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.



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

Still not convinced

Figure 1 – Courtesy of Met Office

I’m still not convinced by the midnight surface analysis from the Met Office. They have clung to this same ‘configuration’ of fronts (east to west) in all their forecast charts this week:

  1. Warm
  2. Upper cold (in the warm sector)
  3. Cold (returning warm)
  4. Cold (short)
  5. Trough (concave to west of Ireland)
  6. Occlusion
  7. Occlusion (bent back)

By sticking to this analysis they are certainly being consistent, but consistently wrong, because I can see little evidence of the frontal features 2, 4 or 5 in the weather radar at midnight. If they only could publish a synoptic review that explained the various features they have in their analysis and forecast charts and the thinking behind it, we then might be in a better position to understand the rationale behind it, but as far as I am concerned their approach is just far too complicated.

Figure 2 – Courtesy of Met Office

 

Wet day in southeast

Figure 1

I estimate rainfall totals of between 50-75 mm since 06 UTC this morning from the weather radar imagery, in a swath that extends northeast from Sussex, across Essex to Suffolk. Quite something – there must be some local flooding going on in that area I should imagine.

August 8 – not a great day to be on holiday at Cleethorpes

Figure 1

A wet day on Humberside and certainly not a good day to be on holiday at Cleethorpes. My 06-06 total was a little low for Leconfield 47.3 mm estimated, 56.8 mm actual, but a single pixel at this coarse resolution could have been the reason for that (fig 1). There was a large arc of >50mm rainfall totals across South Yorkshire, Humberside and North Lincolnshire, which I’m sure is causing massive problems for farmers and holidaymakers alike at the moment. The yellow warning for heavy rain from showers over Wales and the Southwest was a wee bit over the top, apart from around Milford Haven where 34 mm of rain fell, and Bodmin, a good part of Devon remained mostly dry. Why we stayed dry in mid-Devon when there was so much vigorous cumulus congestus development yesterday morning I can’t fathom.

Figure 2

Plenty of heavy rain around

Figure 1

There’s plenty of heavy rain around today, either in the form of showers or longer spells of rain. In the latest 6 hourly totals, Wittering in Cambridgeshire, is the wettest place in the British Isles with 28 mm (fig 1 & 2).

Figure 2

Estimated accumulations from weather radar indicate totals in excess of 24 mm in a swathe across central and eastern counties, and in excess of 32 mm across parts of Pembrokeshire (fig 3). The Met Office, who already had warnings out for heavy rain across eastern England, have issued another one for heavy showers for Wales and the southwest of England, curiously they’ve not included the northwest of Scotland in this one.  Is it me, or are yellow warnings for heavy rain being issued much more frequently this summer?

Figure 3 – Weather radar estimates

 

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.

 

Met Office more reactive than proactive in the issuing of yellow warnings

Figure 1

I’ve been blogging ever since I retired and that’s now well over five years. I spend a good deal of my time watching how the Met Office performs through its forecasts and the various warnings that it issues, and recently, well in the last few weeks actually, I’ve been asking myself what is the exact purpose of the yellow warnings they issue.

What the colours mean

  • Yellow – Severe weather is possible over the next few days and could affect you. Yellow means that you should plan ahead thinking about possible travel delays, or the disruption of your day-to-day activities. The Met Office is monitoring the developing weather situation and Yellow means keep an eye on the latest forecast and be aware that the weather may change or worsen, leading to disruption of your plans in the next few days.
  • Amber – There is an increased likelihood of bad weather affecting you, which could potentially disrupt your plans and possibly cause travel delays, road and rail closures, interruption to power and the potential risk to life and property. Amber means you need to be prepared to change your plans and protect you, your family and community from the impacts of the severe weather based on the forecast from the Met Office
  • Red – Extreme weather is expected. Red means you should take action now to keep yourself and others safe from the impact of the weather. Widespread damage, travel and power disruption and risk to life is likely. You must avoid dangerous areas and follow the advice of the emergency services and local authorities.

More reactive than proactive

After the recent flash flooding in Coverack (18th July) and that at Okehampton yesterday (30th July), I now realise that many yellow warnings simply aren’t working, and that the Met Office have become more reactive than proactive in their issuing of them. I am convinced that yesterday’s late afternoon yellow ‘warning’ for 1750-2000 BST was prompted as much by Twitter, as it was by mesoscale NWP guidance or observational data from weather radar, river level gauges, automatic rain gauges or AWS.

I’m sure that they had reviewed the situation in the morning, and had duly issued a yellow warning for heavy rain for the north of Scotland, the reason that they gave for that warning of 20-40 mm was that the showers would be slow-moving.

What happened literally on their own door step at Okehampton seems to have caught them totally unawares, perhaps they thought the showers in Devon were moving quickly, and hadn’t anticipated that ‘peninsula convergence’ might just keep them coming along the spine of Devon for much of the day. In the end Okehampton received 84.2 mm in the 09-09 UTC period. I can’t believe that these accumulations were not picked up by any of the earlier mesoscale NWP model runs, and a warning not issued at the same time as the one for the north of Scotland was, I would love to see what the models were indicating.

The thinking must be at the Met Office, that a yellow warning must be in place to ensure that they are covered from any fallout in any severe weather, so no matter how late in the day it is, a yellow warning has to be issued. I recall a similar thing happened with the strong winds on the 6th of June, a warning for strong winds could have been issued with the one for heavy rain the day before, but for some reason known only to themselves wasn’t, two men were killed by falling trees on that day. It wasn’t a warning, it came into force as soon as it was issued, and that’s not much help to anybody, similarly yesterday, everyone in Okehampton knew it had been raining heavily all day, and by the time the warning had been issued the worst was over.

Figure 2 – Flooding yesterday at Okehampton courtesy of Twitter and Gillian Cross

I noticed that in the Coverack flash flooding on the 17th of this month, although there was a yellow alert in force, a red alert was never issued. It may be that localised nature of flash flooding does not meet the criteria of ‘widespread’ for the issuing of a red warning, and that’s why one was never issued. If that’s true, that would mean that if the NSWWS had been in place in the past for the flash flooding at Boscastle in 2004 and Lynmouth in 1952, both events would have only warranted a yellow warning.

In yesterday ‘peninsula convergence’ heavy rainfall event at Okehampton, the Met Office did have time to give an explanation to its Twitter followers of what ‘peninsula convergence’ was, but they only managed to issue a belated yellow alert from 1750 BST, when heavy rain had been falling for much of the afternoon (fig 1 & fig 3), and don’t forget that yellow means “Severe weather is possible over the next few days and could affect you“.

Here is my estimated total from low resolution weather radar images of what fell before the warning was issued from 0900 – 1650 UTC. As you can see from the inset hyetograph, there had already been, in the worst affected areas, around 61.7 mm of rain in less than eight hours (fig 3).

Figure 3

And here is my estimated total of what fell during the duration of the warning (1655-1900 UTC), and as you can see in that time a maximum of 25 mm fell in just over two hours in the same area (fig 4).

Figure 4

Today’s warning

Figure 1 – Courtesy of the Met Office

A bit of an odd warning today from the Met Office for the north of Scotland (fig 1), it was issued this morning for 20 mm to as much as 40 mm of heavy rain from slow-moving showers, between 13 and 21 BST this afternoon (fig 1). Although there were a couple of areas of 16-24 mm accumulations from my weather radar estimates, it didn’t quite make it as high as 40 mm over northern Scotland, although it clearly did nearer to home down here in Devon, and that was entirely overlooked (fig 2).

Figure 2

A string of showers has been continually forming and streaming E’NE along the spine of Devon for much of the day, they may have been moving more quickly than the showers over northern Scotland, but they never stopped coming for the central swathe of the county. Here’s a closer look at the estimated totals from 13 BST today (fig 3), and as you can see, there is a stripe of 16-24 mm accumulations aligned WSW-ENE across Devon, and inside that an inner area of 32-50 mm, with a few red pixels indicating greater than 50 mm over northwest Dartmoor by the looks of it. Showers of this nature are not uncommon in the southwest, and although very localised, the totals do indicate intense rainfall.

Figure 3

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

 

This mornings rain from Zlatan

Figure 1

Quite a wet and windy morning in the southwest as low Zlatan moved across Ireland. Rainfall estimates from the radar from for 0600 to 1200 UTC (fig 2), indicate that generally totals were in the 16-32 mm range across Cornwall, with a large area of 32-40 mm further east in the county (lime coloured pixels), and 40-50 mm across Bodmin (yellow pixels). The chief forecast 10-20 mm in a short time in yesterdays yellow warning for heavy rain, but very cleverly he didn’t put any figure on the totals, which they’ve recently stopped doing – or is that another one of my conspiracy theories that I’ve invented in my dotage – so they can never be accused of either grossly underestimating or grossly overestimating rainfall totals in any event.

Figure 2

I reckon looking at the actual six-hour rainfall totals from the SYNOPs that I’m not far out (fig 3).

Figure 3

Cornwall’s had its turn, now it’s Devon’s.