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

A rather wet lion

There’s been a wet start to March across parts of the country, the wettest place in the first 10 days seems to have been Dublin in the Irish Republic with over 4″ of precipitation (107.7 mm). A large part of these totals would have been from snow that fell in the first few days of the month, in fact the highest precipitation amounts correlate very well with the deepest reported snow depths (fig 1). That shouldn’t be that surprising, but it does make me wonder how an AWS handles the water equivalent in cold and snowy situations such as the one we experienced last week, especially when the snow is blowing around like it was.

Figure 1

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

February fill dyke…

Figure 1

It’s turning out to be quite a wet February 2018 more especially in western areas. The wettest place in the SYNOP observations is Capel Curig as usual with 6.55″ of rain up until this morning (fig 1). There are still some drier spots around though, notably in eastern Ireland, the Vale of York and the southeast of Scotland, although I only have a 79% record for Edinburgh Gogarbank.

The mighty Quinn

Figure 1 – Courtesy of the Met Office

The 18-06 rainfall totals weren’t massive from the frontal system associated with low Quinn, but rainfall since o6 UTC was fairly heavy in parts of the east Devon, Somerset and Dorset (fig 1). The rain might have been behind a couple of road traffic accidents that have led to the closure of the M5, and some local flooding in our part of Devon. It shouldn’t be long though before the clearance behind the cold front brightens things up down here in southwest England.

Figure 2

I would say that a secondary low centre was developing on the triple point in the Bristol Channel at 08 UTC, but I can’t find any sign of warm air! You could almost draw the trough down the 4° line of longitude it’s that straight (fig 3).

Figure 3

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.

January 2018 climate statistics on Spotlight southwest

Figure 1 – Courtesy of the BBC

I wonder where David Braine gets his climate statistics from? I would have thought it was just up the road from the Met Office in Exeter. Last night David looked back at the weather for January 2018, and he displayed a table for Cornwall that showed a total of 115 mm precipitation fell in January and that the average for the month was just 70 mm (fig 1).

What’s the Average?

Take a look at the 1971-2000 rainfall averages (fig 1), and you’ll see that the whole of Cornwall is covered by at least light brown coloured contours, indicating a total of at least 100-140 mm, and over higher ground this increases to as high as 220-340 mm, so an average of 70 mm is woefully low.

Figure 2 – Courtesy of the Met Office

Luckily you can download the monthly climate data for Camborne from the Met Office for free. Here are the January totals there since 1978 (fig 3). I reckon the average rainfall total for January at a coastal site like Camborne is just over 131 mm.

Figure 3 – Data courtesy of the Met office
How much rain actually fell?

Here is my best estimate of precipitation accumulations for January that I’ve derived from the SYNOP reports, I miss the occasional report but the totals should be quite accurate (fig 4). As you can see the three Cornish mainland stations are reporting totals of 183.5, 188.9 and 206.8 mm, much more than the 115 mm in the table (fig 1). Perhaps he was using the total from St Mary’s of 104.9 mm? (ToDo: find out if the Scilly Isles are part of Cornwall).

Figure 4
Was it that wet?

The Met Office rainfall anomaly chart shows that January 2018 was indeed a wet month in Cornwall, and as you can see by the purple coloured contours indicating rainfall anomalies of between 125 and 150% of average (fig 5). I make the approximate anomaly for Camborne 140% (a total  of 183.5 mm with an average of 131 mm) which fits in nicely with the anomaly chart.

Figure 5 – Courtesy of the Met Office
So what should the graphic have looked like?

At a guess these values might be closer to the mark than the 115 and 70 mm shown in his original table (fig 6).

Image 6 – Courtesy of xmetman with data supplied by the Met Office and OGIMET

Regional correlation of temperature, precipitation and sunshine – January 2018

Figure 1 – click to enlarge

I’ve put a bit of development work into creating a panel of three scatter graphs that show the correlation between:

  • Mean temperature and precipitation.
  • Mean temperature and sunshine.
  • Precipitation and sunshine.

I’m using what I call the Met Office 1910 regional monthly data to display how the correlation looked for January 2018. I think these correlation scatter graphs are a quick and easy way to visualise climate for either a selected region since 1910, or as in this case for all regions for a particular, year, season or month as I’ve done with January 2018 (fig 1).

As you can see if you examine each graph in detail (fig 1) it’s easy to pick out the outliers which indicate regions that have seen the more extreme weather that occurred during the month. The temperature-rainfall scatter graph on the left for instance shows how much colder and drier Scotland was than other parts of the UK.

It’s not a new idea, but I personally think it’s a very useful visualisation tool for anyone that’s interested in climate, and shows much more that a line or bar chart could.

Capel Curig wettest and Dyce driest in January

Figure 1

The wettest place in the UK as usual was Capel Curig in Snowdonia, with a total of 318.4 mm of precipitation in January 2018 (fig 1). Meanwhile, Dyce in Aberdeenshire only collected 24.6 mm of precipitation as the dry spell continues in the far east of both Scotland and England. Dry days were in short supply during the month, especially in western areas. Here are both ends of the table for January (fig 2).

Figure 2