Weather Radar

I access all my weather radar images from the Met Office. They now provide 5 minute data low resolution radar images of most of the UK and Ireland. They do this by creating a composite image from smaller tiled images, which I then download and then have to stitch back together rather like a jigsaw.

Rainfall in Lake District tops 150 mm

Figure 1

I reckon that rainfall accumulations in the Lake District in the last 24 hours have topped 150 mm on the highest fells (fig 1). The wettest place I can see from my list is probably close to Ambleside (fig 2), with around 90 mm of rain since 18 UTC yesterday evening, although these are rough and ready estimates derived from the 5 minute coarse data that’s available on the Met Office website. A pixel is a long way at this resolution, and you only have to be one or two out to return a value from over high ground rather than lower down.

Figure 2

Wet in Lake District

Figure 1 – Estimated overnight accumulations since 18 UTC on 10 October 2017

A thoroughly wet night and morning across the Lake District and Snowdonia. Estimates from weather radar show purple pixels which equate to rainfall accumulations of over a 100 mm in the last 15 hours or so across higher ground (fig 1). It was well anticipated and forecast by the Met Office in their yellow warning of yesterday morning. The one thing that does puzzle me about yellow warnings for heavy rain like that is, the much higher accumulations that I’ve seen in western Scotland never seem to get a mention, perhaps the impact there is lessened because by the sparsity of the population, or maybe it’s more likely that they don’t want to get caught out by another mega-orographic rainfall event in the Lake District like we saw in December 2015? The more persistent heavy rain, should clear the area with the passage of the cold front early this afternoon (fig 2).

Figure 2 – Courtesy of the Met Office

Rainfall accumulations – 1st October 2017

Figure 1

Plenty of rainfall across the Western Highlands of Scotland yesterday (Sunday, 1st October 2017), with estimated accumulations from the weather radar in excess of 100 mm over the mountains (fig 1). Scattered areas of light blue pixels (16-24 mm) south of the Forth Clyde valley, but small totals from the wet spots in the Lake District, Snowdonia and the west of Wales, well justifies the Met Office’s cancellation of the dual yellow warning for heavy rain and strong wind they issued on Friday morning (fig 2).

Figure 2 – Courtesy of the Met Office

17 Sep 2017 – estimated rainfall accumulations

I had a look back at rainfall yesterday (17 Sep 2017), for the 17 hour period between 12 UTC to 05 UTC this morning, just to see what the estimated accumulations from the weather radar looked like (fig 1). The wettest place that I could find was Westbury in Wiltshire with 39.7 mm of rain, and although it was wet in Ceredigion and Cornwall, it wasn’t quite as wet as it had been on Friday, so that was good news. Hopefully the showers will be lighter and scattered over the next few days and give these places a chance to dry out. The yellow warning issued for yesterday was certainly justified for Westbury  in the torrential rain the town saw during yesterday afternoon (see the inset hyetograph in figure 1).

Figure 1

Here are the 24 hour totals till 06 UTC this morning from the SYNOP stations for completeness.

Figure 2

Amazingly, Liscombe in Somerset remained dry throughout the 24 hours, which is more than can be said of Jersey.


Showers being restricted from the west

Figure 1

I would say, writing as a non-expert, that the showers over West Wales and the Celtic Sea have lost their intensity from earlier this morning (fig 1), due to encroaching upper and medium cloud that’s streaming south across Southwest Wales and the extreme west of Cornwall (fig 2), from an upper warm front that was lying across Western Scotland at 06 UTC (fig 3). I reckon warmer air aloft is stabilising the atmosphere and limiting the convection. The showers certainly haven’t lost any of their potency across the south of Devon at the moment though, and I don’t think I’ve ever seen the sky so full of towering cumulus and CB, or lines of CB, as I can see looking out of the window at the moment. The showers are sharp, but have been relatively brief so far today.

Figure 2

Figure 3 – Courtesy of the Met Office

Better late than never

Figure 1 – Courtesy of the Met Office

I wondered if the Met Office would issue a yellow warning for today for the southwest of Wales in my last blog, and I think someone finally must have read it (who am I kidding – if only), because a few minutes ago they did finally get round to issuing a yellow alert (fig 1) for heavy rain, so better late than never! I noticed no apology to the people of Ceredigion for the flash flooding of yesterday, but of course the Met Office rarely apologise for a bad forecast or a ‘missed’ warning, which occasionally would be nice to see, and at least would show a bit of corporate humility. They can’t be expected to get it right all the time, and that all too frequently seems to be the case with the output from their mesoscale model at the moment.

I can’t understand why the warning for this event should come into force no earlier that 12 UTC?

As far as I can see this event has already started.

The estimated accumulations for the last three hours or so from the weather radar (fig 2) already show areas of light blue pixels (16-24 mm) around Aberporth and North Cornwall.

I know they like to issue their warnings at around 09 UTC each day, but sometimes the weather doesn’t always fit in with the seemingly fixed timetable the Met Office use in the issuing of warnings from their Exeter HQ. This warning should have been issued much earlier than 0908* UTC, and should come into force much sooner. After the events of yesterday, the people in Ceredigion must be bemused with this yellow warning for today, as am I.

Figure 2

*When I quote the time that I received the email as the time of issue [TOI] of the warning, it’s because as far as I can see, the TOI is never stated on any warning the Met Office issue. So as well as not maintaining an archive of previous warnings they never state the TOI, which it make verification of any warnings totally impossible.

BBC: Yesterday’s flooding in Ceredigion

I was just about to start the twice yearly cutting of our hawthorn hedge yesterday when I saw the line of heavy rain or showers from West Wales extending south to Cornwall. It was about 11 am and I looked at the estimates which I thought then merited a yellow warning for heavy rain and duly posted this blog. Three hours later after doing the hedge I half expected a belated warning to have been issued, but no the totals had increased although the feature whatever it was had become fragmented and less intense. What I’m driving at with this story is if I can notice this happening and cut the hedge and generate the accumulations and write a blog surely someone who is employed to do this as is job at the Met Office would have been monitoring the situation, and could, and very probably should have issued a warning? What kind of impact do these events need to have before a warning is raised? Otherwise what is the point of them at all?

Figure 2 – Courtesy of the Met office

Figure 3 – Courtesy of the Met office

Figure 4 – Courtesy of the Met office

The analysis hasn’t got any less exotic over the last 24 hours as you can see (figs 2-4).  I see little way that you could justify the contorted occlusion in the midnight analysis, other than just running a line and linking all the discrete bands of showers embedded in the north northwesterly flow with it. Troughs appear and change orientation, seemingly at the drop of a hat. Interestingly, with regard to showers activity, the same thing seems to be going on today across Ceredigion as it did yesterday. Here are the estimated accumulations since 06 UTC yesterday (fig 5).

Figure 5

As you can see, the area of 50-75 mm estimated accumulations has grown bigger since yesterday (fig 5), and it’s no wonder that the BBC News item described it at flash flooding in New Quay and Talsarn. A repeat performance of yesterday could only exacerbate the situation, and I wonder what the Met Office mesoscale model rainfall accumulations are for today across this part of Wales and Cornwall, and whether we might even get a yellow warning for today to cover it?

A new kind of trough

Figure 1 – Courtesy of the Met Office & Crown Copyright.

I think the Met Office have come up with a new type of surface feature on the 12 UTC analysis today (fig 2), it’s marked on the chart as a trough and is aligned north-south from Anglesey in the north, extending south through West Wales and Cornwall, before ending in the SW approaches almost in Brittany. It doesn’t fit the definition of a trough as outlined in the Meteorological Glossary as far as I can see (fig 1), and runs parallel with the isobars, in fact if was a little further west, it would be indistinguishable from the 1016 hPa isobar. To be fair, a feature like this would explain the long period of heavy rain that started in West Wales at about 10 UTC this morning, but I have no idea of what it is.

Figure 2 – Courtesy of the Met Office

The rainfall from this feature, was very heavy in places from 10 UTC to 15 UTC today, and the totals from it easily merited a yellow warning, but maybe because its Friday afternoon, or maybe it’s because they’ve already had a bit of a bad week with Aileen, the Met Office didn’t bother issuing one, hoping perhaps that people wouldn’t notice, which is probably a good idea on their part, because issuing a warning retrospectively would only draw people’s attention to the failings of their NWP mesoscale model. Here are my estimated 12 hour accumulations so far from weather radar images from 03 UTC this morning (fig 3).

Figure 3

It’s been a thoroughly wet day in the small town of Llandysul in Ceredigion, which I reckon has seen around 48.3 mm of rain in the last 12 hours. There are fairly large areas of 24-32 mm of rain across West Wales and East Cornwall/West Devon region, and an inner region within that of green and yellow pixels 32-50 mm, with isolated red pixels 50-75 mm across Ceredigion.

Surprisingly no yellow warning out for the rain in West Wales and the southwest

Figure 1

Rather surprisingly, there are no yellow warning in force today for the heavy rain that’s falling in the West of Wales and the southwest of England this morning from the Met Office. Estimated totals from the weather radar, indicate that the highest are in the Southwest of Wales, and range between 32 to 40 mm since 06 UTC this morning (fig 1). The 30.2 mm at Llandysul as an example, is not bad going in just over four hours. The rain is now working southward into east Cornwall and west Devon (fig 2).

Figure 2

There was nothing on the midnight analysis from the UKMO to explain the feature across West Wales, but a trough has appeared now on their 06 UTC analysis (fig 3). This shows a trough (not a convergence line) aligned along the isobars across West Wales to explain away the showers and heavy rain that’s going on there.

Figure 3

Figure 4

Look back at Aileen – the rain

Figure 1

I did expect to say ‘well done’ to the Met Office concerning the yellow warning for heavy rain (that they had adjusted southward) from storm Aileen, which looked like it had covered the wettest areas across Northern Ireland, southwest Scotland and the northwest of England (fig 1). But I found from my estimates of accumulation from the weather radar, that the heaviest rain had turned out to be in the Grampian region of northeast Scotland, with estimates for Fochabers of around 40 mm in the 18 hours between 18 UTC on the 12th to 12 UTC on the 13th. The Met Office did issue a late yellow alert for the area, but as they don’t have any form of archive for warnings, it’s impossible to say when it was issued. The rain certainly came out of left field and caught them on the hop, as most eyes were fixed further south on storm Aileen, it looks like it may well have been the results of some embedded instability, as there a few SFERICs on the archived chart from Blitzortung.

Figure 2 – Courtesy of the Met Office