staplehurst

This one on the Isle of Thanet has just turned thundery...

There have been some suggestions that in the past 10 years (or more) the broad synoptic scale pattern has shifted a little further east, so in general the areas that used to experience plume breakdowns (the phasing of upper troughs interacting with the warm, moist low-level air masses) has shifted more to central and eastern Europe - with an increase in supercell activity in western Russia in recent years, for example - though this may be due to increased awareness through social media etc.

Yep! Though coverage doesn't really warrant a MDT - for now...!

Looks like a radar is down in SW Ireland and so the echoes here are being detected much higher in the atmosphere than at ground level by a radar further away - in other words, unlikely to be as heavy as radar suggests

It wasn't always like that

Don't forget the Stornoway radar is out until December for maintenance, so it won't pick up any rain over the Western Isles etc. NE England coming off as expected, several models suggested an uptick in elevated convection here as the approaching upper trough engages with the northern fringe of warm plume emanating from Wales last night. AROME in a broad sense seems to be handling things better with regards to the SE corner than any other model that i've seen during the past 2 days. EC and GFS suggest an increase in thunderstorm coverage over SE England and the Home Counties to Cambs by the end of this morning - we shall see. Less activity (and cloud cover) in the SE may bode better for later in the afternoon for areas largely north of London...

That is because of the resolution of the models used beyond T+60, it goes from 4km to 25km(?). Hence an individual shower becomes a large blob that covers a large area...

A lot of models struggle with spin-up times - in my experience of using the EURO4, the first 6 to 9 hours can generally be ignored for convection. That's why, as Paul says, it's not necessarily the most recent run that's got the best handle on the weather.

I agree with you there - even when I'm storm chasing in America, I do still get a little jealous of any storms that may be happening back home at the same time. There's something about having a storm on your home soil...

Overhyped would imply the potential was never there in the first place, and 'played up' for no reason - the potential is definitely there for severe (large hail, strong downburst winds, frequent lightning, flash flooding etc) given the atmospheric conditions sampled on the 12z radiosondes. It's all about managing expectations and acknowledging the uncertainty over whether the capping inversions can be weakened and overcome. The best potential has always been signalled for the evening hours, as the greatest instability spreads SW - NE with increasing upper forcing - but it's a question of how much will they phase together to be able to create a severe thunderstorm. Very finely balanced, and by no means guaranteed - am yet to see any forecast stating such severe thunderstorms would definitely occur, just that the potential is there for one or two...

As you say, the 650-700mb nose really is proving a pain in developing any deep convection. Plus the mid-levels are still very dry, and quite dry also at the surface too - all in all serves to inhibit deep moist convection. This was the 12z ascent from Nottingham:

I studied this a few years ago and looked at the period 2002 - 2012. We ran WRF model simulations for those 10 years to examine the CAPE distribution spatially and temporally - the net result was the highest CAPE in that 10 year period in the UK was just over 3,000 J/kg. But this is reliant on a model, since getting actual measurements of CAPE is very difficult given how few vertical soundings are taken in the UK (twice a day at a handful of locations). The model itself can be configured various different ways, and this will ultimately have an impact on its calculation of CAPE. Nonetheless, it is quite likely the values currently suggested by the GFS will be toned down by the time we get to Thursday, as the model frequently over-exaggerates both hot and cold conditions in the medium range. That said, Thursday does look very interesting from a convective standpoint, and given the broad synoptic setup one should keep a keen eye on potential triggers that may tap into some of that instability. You can read more about UK CAPE from the paper: http://onlinelibrary.wiley.com/doi/10.1002/joc.3976/abstract

The text also suggested "In general, potential is only 5-10% in any given location so barely worth a LOW threat level issuance." Still a low chance of medium level instability release overnight in same areas, but more likely over the North Sea getting close to Shetland, most of it probably not sufficiently deep for lightning.

Did some digging and found a model with a glimmer of hope! 12z CMC hinting at some elevated thunderstorms which may just graze east coast of East Anglia Thursday morning, before moving away across North Sea. Otherwise, though, very little support right now for anything more than that. Not writing it off entirely given some uncertainties and finessing over the next couple of days, but chances are looking very slim at the moment.

Not a huge amount of model support for a thundery plume - a few days ago several models were allowing some elevated thunderstorms to flirt with SE England / E Anglia late Wednesday night into Thursday morning, but most models have since backed off this idea and retained most of the thundery weather (which looks quite widespread) over the Continent. That said, residual warm, moist air over E Anglia / SE England and an upper trough migrating east with a weak surface cold front might just be enough to develop some isolated heavy showers / thunderstorms perhaps on Thursday, but this very much depends on the shape of the upper trough, how much warm/moist air is still in place etc. Ideally want the upper trough to be slower and dig further south to advect better unstable air northwards into CS England.

Hi Harry, do you know roughly what size (diameter) the hail was? Very jealous of your downburst!

The date's missing on the end of the URL, should be http://convectiveweather.co.uk/forecast.php?date=2017-04-30

We try our best Agree though, it's a shame John is leaving - but can understand why after doing shift work for such a long time. I'm sure we'll (hopefully) see him again somewhere in the not-too-distant future.

Yes, the raw model comes out at 5am (ish) but it has to go through various modification processes at the Met Office before being sent to the BBC, and so it doesn't appear until late morning. Lunchtime bulletins use 00z data, evening bulletins use 06z data, and breakfast 18z data etc

That'll be because they're using the 18z EURO4 (and perhaps some blend of UKV) at this stage. 00z data doesn't come into the system until late morning

18z high-res models suggesting that a slight NW shift of the Amber warning may be required - EURO4, for example, extends the risk of disruptive rain across Hertfordshire into south Cambridgeshire on Friday morning.

Ha! I'd say you have a reasonable chance, more likely between midnight and say 5am, but probably on the western edge - go any further west and suspect your chances will drop fairly quickly...

I've often commented in the office how storms do tend to develop along the M25, i.e. around London, and then the steering winds carrying them across the capital on a surprising number of occasions. Today wasn't really one of these type of events (as it was most likely UHI driven combined with the convergence) but it does happen a surprising number of times that London tends to be in a hole initially. Or just coincidence?

Would never go red unless I was absolutely certain storms were going to happen - which let's face it, hardly ever happens! But given the concerns over the volume of water and risk of flooding, have upgraded to SVR just for the rain aspect alone (which I've never done before)... Lightning is not a-given with the activity later tonight, it could end up being a large convective mess with not much lightning within it, but nonetheless things will pick up largely post-midnight, and more especially towards the end of the night in terms of rainfall at least.

Further to my previous post, the Met Office are considering issuing an Amber warning for London and environs down to Hampshire for these reasons...