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Ashton Court, Bristol: Thursday 12 - Sunday 15 August 2010

This forecast will be regularly updated. This update: Sun.Ìý15 Aug., 10:30BST

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Changes of Emphasis since last update:Ìý Overcast, breezyÌýmorning on Sunday will be replaced by much sunnier weather PM; feeling warm, staying dryÌýand winds moderating. Good chance for the evening mass ascent.

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Nowcast Updates

(These have proved popular with my Formula One blog forecast audience, so here goes!)

SUN 1020BST: Well, the forecast for cloudy skies early this morning proved correct, as a veil of fairly thick stratus spread southwest all the way from the North Sea overnight. This, combinedÌýwith a rather brisk (10kt) northeasterly wiind, prevented the early mass ascent but I'm much more hopeful for later today. Post-midday, the stratus sheet should fragment and dissipate - it's already doing so anywhere eastÌýof Wiltshire and we'll gradually see this thinning process ease across Bristol, too. So, the afternoon will turn sunnier and warmer, with a maximum temperature of 23C. Should be good conditions for the Red Arrows after 4pm! Also, the surface winds should moderate later. High-resolution modelling shows that by 6pm, Ashton Court will have only 5kts surface wind (see Met Office model output for 6pm local time, below)Ìýshifting to due north rather than the earlier NE and with a possible westerly componentÌý through sea breeze influences. I am thus advising the Fiesta directors to assume that the evening mass ascentÌýstands a good chance ofÌýbeing a 'go'.

SAT 2200BST: I've spent much of the day forecasting on-site at the FiestaÌýwith Flight Director Don Cameron and Fiesta Director Clive Bailey. The showers duly appeared as expected, but nonetheless the crowds enjoyed a range of displays including Royal Navy Lynx helicopters and a veryÌýtalented powerglider pilot! The Night Glow should be underway now as hoped; the last of today's dying thundery showers are tracking westwards upÌýnorth of Ashton Court, with skies largely clearing now. Tomorrow's prognosis is far better, with the main weather issue for the planned early ascent being twofold: wind vector taking balloons towards Bristol Airport, plus perhaps early low cloud and mist which should readily lean towards dispersing. The rest of Sunday gets better by the hour: by mid-afternoon, sunny spells, dry and warm as temperatures reach 23C. So, it looks promising, despite the wind still vectoring balloons towards the airport... but I'm very, very hopeful we will see some flights take place.

SAT 1035BST: Unfortunately (albeit fortunately for me), this morning's spell of fairly heavy rain appeared very muchÌýas expected, associated with an and westward extension of aloft, which had the effect of increasing the rainfall intensity. It's all now eased southwards into Somerset, with dry but cloudy conditions following across Ashton Court. The combined poor weather prevented any mass ascent, obviously, but we should see spells of brighter weather into the afternoon. However, the next weather-related issue comesÌýin the form of towering shower clouds after midday. The today suggest these could readily form cumulonimbusÌýand turn thundery widely across southern England, if we see temperatures of 17C+. Cloud tops of these cellsÌýcould extend up past 30,000ft, with a convergence zone aligned north-south gradually moving westwards to lie close to Bristol by early afternoon.ÌýI expect this threat of heavy showers to haveÌýpassed by this evening's Night Glow, which should be dry with clear spells. Tomorrow continues to look a much,Ìýmuch better day in every respect. Well, I'm heading off to the Fiesta today to help with on-site forecasts and nowcasts - might see some of you there!!

FRI 1735BST: Ashton Court has thankfully dodged most of the rash of showers that have developed widely across the West Country today, but that luck might come to an end within aboutÌýthe next 30-40mins or so. An area of showers is now runningÌýdown into SE Wales and is likely to cross the Severn and trackÌýinto the Bristol area. Some moderately heavy rain is possible in places from this feature. Meanwhile, the forecast signal for a spell of wet, overcast weather early Saturday morning remains firm, which is not good news for any planned early mass ascent...

FRI 0950BST: Apparently 81 balloons managed to lift-off this morning, with Fiesta Director Clive Bailey telling me how his own flight eventually landed south of Chew Valley, encountering winds aloft around 15 to 20kts, but only 3kts at the surface on landing. So, the Met Office modelling of local windfield direction and strength has been very accurate this morning. You can see highlights of his flight - with reporter Will Glennon - on this evening.

FRI 0710BST: Latest conditions are an increasingÌý8 knot wind, visibility over 10km; scattered clouds at 4100ft and temperature 12C.

FRI 0700BST: A number of balloons up and away; some already touching-down in Hengrove. The wind turning brisker now both aloft and at the surface.

FRI 0636BST: They're up! The first balloon is away... more to follow. Fantastic.

FRI 0626BST: A light, small-scale shower currently easing south right now past Bath is the first hint of growing instability aloft. With the breeze also picking-up steadily from now on, the balloons need to getÌýup and away imminently, or the chance, I fear, will be lost for today...Ìý

FRI 0600BST: Looking promising! Clive Bailey says first balloon should be aloft in the next 30 mins. More to follow en-mass shortly after. Latest conditions at Lulsgate: Wind 270deg(W) at 3 kts; cloud ceiling and visibility ok; temperature 11C. Forecast all going to plan... so far!!

FRI 0500BST: A bit more cloud is now easing southwards, but with surface winds still effectively light at 7kts over at Bristol (Lulsgate) Airport, conditions still remain flyable for this fairly brief window of opportunity. I've discussed the forecast with Clive Bailey (Fiesta Director) an hour ago and he's certainly aiming for a 'go'. If they do get up, the balloons will head eastwards initially towards Bedminster; those ascending higher will then swing more to the SSE in the 15 kts gradient flow, off towards the likes of Pensford and Whitchurch.

FRI 0400BST: Conditions looking promising for this morning's planned Mass Ascent.ÌýRecent conditions have been light (5 kts) wind from the WNW; a little patchy cloud at about 4000ft and wholly dry. I'm very hopeful they'll lift-off as expected... incidentally, yesterday evening's Night Glow went ahead as hoped, despite being a bit breezy.

°Õ±á±«¸éÌý1800µþ³§°Õ: Positive news this evening, as per the forecast expectations outlined below. The special shape balloon will be displayed tethered, but some other balloons are set to fly. The Night Glow later stillÌýlooks good for this evening, as predicted. Enjoy!

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Forecast Summary:

The broadscale weather for this year's Fiesta isÌýlookingÌýreasonable at times but also very changeable: sometimes rather breezy and with an occasional chance of some possibly disruptive showers, especially later on Friday and Saturday.

After an inclement start to the week, we'll gradually seeÌýan area of low pressure shift eastwards and then southwardsÌýthrough the North Sea, while at theÌýsame time,Ìýhigh pressure establishes outÌýto the westÌýof the British Isles.

It won't prove a very clear-cut transition, however. An upper vortex dominates across the North Sea, eager to 'block' the high development from the west; the strong jetstream winds way up aloft will bow-downÌýsouthwards overhead Ireland Friday-Saturday; and one consequence of this somewhat messy 'squeeze'Ìýwill be moderately breezy conditions at times during the Fiesta. The broadly northerly flow across EnglandÌýwill also, at times, swing some showery outbreaks southwards within shortwave features, albeit Ashton Court should sit at the western extreme of this potential, with largely dry weather dominating.

The precise positioning and westward expansionÌýof the low pressureÌýcentre out east remains subject to someÌýflux in recent forecast models. This isÌýa veryÌýcritical aspect, as theÌýlocal nuances of hour-by-hour cloud cover, windspeed and occasional chance of showersÌýwill proveÌýthe trickier elements ofÌýmy forecast blog. Friday afternoon and much of SaturdayÌýcarry the greatest threat of showers, some possiblyÌýturning rather heavy across parts of the West Country.

One certainty is thatÌýthroughout the Fiesta, winds will be largely from the NW/NNW/N (on Sunday, NNE)Ìýacross Bristol and environs.

Flights - when they'reÌýpossible -Ìýwill thus head out towards the ESE /ÌýSSE / SE /SÌýof Ashton Court, i.e., away from Bristol city centre; broadly across towards Bedminster / Bishopsworth / Dundry (sometimes a bit more eastwards)Ìýand into parts of North Somerset andÌýB&NES.Ìý

Conditions are expected to see variable - and sometimes ratherÌýwidespread -Ìýcloud coverÌýcoupled withÌýbright / sunny spells; temperatures about average for the time of year and winds largely light to moderate, at leastÌýdown at ground level in Ashton Court itself.

The precise windspeeds / vectorsÌýat ground level and aloft - plusÌýa high chance of theÌýprevailing flowÌýrouting balloons towards Bristol International Airport airspace -Ìýare two of theÌýweatherÌýissues I can forseeÌýbeing problematic at timesÌýfor organisers and pilots.

It's not a unique situation forecast-wise, by any means.

isÌýthe Fiesta's Director and a hugely experienced pilot (as well as being great fun to be up aloft with, as I can recently attest!). He's overseeing the event weather forecasts for pilotsÌýand -Ìýin respect of the possible incursions towards Bristol Airport -Ìýtells me:

"We can possibly go over the top (of Lulsgate) at 3,000 feet... the best thing is being surrounded by people that want it to happen."Ìý

The airport, incidentally, has always been a very willing participant in assisting theÌýFiesta, whatever the conditions.

"They are great, and will do everything possible to get us away," says Clive.

The latest Met Office modelling of windfields up aloft at 600m for Thursday / Friday / overnight into SaturdayÌýsuggests a pretty steady 15+ kts or so (17 mph+) and sometimes 20+ kts, so yes, it'll be brisk at times and perhaps too much for some of the flights. ButÌýhopefully, we'll see decent flyable (and tetherable!)Ìýwindows of opportunity, too. For example, the low-level winds on early Friday morning look ok for the planned mass ascent at that time. The local nitty-gritty of wind forecasts will become clearer only much nearer to the time of each ascent: so fingers-crossed!

The cloudbase, although at times extensive as we see a fairly routine process of (and especially during afternoons), shouldÌýbe high enough to not prove aÌýcontinuousÌýconcern; equally, visibility should be OK.

However....

The third problem for pilots and organisers comes in the shape of some potentially very beefy shower clouds.Ìý

On Thursday, it's very likely to remain dry: any showers should be fairly light, small-scale, restricted to the early-mid afternoon period and probably affecting districts further to the north and east.

But the very latest (Thurs PM) forecast modelling from the UK Met Office continues to throw-up a pronounced possibility of a few showers during Friday afternoon, with Saturday also rather shower-prone at times, essentially throughout the whole day.

By Sunday, I'm expecting the threat of showers to have largely passed. Bar perhaps one or two isolated light showers, it should turn into a fine, dry, fairly warm but (at times) somewhat breezy day.

Here's the current thinking on day-to-day prospects...obviously, I'll be providing greater detail as each day nears:

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Thursday: Partly cloudy; some sunny intervals. Prolonged dry spells expectedÌýwith just a low chance of scattered light showers during the afternoon,Ìýthese mostly out in districts to the east / NE of Ashton Court. Dry by the evening andÌýwith lighter winds. Max. temp. 20C. Wind moderate WNW; likely to be breezier through the middle of the day.Ìý NB: Gates open 12:00pm

Flying: 18:00hrs, Special Shapes Ascent. Conditions essentially good, but the chance of flyingÌýwill be completely dictatedÌýby local wind parameters (turning moderate-fresh at times, which could be problematic) and possibly a scattered shower or two at that stageÌý(very low probability). Flight direction, if we get the launch,Ìýwill be to theÌýESE / EÌýof Ashton Court and somewhat modified by sea breeze influences, towards Bedminster / WhitechurchÌýand environs.

Thursday Evening: Night Glow, 21:30hrs. Should proceed as planned. Remaining settled and dry for the event; winds falling a good deal lighter and the east-facing slope at the Fiesta offering further shelter from the forecast wind direction.

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Friday: Starts dry, with someÌýfairly clear skiesÌýat this stageÌýand winds fairly light from the W/WNW. Looks good for the early Mass Ascent. But as the day progesses, convective developments will ease south towards midday, bringing a risk of scattered showers, some moderately heavy but risk of thunderstorms considered very low (these will occur in eastern parts of the country).ÌýA fewÌýbrighter spells too but generally rather cloudy PM.ÌýÌýLater overnight, some further showery outbreaks of rain will pass through at into Saturday. Max. temp 19C and a moderate to freshÌýbreeze from the NNW / N.

Flying: 06:00hrs and 18:00hrs, Mass Ascents.ÌýEarly ascent should be OK with a good chance of lifting-off,Ìýsubject to exact wind speed /Ìýdirection. However, the evening planned ascent could be curtailed by a combination of local convective development and winds.

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Saturday: Likely to be a rather grey and dampÌýstart, with extensive cloud cover; then developing into a day of fairly prolonged dry intervals, variable (probably rather extensive)Ìýcloud cover at timesÌýwithÌýsome sunny intervals; andÌýa fair number of showers across local districts, especially PM, some turning heavy in places. A real lottery in terms of the hit-or-miss wet weather probability. Winds moderate; fresher aloft; with gradient wind direction mostly fromÌýNNE to N.Ìý Max. temp 20C.

Flying: 06:00hrs and 18:00hrs, Mass Ascents.Ìý Early ascent currently looks very unlikely. Evening one might also prove difficult based on current forecast expectations;Ìýit will be subject to exact wind speed and direction and any local shower cloud development.

Saturday Evening: Night Glow, 21:30hrs. Still a low-ish shower potential but hopefully most will have faded to offer improving conditions: i.e., dry or mostly so; winds expected to be light-moderate at this stage. Assuming most (if not all) shower threat has passed; ground not too soddenÌýand winds fall lighter, should proceed as planned assuming tethered control of balloons is not problematic.

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Sunday: Early cloud tending to disperse to offer a settled, largely dry and generally much sunnier day; the wind expected to be more from the NNE, moderately breezy at times.ÌýFairly low probability of showers; around 30% and they'll be light and well-scattered. It will feel pleasantly warm in the sunshine; Max. temp 21C.

Flying: 06:00hrs and 18:00hrs, Mass Ascents. Subject to exact wind speed and direction (i.e., to avoid possible conflict with Bristol International Airport and excursions drifting beyond the Somerset coast!). Flight direction will be essentially to theÌýsouth / SSW of Ashton Court, into North Somerset.

Red Arrows: 16:30hrs: The spectacular finale to this year'sÌýFiesta should be unhindered by the weather; should be a full or largely full display from the RAF's maestro aerobatic team, as planned.Ìý

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More updates will follow...

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