Rainfall 14-15 March 2018

Figure 1

It’s been a very wet 24 hour’s down here in Devon, although there seems to be a curious mismatch between the estimates of what the radar saw (fig 1), and the rainfall that found its way into the gauges of the AWS at all the various SYNOP stations across the country (fig 2). Capel Curig just 2mm? The river Culm has certainly burst its banks locally and flooded all the water meadows that surround it.

Figure 2

Locally in Bradninch I only measured just over 19 mm in the 24 hours, but Exeter had almost 30 mm, and Dunkeswell close to 40 mm, so either the rainfall was very sporadic, or I’ve still got a spider’s web in my AWS connected to my tipping bucket.

Figure 3 – Station road, Hele
Figure 4 – looking down the Culm
Figure 5 – Station road, Hele
Figure 6 – looking across the Culm Valley

The sheltering effect of Dartmoor

The sheltering effect of Dartmoor seems to have had a long reach if you look at the estimated 24 hour rainfall totals for yesterday from the weather radar network. Conversely the funneling effect of the Bristol Channel, and the resultant large rainfall totals across south Wales, with a finger of higher totals extending northeast towards the Wash was also evident (fig 1).

Figure 1

You would have expected the upper winds to be from around 220° which they were from the 2317 UTC ascent from Camborne (fig 2).

Figure 2

Rapid thaw on Dartmoor and Exmoor

Figure 1

There’s no quicker way to strip snow of high ground than a combination of mild air and heavy rain. The rivers must be in spate after a rapid thaw of the snow on Dartmoor and Exmoor brought about this way overnight by a band of heavy early morning showers tracking N’NE across the southwest (fig 1).

Figure 2

The cold air is still hanging on for dear life further north, so there’s quite a temperature contrast in this 09 UTC chart (fig 2). The Met Office analysis for 06 UTC reckon the rain is from an occlusion.

Figure 3 – Courtesy of the Met Office

Red warning for snow

Figure 1 – Courtesy of the Met Office

Well, what I thought might happen has happened. I can’t remember seeing a red warning for snow being issued by the Met Office before so this could be a first for the central belt of Scotland (fig 1).

I’ll be my usual picky self and say that they should have extended that red warning southward to include most of the east coast down to Berwick-on-Tweed by the looks of the latest radar images. There are some red pixels mixed up in the intensities that extend over most of the Cheviot’s (fig 2), certainly a day to be staying at home round the fire, and not out on the A1 which almost certainly will get blocked later today, I feel sorry for the hill farmer whose sheep are out in it.

Figure 2 – Courtesy of the Met Office

10 February 2018 – heavy rain over Northern Ireland

Figure 1 – Courtesy of the Met Office

The Met Office may have purposely forgotten to issue a strong wind warning for Philine yesterday, but the NWP model must have been indicating some heavy rain for Northern Ireland so they did issue a yellow warning of heavy rain for the province (fig 2). As you can see there was indeed some intense heavy rain, with rainfall rates of over 32 mm per hour at 1810 UTC (fig 1) close to where low Philine was developing a discrete centre. This shows you just how good the NWP models can be.

Figure 2 – Courtesy of the Met Office

Weather Radar Estimates

As far as I can see from my estimates from weather radar images there was a largish area with totals in excess of 50 mm for the period 10-1200 to 11-0845 (fig 3).

Figure 3 
Evidence from the rainfall gauge
Figure 4

It’s clear from the gauge network totals for the 24 hours ending 06 UTC (fig 4) that the weather radar may have been over doing the intensity of yesterday rain for some reason. The freezing level was low enough for the rain to be falling as snow down to low levels ~2500 feet, so the white intensities (fig 1) could have been some form of bright banding effect.

The warning itself was just about perfect for Northern Ireland, but there were even higher totals across northwest England and southwest Scotland that didn’t get a mention.

24 January 2017 – line convection on cold front

Figure 1

One memorable thing that did come out of storm Georgina, was the line convection that ran southeastward across the country during the morning, it was associated with an active cold front that was aligned SW-NE, from Exeter in the southwest to Hull in the northeast at 09 UTC (fig 1). If you take a look at some of the times of the approximate peak gusts across the country, many of them occurred on the passage of the cold front (fig 2).

Figure 2 – Approximate hour of peak gusts

Flash flooding at Combe Martin

Figure 1 – Courtesy of BBC News

Apparently there were reports of what looks like flash flooding at Combe Martin in north Devon on Sunday according to the BBC (fig 1). Here are the 24 hour totals from 06 UTC on the 21st of January the day that it happened.(fig 2) with the two nearest rainfall stations ringed in red.

Figure 2

The weather radar just seemed to have missed the rainfall event across the north of Devon yesterday for some reason, if you remember a warm front was pushing eastward across the southwest during the night. Estimates from the radar suggest totals of around 16-24 mm over Exmoor, but Liscombe measured a little more than this, with a total of 29.2 mm in 24 hours [06-06] (fig 2). Radar rainfall estimates for other places across Wales were generally very good (fig 3 & 4).

Figure 3
Figure 4

Judging by the pseudo hyetograph for nearby Arlington Court (fig 5) the heaviest rain occurred in the morning between 0730 to 1000 UTC, at a guess 6 mm fell in that time, which I’m sure wouldn’t have been enough to cause the flash flooding seen at nearby Combe Martin.

Another possible cause is that the swollen stream somehow became blocked above the village, maybe at a bridge or in a culvert, and the water dammed up for a short while before being suddenly released as the blockage cleared. It happened in a much larger way at Louth on the 29th of May 1920 so there’s no reason to say that it couldn’t happen at Combe Martin, although I’ve not seen any reports to indicate that this was the case.

Figure 5

Steep valleys do run northward of the high ground of Exmoor, but the stream that runs through Combe Martin doesn’t seem to originate on Exmoor though, and looks to be quite a short affair (fig 6). I’m sure that this area has seen much heavier rain and not seen flash flooding the likes of this, so all I can put it down to is a very local intense period of rainfall, or even a cloud burst if you like, that somehow went unnoticed by the 5 minute radar images.

Figure 6 – Courtesy of Bing Maps and the Ordnance Survey

Bands of showers across SW

Figure 1

There have been three (soon to be four) bands of quite intense showers associated with short troughs and large CB’s moving E’SE in the strong W’NW gradient across our part of mid-Devon today. The sky has gone intensely black with each of them with a short intense burst of rain, the last one being the sharpest. From my Vantage Pro, I can see that the first one occurred at 0810 UTC, another at 1020 UTC, and the last around 1300 UTC. The fourth band is almost on us judging by the latest frame of the weather radar (fig 1). There have been no SFERICs associated with them. You can see a number of them waiting in line across the Celtic Sea in an earlier visible satellite image (fig 2).

Figure 2

A sting jet or the work of an overactive imagination?

It maybe that Storm Dylan has developed a bit of a sting jet in the last few hours. I’m no expert, but this was anticipated by some but came as a surprise to me, if that indeed is what the latest frames of the IR satellite image (fig 1) and the weather radar (fig 2) are showing across western Scotland at 07 UTC.

Figure 1 – The arrow and the garish colours are mine!

Of course this all may be the work of an overactive imagination and just a curl of frontal cloud and precipitation from the bent back occlusion getting sucked into the vortex of Dylan.

Figure 2