Setting the Scene

In the early hours of Thursday, a warm plume of exceptional moisture wraps into an area of low pressure developing over NW France. Having once looked to arrive late Wednesday, the approach has since been slowed down, but this has only served to raise the potential precipitation amounts further from what were already concerning amounts.

Following this initial event, any breaking up of the cloud over England and Wales during Thursday daytime has the potential to produce a sharp rise in temperatures followed by some slow moving home-grown thunderstorms. Most models are not that keen on this idea, but those that run with it produce very high totals but very localised in nature.

Then, going through Thursday night and well into Friday, we see another surge of moisture from the S or SE which may be accompanied by some severe storms across the far S and/or SE. Confidence in this is currently low so stay tuned for updates.

That surge of moisture then interacts with an occluding frontal boundary across western parts of the UK, greatly invigorating it, with the resulting very active frontal rain - probably thundery - tracking fairly slowly east, hitting a swathe of the UK with some more exceptional rain totals. The exact location won't become clear until late Thursday I imagine.

What follows is an early assessment of the potential magnitude of these weather events based on a combination of high resolution model outputs.

Early Thursday: 

Elevated storms are likely to develop in that tongue of very moist air and head inland across S. England (western 2/3 most likely) by dawn Thursday, bringing frequent lightning and some torrential rain in places - but generally weakening as they head north.

Models vary greatly on the intensity of the rainfall, with the lower end bringing some 20 mm or so by the time the storms clear, but the upper end dropping as much as 80mm in the space of 9 hours... Euro4 being one of those models (http://expert-images.weatheronline.co.uk/daten/proficharts/en/euro4/2015/08/11/basis12/ukuk/rsum/15081312_1112.gif)

Measures of storm rotation (helicity) suggest that these storms have the potential to organise rapidly, resulting in some wild conditions for some parts of the region while others escape with a (or some) fairly typical summer downpour(s).

Worth noting that the likes of WRF-NMM and GFS have more of a banded structure to the convection - like an organised line advancing north - which avoids a chaining of storms S-N, so reducing the highest totals (they're the ones with near 20 mm or so). Past experience suggests to me that the reality tends to be a real mix of banded features and independent cells, with a chaining of storms seeming to be more unusual when they are elevated. Not willing to rule out Euro4 based on that alone, though!

Skipping ahead to Friday (given the lack of agreement regarding home-grown action on Thursday), we have this from WRF-NMM:

That sure is an intense system moving in from the SE in the early hours. Worryingly, GFS shows the same sort of thing taking place. WRF-NMM and GFS producee more than 100 mm from that thing alone... with not far off 130 mm in the vicinity of Reading from the NMM. I daresay they're overcooking things as usual, but even so, 50-60 mm seems a reasonable bet if such a feature does develop.

Some models have it arriving further southwest - across The Solent, for example, with similar totals shown.

The 'final act' of the low pressure system responsible for all this is the very active frontal boundary during Friday daytime. NMM throws down 30-40mm for the areas affected (these shown well by the snapshot taken above-right).

The most extreme output I've seen is from a model based on NMM that seems to also have something in common with GFS. It belongs to a rival site so I'm just going to summarise the worst of it; Thursday's elevated storms drop some 70-80 mm across western Hampshire (i.e. where I am currently in residence...), after which Friday's active frontal boundary drops another 70 mm or so. Even with the import from the SE appearing to strike east of my location, I'm still left with a projection of 140 mm or so in the space of 48 hours. About 1/5 of a typical year's rainfall.

Although my house is very well placed to avoid flooding, I sincerely hope that model is overcooking things big time. Same goes for Euro4, as 80 mm in 9 hours over a large part of west Hants would surely turn the local streams into torrents.

Looking set to be quite moist for some of us. I'm expecting my turn here at Kent to occur on Thursday evening/night with around 30 mm and perhaps some thunder but it's like what you said, it really is subject to further fine tunings. However, I wouldn't be surprised if somewhere over Southern England will pick up over 100 mm before the weekend arrives - most likely the Hampshire/Berks area so it seems.

A lot of talk about this rain but i will believe it when i see it . I dont think you can pin forecasts on the euro 4 precipitation model as it was way out yesterday so has a poor starting point.  On the latest Beeb i sensed a slight downgrade in extremity. 

There hasn't been any talk of 'extreme' rainfall by the 'professional' agencies as far as I'm aware, just amatures talking of 'extreme' rainfall. 

The metoffice have only put warnings of 15-30mm with some localised totals of 50mm possible. None of this is 'extreme' by any stretch, but does have the potential to be problematic with regards to flooding.

The Met Office warnings released yesterday mention up to 40mm in the space of 6 to 12 hours with up to 70mm locally by Friday. I assume the warning details will be updated later today but this is the current text: 

"An area of low pressure is expected to push north then east across England during Friday. With very warm air entrained into the system, locally heavy rainfall is likely to continue as the system pushes across England and Wales. 25 to 40 mm of rain is possible in 6 to 12 hours, with as much as 70 mm possible very locally by the end of Friday. Localised thunderstorms may develop across the south and east of England during Friday afternoon, these potentially giving 15-25 mm of rain in a few hours.

The development and subsequent track of this system is very complex, and uncertainty in the details of this event is relatively high. As such this warning will be updated in the coming days as confidence increases."

 http://www.metoffice.gov.uk/public/weather/warnings/#?tab=warnings&map=Warnings&zoom=5&lon=-3.50&lat=55.50&fcTime=1439506800&regionName=uk

Ah so there is, I hadn't realise there was an additional warning towards the bottom, I'm using the mobile version.

So potential for up to 120mm in 48 hours in the most extreme case for an isolated spot or 2. Fair enough to call that extreme given that most could fall in short periods of time. 

As mentioned the situation is a complex and difficult one to predict accurately with Thunderstorms as well as very heavy frontal rain, and details are changing run to run as we get nearer.
Latest GFS indicates the heaviest rain along Western coasts and by the SE coast around Thursday midnight. Really heavy rain then develops up the western side of the country Friday morning. Flash flooding is likely in some areas throughout the period.
All I would say wherever you live expect the unexpected. In my neck of the woods and nearest the greatest area of instability, we could see potentially some severe weather here joining the western side of the country in disruption.

http://www.wetterzentrale.de/pics/Rtavn424.gif
http://www.wetterzentrale.de/pics/Rtavn484.gif

http://www.wetterzentrale.de/pics/Rtavn3611.gif

The latest EURO4 is showing some of the heaviest rain to be from the thunderstorms tomorrow morning.

http://expert-images.weatheronline.co.uk/daten/proficharts/en/euro4/2015/08/12/basis06/ukuk/prec/15081312_1206.gif

But all of the high-res models have utterly different rainfall distributions even 24-36 hours out.

GFS has swung from 108mm of rain here in 72 hours to 8mm :)

http://www.wetterzentrale.de/pics/Rmgfs72sum.gif

That's the beauty of thunderstorms, so unpredictable.

Yep 

I'm hoping we get a decent soaking, but not to the extent of flooding. My garden is still dry even after yesterday's lengthy drizzlefest.

The highest totals will likely be from plain rain in the west and north, with scattered storms/downpours 

to the east of it 

http://modeles.meteociel.fr/modeles/wrfnmm/runs/2015081212/nmmuk-1-37-0.png?12-19 

15z HIRLAM and GFS 12z look very different tomorrow morning. HIRLAM as you'd expect seems pretty much in line with the Beeb forecasts this afternoon. GFS has been very good at times this summer so I'd not write it off.

To be fair, all the models have been at variance today - and if any get it right I think it will be by luck.  Either the Scottish Borders, Wales, SW, Midlands, East Anglia, South Coast or South-east will get a lot of rain in the next 48 hours.   And any of those areas may not get much.  Beyond that, just watch the radar    Fun times!

Yes they have. Don't forget the WRF / NMM usually feeds off GFS data in Europe and so is likely to be very similar. 

I have a feeling its going to be a damp squib here and come Saturday I will be lucky if we are even close to average rainfall for the first half of the month.   Obviously not expecting any thunder!

Don't listen to him George! If dirty high pressure doesn't read stupid chart properly and make epic fail on your shed roof, then you have every right to say so!