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2010 Bristol International Balloon Fiesta: Weather Forecast

Ian Fergusson | 13:32 UK time, Monday, 9 August 2010

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

This forecast will be regularly updated. This update: Sun. 15 Aug., 10:30BST

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Changes of Emphasis since last update:  Overcast, breezy morning on Sunday will be replaced by much sunnier weather PM; feeling warm, staying dry and 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, combined with 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 east of 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) shifting to due north rather than the earlier NE and with a possible westerly component  through sea breeze influences. I am thus advising the Fiesta directors to assume that the evening mass ascent stands a good chance of being a 'go'.

windfields-sunday-6pmlocal.jpgSAT 2200BST: I've spent much of the day forecasting on-site at the Fiesta with 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 very talented powerglider pilot! The Night Glow should be underway now as hoped; the last of today's dying thundery showers are tracking westwards up north 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 much as expected, associated with an occlusion and westward extension of shear vorticity 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 comes in the form of towering shower clouds after midday. The atmospheric profiles today suggest these could readily form cumulonimbus and turn thundery widely across southern England, if we see temperatures of 17C+. Cloud tops of these cells could extend up past 30,000ft, with a convergence zone aligned north-south gradually moving westwards to lie close to Bristol by early afternoon. I expect this threat of heavy showers to have passed by this evening's Night Glow, which should be dry with clear spells. Tomorrow continues to look a much, much 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 about the next 30-40mins or so. An area of showers is now running down into SE Wales and is likely to cross the Severn and track into 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 Points West this evening.

balloon-mass-ascent-fri-dav.jpgFRI 0710BST: Latest conditions are an increasing 8 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 get up and away imminently, or the chance, I fear, will be lost for today... 

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. Recent 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.

THUR 1800BST: 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 still looks good for this evening, as predicted. Enjoy!

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Forecast Summary:

Balloon-ian_fergusson_2010.jpgThe broadscale weather for this year's Fiesta is looking reasonable 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 see an area of low pressure shift eastwards and then southwards through the North Sea, while at the same time, high pressure establishes out to the west of 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-down southwards overhead Ireland Friday-Saturday; and one consequence of this somewhat messy 'squeeze' will be moderately breezy conditions at times during the Fiesta. The broadly northerly flow across England will 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 expansion of the low pressure centre out east remains subject to some flux in recent forecast models. This is a very critical aspect, as the local nuances of hour-by-hour cloud cover, windspeed and occasional chance of showers will prove the trickier elements of my forecast blog. Friday afternoon and much of Saturday carry the greatest threat of showers, some possibly turning rather heavy across parts of the West Country.

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

Flights - when they're possible - will thus head out towards the ESE / SSE / SE /S of Ashton Court, i.e., away from Bristol city centre; broadly across towards Bedminster / Bishopsworth / Dundry (sometimes a bit more eastwards) and into parts of North Somerset and B&NES. 

Conditions are expected to see variable - and sometimes rather widespread - cloud cover coupled with bright / sunny spells; temperatures about average for the time of year and winds largely light to moderate, at least down at ground level in Ashton Court itself.

The precise windspeeds / vectors at ground level and aloft - plus a high chance of the prevailing flow routing balloons towards Bristol International Airport airspace - are two of the weather issues I can forsee being problematic at times for organisers and pilots.

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

Fiesta Flight Director, Clive BaileyClive Bailey is the 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 pilots and - in respect of the possible incursions towards Bristol Airport - tells 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." 

The airport, incidentally, has always been a very willing participant in assisting the Fiesta, 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 Saturday suggests 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. But hopefully, we'll see decent flyable (and tetherable!) windows 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 convective infill (and especially during afternoons), should be high enough to not prove a continuous concern; equally, visibility should be OK.

However....

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

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 expected with just a low chance of scattered light showers during the afternoon, these mostly out in districts to the east / NE of Ashton Court. Dry by the evening and with lighter winds. Max. temp. 20C. Wind moderate WNW; likely to be breezier through the middle of the day.  NB: Gates open 12:00pm

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

balloons-nightglow.jpgThursday 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 some fairly clear skies at this stage and 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). A few brighter spells too but generally rather cloudy PM.  Later overnight, some further showery outbreaks of rain will pass through at into Saturday. Max. temp 19C and a moderate to fresh breeze from the NNW / N.

Flying: 06:00hrs and 18:00hrs, Mass Ascents. Early ascent should be OK with a good chance of lifting-off, subject to exact wind speed / direction. 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 damp start, with extensive cloud cover; then developing into a day of fairly prolonged dry intervals, variable (probably rather extensive) cloud cover at times with some sunny intervals; and a 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 from NNE to N.  Max. temp 20C.

Flying: 06:00hrs and 18:00hrs, Mass Ascents.  Early ascent currently looks very unlikely. Evening one might also prove difficult based on current forecast expectations; it 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 sodden and 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. Fairly 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 the south / SSW of Ashton Court, into North Somerset.

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

 

More updates will follow...

 

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