Satellite

I source most of my satellite imagery courtesy of the Met Office and EUMETSAT. They provide 15 minute data low resolution visible and infrared images of the British Isles and Western Europe. They do this by creating composite images from smaller tiled images, which I then download and then have to stitch back together rather like a jigsaw.



Is the Brown Willy effect on the increase?

Figure 1

Yesterday’s showers up through the spine of Cornwall and Devon were another classic example of showers caused by peninsular convergence. It also goes under the less prosaic name of the Brown Willy effect, it’s amazing what you can find courtesy of Wikipedia. The first thing you will notice that yesterdays string of showers seem to originate a little south of Brown Willy, if my estimates from the weather radar are correct (fig 1), and are aligned around 240-060°. Rainfall accumulations are not particularly high along its length for the period 08-21 UTC, generally between 8 to 16 mm with a few isolated light blue pixels indicating accumulations greater than 16 mm in Somerset. The first band of showers seems to fizzle out, as a second band forms a little further to the north for a short distance, before the southerly band re-intensifies somewhere over the Blackdown hills. The band of showers stretched as far as East Anglia before losing its coherence. We never got anything more than a few spots out of it here in Bradninch during the day, 14 km to the northeast of Exeter, and the cut off between wet and dry was quite sharp.

The recent flooding in Okehampton on the afternoon of the 30th of July was another good example of peninsula convergence, which the Met Office NWP model just didn’t seem to get a grip on. Has the occurrence of peninsular convergence increased in recent years, or is that just down to extra vigilance on my part?

Figure 2

For once there was an upper air ascent in the right place, and for the right time (fig 3). The wind direction aloft in the 11 UTC Camborne ascent are remarkably constant, between 240 and 260°, well up to 15,000 feet.

Figure 3

Sitting in an English garden waiting for the sun…

Figure 1 – Courtesy of EUMETSAT

Summer seems to have taken a bit of a holiday and gone south this August, with cool, showery air over the UK, and clear skies top to toe across Italy again this morning (fig 1). I bet there would be many Italians who would be thankful to swap a day with maximum temperatures of 20°C that we will see in the UK today, with the 40°C many of them are likely to see again today in Italy (fig 2).

Figure 2 – WMO Block #16

2 August 2017 – frontal analysis

Figure 1

The frontal positions look fine on a plotted surface chart for 11 UTC

Figure 2

…but there doesn’t seem much connection with the weather radar (fig 2), or the visible satellite images come to that (fig 3). Much of the heavy rain over central southern England seems to be contained within the warm sector, and there’s little if any precipitation on where I’ve put the occlusion. The satellite image does seem to hint at some kind of wave on the cold front in the SW approaches.

Figure 3

The south coast of Devon and Dorset and the Channel Islands, seem to have taken the lions share of the rain since 06 UTC, with totals of over 24 mm on the hills overlooking the Chesil beach.

 

Spot the cold front

It’s almost impossible to spot the cold on the 08 UTC weather radar this morning across Ireland (fig 1).

Figure 1

I had a stab at where it was on the plotted SYNOP observations for the same time (fig 2).

Figure 2

I then had a go using the latest visible satellite image, which ends up a little more advanced than I had it using the surface observations (fig 3). The cold front is certainly very weak, and will probably be no more than a few spots of rain when it passes through later this morning.

Figure 3

 

The poor old southeast

Figure 1

It’s not often that you can say this, but the poor old southeast didn’t score very high on yesterday’s (24 July) summer index. It was cloudy and cool in the moderate northerly airstream, I’m sure that there’s a much better chance of the cloud clearing today. I thought that Exeter might be the sunniest place in the UK (fig 2 & 4), but the time of year and the 23.5° inclination of the Earth put paid to that, and Edinburgh, with 15.3 hours took the honours.

Figure 2

As regards highest temperature, again I thought Devon might have topped that table, but Hurn in Dorset, with a late burst of 6.6 hours of sunshine in the afternoon took that accolade with 24.5°C (fig 3). I reckon it’s about time that the Met Office put an AWS in mid-Devon, because in yesterdays northerly flow down from off Exmoor, gave Bradninch a maximum of 25.9°C, which although a bit on the high side, are seldom reflected in temperatures from Exeter airport just 10 km to the south.

Figure 3

Figure 4

 

Storms brewing to the southwest

Figure 1

There are two large CB cells to the southwest now, one just south of Plymouth, and the other close to St Malo. The cirrus that’s plumed of the top of the more northern one has just about covered most of Cornwall and Devon bu 12 UTC (fig 1). The shield of cloud thickened up here in mid Devon from CS to AS in the last hour, but the lightning activity is still well out to sea (fig 2). I estimate from the visible satellite image, that the closer one of the two cells, is moving 030° at around 30 knots, although the rain seems to be tracking almost due north.

Figure 2 – 1219 UTC courtesy of Blitzortung

The rain looks quite intense as you would expect (fig 3), but because the cell is moving quite quickly it shouldn’t cause too many problems, having said that there are some white pixels now showing up in the image (>32 mm/hour intensity) so famous last words. Whilst I’m busy putting my foot in it, I suppose that once this one has cleared away later this afternoon, that might be it for our part of the world, although the radar is now showing some more scattered showers to the southwest of the Lizard.

Figure 3

Story delayed due to power cut –  what’s wrong with the infrastructure of this country when a lightning strike 70 miles away takes out the whole county?

Multilevel flow in action

Figure 1

Fascinating to watch the low-level flow across the bay of Biscay blow the low cloud westward (fig 1), as the medium level instability marches northeastward (I estimate 031° at around 14 knots or so) above it (fig 2). This instability may develop a little further this afternoon, and is the precursor of the thunderstorms that will affect the southwest of England later tomorrow.

Figure 2

Medium level cloud reluctant to clear from southwest

 

Figure 1

Medium layer cloud is still much in evidence across the southwest this morning, possibly the remnants of yesterdays weakening cold front that marks the boundary between air with dew points of 9°C in Devon to 17°C in the north of France (fig 1). There is some rather fine altocumulus (fig 2), and despite the surface flow being east of north, I reckon the flow at 17,000 feet (LCBR at Exeter airport at 09 UTC) is from around 248° at 27 knots, if my rather crude estimates are anything to go by.

Figure 2 – 0845 UTC 17 July 2017 looking south

Low cloud finally clears at Gibraltar

Figure 1 – Courtesy of the Met Office & EUMETSAT

The low stratus that’s been clinging to the rock of Gibraltar for much of the day (fig 1) has finally cleared at 15 UTC this afternoon (fig 2), up until then, the temperature had been pegged back to around 23°C. I tried to find a decent image of the cloud around the rock, but webcams pointed at interesting places seem to be in short supply these days on the internet. What’s even more depressing is that Gibraltar no longer do ascents using radiosondes these days. This is all probably connected to the easterly wind that blows in this part of the world and is known as the Levant wind, and gives rise to the Levanter cloud that forms round the rock, but I am no expert.

Figure 2

A little further north at Cordoba in Spain, it’s been considerably hotter this afternoon, with a temperature at 15 UTC of 45.1°C (fig 3), which is not quite as hot as the 46.8°C it was at the same time yesterday (06-18 maximum 46.9°C).

Figure 3

Observational and forecasting cloud problems

Figure 1 – Courtesy of the BBC

The Met Office seem to be having problems with both forecasting and observing just how much low cloud there is across the southwest of England again today. Although it’s cloudy across Cornwall at the moment, there is little in the way of low cloud at 09 BST over much of Devon as you can see from the visible satellite image (fig 2), and this at odds with the NWP graphics in the BBC forecast for this time (fig 1). No doubt the SC sheet will roll ENE’ward this morning and things will cloud over here, but in the meantime temperatures are around 24°C and its a lovely sunny morning. A similar clearance of cloud occurred yesterday at lunchtime, when the cloud cleared and the early afternoon was sunny, after another forecast of a mainly cloudy day from the graphics. The model does seem to overgenerate low cloud at times, and it doesn’t seem to make any difference if the base is at 2,000 or 7,000 feet.

Figure 2

The AWS the Met Office use are far from perfect either, both Exeter airport and Dunkeswell were at it again this morning overestimating how much low cloud there was. In my mind, there is absolutely no way there was 7 or 8 oktas of SC at 5,000 feet at either of these sites at 08 UTC this morning (fig 3). I can see why for safety reasons the AWS is programmed to err on the high side at an RAF station, but this is just plain misleading.

No wonder the presenters get the wrong idea about the weather across the region from observations like this. I think it might be a good idea if the Met Office introduced weather or skycams to these AWS as they did when they initially started trialling them, it’s either that or I drive down to the airport and start my observing career over again to see what’s going on.

Of course any outstation forecaster who uses one of these AWS will know full well its shortcomings and limitations when it comes to reporting cloud amounts, thankfully my observing career had come to an end before one was introduced at Kinloss. I should imagine that it can get a bit fraught at a fast jet station when the AWS suddenly announces that there’s 7 oktas of ST at 200 feet when it’s just a patch sat over the LCBR.

You could conclude, that the over forecasting and the over observing of low cloud, are some how linked, but as far as I know low cloud amounts from AWS don’t find their way into any NWP model, or maybe things have changed.

Figure 3