Seems like there ought to be a dedicated thread for the rainfall events coming our way this week.

To push the boat out, here's my analysis of the 12z model output that I posted earlier in the MOD thread:

This is what Euro4 predicts will fall prior to the arrival of the second frontal system tomorrow. This includes what has already fallen today in the SW since noon. Quite a sharp drop off in amounts heading inland from the coasts in the affected regions, except of course where high ground gets involved.

The model develops countless areas of heavier precipitation which stream across the affected area. Their exact locations will vary around what is shown on this individual run, which is why forecasts such as those from the Met Office choose give a range of possible precipitation totals (latest values in the warnings here: http://www.metoffice.gov.uk/public/weather/warnings/#?tab=map)

Based on the Euro4 chart above, coastal counties look to be in for 10-25 mm while high ground may see as much as 40 mm.

Then the second frontal system comes along, and Euro4 makes more of this one, which appears to be due to the presence of more convective elements, particularly showers Wednesday daytime, which are still ongoing at the point the run ends.

Totals for 12 noon today through to 12 noon Wednesday:

It looks like a large part of Northern Ireland is in for a real soaker, with up to 50 mm possible from the two fronts combined.

Across England and Wales, the distribution of totals remains similar to the previous chart shown, but the accumulations now extend a lot further inland, as the fronts make more progress NE Tuesday night, after which convection initiates across much of the UK on Wednesday.

By noon Wednesday it appears that southern coastal counties will have seen 25-40 mm, while the mountains of Wales could receive as much as 60 mm. That ties in closely with the Met Office warnings, as you might expect.

Now for another take on things - the 12z WRF-NMM run:

The distribution of the areas seeing 5-20 mm is actually very similar to what Euro4 shows, but the regions with over 30 mm are larger, particularly across the southernmost counties over which the leading front is expected to stall tonight (W and NW of the Isle of Wight).

Within those regions, the model suggests that totals of 40-50 mm will be widespread. This is getting beyond what the Met Office have warned for, suggesting that WRF-NMM might be overdoing things a bit... either that or it has trended towards higher values since this morning - I did not see the 00z or 06z so can't give an opinion there.

Regions with over 50 mm projected are largely those with higher ground, with a few exceptions. One of those extends up towards my house. 50 mm in the next 48 hours would leave me with over 110 mm at less than half way through the month 

At least the model does go on to show a respite Wednesday evening through to Thursday noon, during which little is added to the above totals for England and Wales.

Addition at 23:17 10th Nov: The 18z WRF-NMM run removes most of the 50 mm + areas in the south of England, but produces more than 80 mm over the Beacons in Wales.

What about GFS then? Well, despite the lower resolution, it tells a similar story, and it throws down 62 mm onto the coastline between Weymouth and Bournemouth with 45-50 mm across Dorset and 25-40 mm across neighbouring counties. Steer clear of Bournemouth beach unless you like being drenched to the bones  

Addition at 23:16 10th Nov: the GFS 18z op draws in even more warm, moist air on Friday, which produces even larger precipitation totals on that day. I would add in an 18z accumulated precipitation chart, but for some reason they haven't updated on Meteociel.fr

Oh and here's the 12z accumulation for the next 6 days, which adds the Thursday-Saturday stalling front events to the above:

At this rate, the UK will start to tip over sideways  

Great analysis

Many problems involved including warm maritime air, warm SST's, slow moving weather fronts and frontal wave potential

Ark required

Purely out of interest, here's a bit of verification of the 00z model runs for my location, the list format for each time period being

Midnight to 6 am: Euro4: 2 mm,   WRF-NMM: 3 or 4 mm,   GFS: 7 mm,   Actual: 2 mm

6-9 am: Euro4: 4 or 5 mm,  WRF-NMM: 0 mm,   GFS: 0 mm,   Actual: 8-12 mm (based on two hours of 4-6 mm per hour rain rates)

9-12 noon: Euro4: 5 mm,   WRF-NMM: 0 mm,    GFS: 2 mm,   Actual: one or two mm.

From the results it can be deduced that Euro4 has done a decent job so far though it may have been a little slow with the area of heavy rain this morning. By contrast GFS was too fast with it. Both models have close to accurate totals for midnight to noon today, both at most a couple of mm out. WRF-NMM has not done well, and that's because it has the frontal boundary positioned too far west.

Accounting for the precipitation between noon yesterday and midnight today, the latest WRF accumulation out to the end of this first event has adjusted the totals down a little bit, but not as much as might initially seem the case when comparing the two outputs:

More than 20 mm widely for the coastal counties of Southern England, and for much of Southwest England and Wales. More than 30 mm along the South Coast as far east as Southampton and extending inland some 20 miles in places. Still some areas of over 40 or 50 mm, these largely associated with high terrain.

It's a very similar story from Euro4 so little need to post that chart. GFS follows the same sort of pattern, but the lower resolution limits the detail.

1pm update: the event total stands at about 14 mm so far, with around 9 mm since midnight. More or less on track I reckon - but a long way to go yet, with the last of the action being tomorrow afternoon.

The area within the gap may be in line for what's currently over NW France, which is tracking mostly north but with a slight eastward motion too.

Good to see a break in the rain though.

MetO warnings not only tonight but more extensively Thu-Fri 

 "Many places may well see 20-30 mm during the period, with as much as 50-70 mm over higher ground of Southwest England and Wales"

http://www.metoffice.gov.uk/public/weather/warnings/#?tab=warnings&regionName=se&fcTime=1415923200

Not much interest in this event it seems. Admittedly it's been a bit boring for most parts this afternoon with sporadic outbreaks of light rain and drizzle, the exception being across parts of Devon in particular where the rain has often been heavier, leading to a few flooding problems according to the 10:35 pm forecast on the BBC.

That forecast went on to suggest that it will produce some excitement tomorrow morning across a swathe of England in the form of thundery showers with some intense rain rates at times, though there's not much certainty as to whether it will actually happen, let alone where it will go if it does.

Euro4 is the only model I can find that really shows the feature:

The same goes for some afternoon showers in the same area:

GFS shows a lot of convection over the Channel but carries none of it inland, while WRF-NMM has a few scattered showers heading SW to NE as opposed to more S-N as Euro4 shows. Both of the models have the same pressure pattern, yet they differ in cloud motion... how strange.

With often very heavy, even torrential rain streaming north through here since 4:30 am and looking to continue on and off for quite a while longer, it's hard to believe that GFS entirely missed the inland movement of this system, or that WRF-NMM only had light rain showers.

Even Euro4 looks to have underdone the intensity, but the model has to be commended for not only spotting the convective system but picking out the focal point as well. It also predicted the area of convection currently headed for London.

A mass of heavy rain to the south of here moving this way.

More than 20 mm so far today, after 12 mm yesterday.

The event total is now in line with what Euro4 predicted on yesterday's 00z run. Not bad going, but some more showers appear likely this afternoon and evening and they could pack a punch, though probably well below what's been going on this morning, which has been right at the top end of November rainfall events due to the sheer intensity of the rain at times - which can be attributed at least in part to a combination of a relatively warm fetch of air from the Atlantic and the anomalies of +1 to +1.5*C that are widespread to the south and southwest of the UK.

In my records going back to 2003, there are very few events between November and February which have produced such high totals. On the other hand, the total of over 70 mm seen in a single event just before Christmas 2013 far surpasses the current one...  that was a bonkers day with severe gales as well.

I was struck by how much it had risen when walking past it late this morning. I was passing a wooden walkway that crosses a water-meadow fed by a stream, and saw that the water level was almost over the top of it. When the waterway is submerged, that is my criteria for a flood level on that stream. It looks to have risen some 10-15 cm since last Friday, though I haven't made accurate measurements.

The water then takes a sharp right hand turn and leaves the meadow via an old humpback bridge. The water level late this morning was about 10 cm from the top of the associated tunnel. The stream rarely gets above the top of that tunnel, and in that situation the steep banks are still just about high enough to keep steering sharply to the right.

Last winter, the water level came within mere centimeters of over-topping those banks. This was second only to the autumn of 2000 as far as I know, when the top few centimeters of water cleared the banks and just powered straight on ahead, flooding countless properties and pretty much obliterating a large vegetable farming patch. This was prior to the wooden walkway being constructed - it was part of project to clear the meadow of excessive mud and weeds, simultaneously opening it to the public and giving the stream a sort of miniature flood plain. Had this not been in place last winter, I'm sure the flooding would have been even worse than that of 2000.

The River Allen has a fast response rate, hence the rapid changes of late.

By contrast, the River Avon has a very slow response rate. Despite this, it looks to have risen close to 20 cm over the past 24 hours alone. For some reason, this is the only up-to-date station information I can find between Salisbury Plain and Fordingbridge.

Still nine tenths of a metre to go to hit record flood level though. That's almost impossible to imagine - even last winter it stopped 24 cm short of that 

Lastly, here's an event summary and brief model appraisal:

The event total ended up at 34 mm here. That final value was well predicted by Euro4 and WRF-NMM, while GFS wasn't far away, and I suspect it was only really held back by its lower resolution. As for the detail, the former two handled convection a lot better than GFS, which often failed to capture it inland - the final total only managed to be close because the model over-predicted the initial frontal rain

River Arun at Billingshurst bankfull yesterday and beginning to spill into the fields. It won't take much more.