prix qualifying.

On venturing (somewhat bleary-eyed)Ìýoutside later about 9am - and greeted byÌýthe expected blue, cloudless skies -ÌýI 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.Ìý

RoamingÌýfurther afield to some local shopsÌýaÌýshort whileÌýlater, it was evident how the phenomenon had beenÌýrepeatedÌýon every car parked outdoors overnight,Ìýacross a swathe of .

But no rain had fallen overnight. And no prevailing southerly flow existed aloft, so noÌýchance ofÌýSaharan sand deposition -Ìýas often happens here into the warmer months.Ìý

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 fallenÌýlower through the atmosphere. Overnight, it settled - albeit very finely -Ìýonto cars and other surfaces. The uniform coating was subsequently disruptedÌýand streaked by the formation of dew before dawn; this later evaporated away after sunrise to cause a myriad of spots and drip-like streaksÌýacross car bonnets and windscreens.

AÌýspectacular sunset over Westbury, Wiltshire, where a veil of volcanic ash hungÌýaloft at around 6000ft by Friday evening (Photo:Dwayne Alexander)Ìý

The current and forecast extent of the ash cloud - and the altitudes affected for aviation - can be viewed through .

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

It's more bad news for air travellers this morningÌýas Britain's airspace continues to be restricted - alongside similar .

Ironically, it's the very same high pressure cell destined toÌýyield some glorious spring weather for usÌýthis weekend thatÌýkeeps the volcanic ash problem persistingÌýfor 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 ofÌýIreland - will continue feeding ash southwards from Iceland,Ìýover 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 (Saturday).

And it's by no means the end of this matter: as of 7am this morning (Fri.), theÌý was still throwing more ash up to around 18,000ft, but it's onlyÌýdoing 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-southeastwardsÌýwill 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 weekÌý- especially across northern parts of the British Isles -Ìýbut we'll have to await 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 (MarkÌýpresentsÌý³ÉÈË¿ìÊÖ Radio Gloucestershire's Breakfast Show).

Ìý(ABOVE) Thursday's colourful sunset, as seen from Tog Hill,Ìýnear BathÌý(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 .

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, 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 chatting to Steve LeFevre on ³ÉÈË¿ìÊÖ Radio Bristol's Breakfast Show this morning.

Eric was flying a British Airways Boeing 747, Flight 009,Ìýat night near Java in June 1982, when - at a cruising altitude of 36,000ftÌý-Ìýhis 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.

when the crew's persistence eventually paid-off,Ìýmanaging to re-lightÌýthe damaged engines after a powerless descent seemingly towards inevitableÌýditching in the dark Indian Ocean. Their close escape emphasisedÌýthe dangers posed by such volcanic clouds to aircraft and so believe me, if your flight has been cancelled today, it's beenÌýfor very sound reasons!

The wider weather situation todayÌýis 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 slowlyÌýas 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 asÌýbrought to us occasionally on warm plumesÌýfrom 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 atÌýdawn 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 devastatingÌý.

Obviously,Ìýthis time it'sÌýa 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 sunsetsÌýor sunrises,Ìýdo send the photos through to us and I'll put them on the blog - you'll find instructions for sending these here.

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Shanghai, 16-18 April 2010

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

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SUMMARY: Practice 1, 2, 3: DryÌýÌý Qualifying: DryÌýÌý Race:ÌýRainÌýexpectedÌý

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NOWCAST/FORECAST UPDATE: Sunday 06:00hrs BST: OK - here's the final forecast and it's essentially edging back more definitivelyÌýtoÌýthe thrust of our forecasts leading-up to today:Ìýwet running. It's much as we'd expected from the Met Office's Global Model, which held firm and was far more bullishÌýon this, whilst some other models (notably the GFS) continued inter-run 'flipping' right to the lastÌýand 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 andÌý - albeit dry right now at the circuit - current trajectoriesÌýshould see wet weather affectingÌýit 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

Enjoy the race!

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Compared to steamy, showery Sepang a fortnight ago,Ìýit's a very different weather scenario forecast in ShanghaiÌýfor 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 -Ìýand aÌýnod towards the conditions likely to prevailÌýat 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,Ìýwith a range from possibly as low as 12-14C on Friday, peakingÌýaround 20-22C by Sunday, it's a notably coolerÌýprospectÌýchallenging the teamsÌýcompared 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 presentingÌýa 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 aÌýcar-v-car dynamic we've yet to see, despiteÌýthe 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 duringÌýmost 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-froÌýand 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 afternoonÌýremains elusive.ÌýThe latest forecastsÌýcollectively emphasise a threat of showers at any stage of the race.ÌýHowever, 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, , 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 sufficientÌývariationÌýto 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 Ìýmedium-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 ofÌýforecast 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.Ìý What a nuisance...

The broad forecast theme is thus:

FRIDAY: I wouldn't be surprised if there's some generally low visibilityÌýaround beneath a weak Ìýby daybreak. Shanghai will lie beneath the ridge axis of an area ofÌýhigh pressure, drawing cool air off the Yellow Sea andÌýsome 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 howeverÌý(and feeling that way too, with low relative humidity);Ìýperhaps up to around 16C given sufficient insolation by mid-afternoonÌý(and probably closer to 12C for Practice 1).ÌýTrack temperature will clearly be rather low, too - perhaps 25+C but certainly way, way below the values experiencedÌýatÌýprevious race weekends this season. Breezy at times,Ìýin the E-SE flow.

SATURDAY: The inversion is still fairly pronounced by dawn, so rather a murky, hazyÌýstart is likely again. With the ridge axis just starting to topple eastwards and a more southerly component to theÌýprevailing flow, this - combined with higher (circa 14C)Ìý air - suggests not as chilly a morning. Otherwise, it's a continuation of the dry story this time around - so no repeat of Ìý Pitwall radar screensÌýshould 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 theÌýafternoon)Ìýand warmerÌýat around 20C air temperature. Track temperatureÌýshould readily recover to 25+C or soÌýin these conditions. Wind predominantly SE and breezy at times.

SUNDAY:

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

ModelÌýruns from various forecast organisations tend (even at this rather late stage)ÌýtoÌýdisagree onÌýthe rainfall distribution, accumulationÌýand timing of eastward clearance.ÌýThis rather poor continuity, both inter-run and between eachÌýmodel,Ìýhas notÌýbeenÌýunusual. Nonethless,Ìýthe combined ensembleÌýcollectivelyÌýaffords onlyÌýmoderate precision forÌýthe precipitation aspectÌýof Sunday's forecast (at best) and I do stress this! Crucially, for example, one model offers a dry window effectively throughout the race.ÌýA real forecast headache, for sure. It'll go to the wire in terms of pinning this one down with higher confidence.

On balance though,ÌýtheÌýrace does look likely to be rain affected at some stage, but just how wet (or not)Ìýmight the prospects be?ÌýThe teams will be very conscious of how much of theÌýdata assimilated through the dry practice and qualifying sessions could prove negated, come Sunday.ÌýIf 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 circuitÌýseeing 18-20C ambient. Aided byÌý, higher ÌýandÌýa 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.ÌýÌý

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 brightÌýreturns (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 - from which we can obtain very accurate time and location mapping for each cloud-to-ground flash acrossÌýEuropeÌý- 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 thisÌýpart of South Gloucestershire.

During my morning shift yesterdayÌýon tv and local radio, I'd been mentioning the strong likelihood of some areas across the West CountryÌýseeing thunderstorms with hail during the afternoon. As a storm aficionado, IÌýwas 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 decayÌýof the showers through the evening, however... we'd originallyÌýforecast them to dissipate more readily by early to mid evening, but a convergence zone kept them stubbornly persistant for some districts wellÌýpast 9pm. Such is the nature of providing the breakfast time forecasts - in developmental situations such asÌýyesterday, 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 acrossÌýmuch ofÌýEngland and especially for southernmost counties, such as Dorset and Hampshire,Ìýacross to the southeast, where the heaviest downpours are forecast through this afternoon. Some parts of theÌý³ÉÈË¿ìÊÖ Counties, for example, can expectÌýrainfall to exceed 25mm during the rest of the day.

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

UPDATE, MONDAY 5 APRIL:

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