fergieweather
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Posts posted by fergieweather
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UKV also generates spot totals around 30mm+; narrow corridor of heaviest rain is v similar to NAE and EC OP: approx. W/N Cornwall up across to Pembrokeshire, through W Wales, and on NNE towards Lancashire/S Cumbria. Meanwhile, to the S, any storms tomorrow could generate 25mm p/hr.
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Warning polygon extends from approx N Somerset/Bristol/N Wilts up N/NE through Midlands to NE England, valid from 1400hrs. UKMO website still kaput, hence warning graphics were emailed directly to us. Key point to stress, as Chief Forecaster says, is the "...low point probability but medium impact" of any such development. Latest NAE keen to pop-up cells broadly M4 (W Country) northwards into Oxon & beyond to areas of Midlands etc, circa 14-15z onwards especially. However, UKMO high-res models remain less keen on all this, so Ops Centre has taken precautionary approach, not least as their modelling supports threat of some isolated torrential storms, with a good dose of shear & hail likely. HOWEVER, must be stressed that models continue to support notion of a dry day for vast majority of districts across W Country & more especially so further E & SE. UKMO goes for hottest temps tomorrow likely Herts/Cambs.
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Hi Barb, yup, latest NAE ramps-up totals again; 30mm or so in places. Re positioning, given the broadly good agreement between EC & UKMO-GM & NAE, that's the story they're running with at Exeter for now. Chief makes no comment re GFS prog, but they'll of course considered it (albeit EC tends to be favoured anyway in many respects... we'll often see comparative analyses sent to us comprising UKMO output made alongside those from ECMWF, GFS, ARPEGE & sometimes JMA... they rate the latter v highly indeed). I can't post the EC output here graphically, so you'll just have to take my word on it, descriptively!
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Hi Coast, re the SE, UKMO suggests 'low risk' isolated storms in that area overnight into Sun & again later Sun, but notes that low risk is tempered by potential high impacts. However, new analysis (just in) carries similar story to before: early AcCas showers frm S, some possibly thundery into W Country, with then the frontal/wave showery rain aligned roughly Cornwall-Pembrokeshire-Cumbria into day (some heavy), whilst convergence feeds showers/storms (potential severe) in area by mid afternoon from approx Wilts/Glos N'wards into Midlands. This latter concern will be focus of warning which I understand will be issued at some point later this morning. Potential for isolated storms further SE later Sunday remains, but is, as noted above, considered "...low risk, but high impact". It's a tricky one, frankly, much as we dealt with yesterday as the chaotic elements of upper forcing kept us second-guessing what would happen each hour!!
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Yes, Sunday's concern is potential for heaviest rain in similar areas to those affected yesterday, as the final wave (with shortwave accompanying) runs up NE from SW peninsula. Latest EC Op rainfall accumulation for 06-18z Sunday shows strong signal for this very similar distribution to yesterday, with a very muted signal in SE, much akin to UKMO-NAE, GM & extended UKV.
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Yes, the key area, at least in UKMO analysis, remains northern/northwestern boundary of developing heat low through Sunday. The last 2 NAE runs have been consistent in this respect, with 06z analysis & high-res (UKV/UK4) output due in next 40mins. In broad terms, AcCas showers Sunday AM expected to spread N from S Coast early AM, primarily into W Country environs, with subsequent deeper destabilization into midday on periphery of frontal boundary as a marked area of convergence (signalled in last 3 runs) threatens to fire things up in a broad zone extending roughly Bristol / Wilts N'wards & NE'wards across the flank of the heat low. However, confidence on distributional bounds for threat of torrential downpours has been low based on recent runs, so UKMO Chief Forecaster awaiting better continuity before warning is given consideration (might be issued this morning). None of the last 3 runs keen to generate storms in SE quadrant, but detail not wholly trustworthy. Will update on sight of next analysis. Cheers, Ian.
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PS just to emphasise that the more likely snow threat is Monday. Latest UKMO-GM clear on this. As for first part of Feb, well..... a point of great interest now at UKMO! Ian
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Tricky one. Latest modified NAE just arrived from Chief Forecaster at Exeter; along with last UKMO-GM run from his Medium Range colleagues. Little significant to add, other than frontal progression remains problematic element later Sunday. Easterly bounds hard to judge; latest run has it into Bristol / M5 corridor by around 19z Sunday evening. Exactly how far east it extends thereafter... tough call, but the GM has more extensive snowy signal into the likes of Wilts / Glos northwards by 12z Monday, which is supported too by mapped ECM snow progs. But certainly NOT to be taken literally. Indeed, it's worth noting that even at some stage prior to the colder continental air advecting westwards, the prospect of significant snow for western uplands (especially Wales) and quite possibly to some lower levels exists through latter stages of Sunday into Sunday evening and night, as the boundary layer cools-off in the precipitation and aided by light winds. So, the easternmost bounds of the wintry ppn remains the key f'cast uncertainty then into Monday, but the UKMO products show a very clear and sharp decline of WBFL's on this leading edge flank, suggestive of a rather knife-edge situation re when and where the rain/sleet turns to snow, and how any waves causing frontal enhancement impact the ppn rates. The progged rates look very light on the leading edge; exactly how much snow open to question, but we'll see how next runs handle it and also the high-res (UKV/P) models.
Gotta go get ready for broadcast - cheers all, Ian.
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Jst off UKMO conference. Still same broad tenor of forecast for W Country: 2-5cm Cotswolds (locally more); N Wilts, M4, etc. Some into S Glos, Bristol / Bath higher ground especially, early - say 5-7am.. M4 could be v difficult across Wilts into Berks rush-hour: wet snow; settling amounts uncertain but certainly perilous on roads for a while. Chance of further wintry PPN Cotswolds later into Fri eve as feature runs SE out of Wales. Gotta dash! Cheers Ian
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UK4 additional snow depth to 10z Fri gives fair steer....prob underplayed in E Midlands.
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New NAE run gives key changes to any snow distribution and I've now checked the latest UK4 snow depth prog to 10z Fri.
E Mids now more threat; W Country somewhat less (now mostly Cotswolds E'wards, as per previous forecast emphasis); more for Chilterns etc. Totals are generally less too. Hopefully therefore, this won't prove an issue for rush-hour through a fair part of M5 corridor (Bristol, Glos etc.) - maybe some wet snow not easily settling, prior to 0800hrs - but more of a problem M4/M40 corridors eastwards. Tricky call re amber warning (they're all impact-based nowadays, as you will all know) but will await UKMO conference in a few mins.
Ian.
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yes there are quite a lot of unkowns yet Ian-much as I chunter on about-initial forecast wrong, amended forecast wrong, actual weather is between the two!
Exactly, John! Totally agree...!!
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Additional important thoughts from learned Exeter colleagues: "There has been a general run-by-run trend for models to follow this less developmental more zonal
track…and yet, there must remain a significant possibility that dynamic instabilities in the cross-Atlantic flow and/or more engagement of upper troughing east of Newfoundland will induce much more development." In other words: a LOW probability, BUT a high impact event if it occured.... and one hopefully that won't happen. Fingers-crossed...
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Out of interest after reading that page, in particular the sentence "The French meteorological office used a different computer weather model to the British, and the French model proved more accurate in predicting the severity of the storm in the Channel." do we know what the French are forecasting for Thursday/Friday?
Yes, Arpege broadly similar to other models with southerly track. BUT caution required all-round. As the UKMO forecasters advise: "
....there is still considerable uncertainty over the details and the situation will need close monitoring. Depends strongly upon the track and development of the system…and sudden and rapid deepening of a perturbation in the strong cross-Atlantic baroclinic zone remains possible, even if not captured by models..." They also note (from EC / MOGREPS and other analysis) that a more southerly track is plausible; a more northerly very much less so. But a lot to play for. V tricky for our colleagues in Exeter and they've played this one absolutely correctly in terms of modifying the forecast message to-date.
I think we are all making the classic '87 mistake of relying on the models for sub-tropical features and instead will have to solely rely on satellite/obs data even as far out as 12-24hr before the event in determining the systems final position. The same reason why the MetO got caught out before, i'm sure there not making the same mistakes twice with eyes already on the western side of the Atlantic.
exactly the situation, yes.
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It would be interesting to see if any site investigations in Darwen and Breighton pick up on any tornadic damage. Looking at the damage pics in Stockport I can say imho this was more a case of localised SLW, possibly exacerbated by a brief downdraught. Than anything Tornadic in nature..the damage pattern doesn't fit and I can see how easily the Public may have *thought* it was a Tornado especially when the front came suddenly and without warning, giving these convective bursts that would have attributed to the fallen chimney that caused one to be injured. Even a small tornado would be trackable on the ground, yet the only damage seen is within this row of shops.
Agreed. SLW is so often misreported as tornadic. That's not to rule the latter out - but these things are often reported uncritically....
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We've had another tornado report at Breighton, E Yorks - an HGV blown-over and reports of various damage in the locality....
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Just in on wires from colleagues at BBC Manchester, for info - (note on radar line convection elements over this site report) - -
"What's being described as a mini tornado's hit Stockport. One woman has been taken to stepping hill hospital after a chimney stack collapsed on her. Emergency services are on the scene. Tiles are said to have blown off the roof damaging cars .. and tables and chairs have been blown up the street. Dave Ashton works in Heaton Moor and saw what happened...."
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Thanks - yes, bit more time on my hands now with the F1 weather duties finished for another year!Hi Ian and welcome (back?) to the forum! -
For the Saturday and Sunday low all the models are agreeing on its path although its exact position is still unknown we are increasingly getting confident on its placement. How deep it will go is still unknown at this time but at the moment only the far North of Scotland would see the strongest winds. For the Monday storm not all the models are agreeing on its path or strength but the ECM and GFS show something similar meanwhile the UKMO doesn't show anything like it.
Indeed so, at least based on the latest UKMO-GM, but the new run and analysis is awaited currently. However, Exeter's medium range folks are well aware of the potential from this set-up into Sun-Mon, but the current uncertainty needs stressing, as per in their own words earlier today: "Some uncertainty regarding the development of a depression expected to cross S’ern parts of the UK during Sunday. A more developed system would bring a risk of heavy rain and strong winds across S’ern areas and perhaps some snow across N’ern areas. Confidence not high enough for any alerts at this stage...."
Watch this space...
Ian
Uk Convective General Discussion & Forecasts, 16th August 2012>
in Storms & Severe Weather
Posted
Cambs sounding, 15z Sun... note cap to overcome at circa 750hPa: IMG_0870.PNG