13 September 2016 – Gravesend

Gravesend - Broadness (courtesy of Tallbloke.com)

Gravesend – Broadness (courtesy of Tallbloke.com)

The maximum temperature at this time in September usually occurs around 13 UTC or possible 14 UTC. The hottest place in the SYNOP observations in the UK on the 13 September 2016 was at Gravesend. Its maximum temperature of 34.4°C made it the warmest September day since 1911. Here’s a snippet from the Monthly Weather Report for September 1911 (courtesy of the Met Office) about that warm spell. Since 94°F is 34.444°C so the Gravesend reading just falls short.

capture1

The thermograph based on hourly SYNOP data was quite peculiar. The temperature instead of peaking at 13 or 14 UTC as you might have expected on such a hot day just plateaued.

thermograph-for-03784-gravesend-broadness-00-utc-on-13-sep-18-utc-on-14-sep-2016

The temperature hit 32.0°C at 1300 UTC and again at 1600 UTC, but between those times (at 1400 and 1500 UTC) it dropped slightly and levelled off at 31.4°C. In that time period the temperature must have peaked to produce the 34.4°C maximum, so it must have climbed and fallen by at least 2.4°C in the space of an hour. It certainly can fall and rise very quickly at Gravesend because in the morning it rose 4.7°C in an hour (between 09 and 10 UTC) and later it fell 6.5°C in an hour (between 17 and 18 UTC). I would have said that the latter may have been down to a sea breeze, but there had been a light flow from the east for most of the day which confuses matters. The one minute data that the Met Office collect from Automatic Weather Stations like the one at Gravesend would clarify matters, but I doubt that this data would ever be released.

synop-grid-03784-gravesend-broadness

03784 Gravesend Broadness 13 Sep 0600 UTC – 14 Sep 1600 UTC

A similar hiatus in temperature occurred at Heathrow that day too, at 1200 UTC the temperature was 32.1°C but by 1300 UTC instead of being a little higher at 1300 UTC it had fallen by 2.2°C to 29.9°C. The temperature recovered a little by 14 UTC and recovered again at 15 UTC to reach 31.8°C. Somewhere between these times a maximum of 32.8°C occurred.

thermograph-for-03772-london-heathrow-00-utc-on-13-sep-18-utc-on-14-sep-2016

synop-grid-03772-london-heathrow

03772 London Heathrow 13 Sep 0000 UTC – 14 Sep 1800 UTC

A drop in temperature at either 1300 or 1400 UTC on the 13th is also noticeable at a lot of other sites across the southeast, I suppose it could have been caused by some medium or thick high level cloud, but there certainly was little evidence of it as far as I can see from the visible satellite images apart from a okta or two of cirrus or medium level castellanus.

uk-5-2016-09-13-1300

Visible Satellite Image  13 Sep 2016 1300 UTC (courtesy of the Met Office & EUMETSAT)

As far as I can deduce this is the location of the meteorological enclosure and Stevenson screen at Gravesend, and yes that it is some kind of communications mast next to it!

03786-gravesend

And here is a zoomed out view of its location with a little more perspective of where it lies, east of the city of London on the south bank of the Thames which of course is tidal – but I don’t intent to dig around for those details! If you are interested the Talkbloke blog has an interesting article about the location of  the site.

03786-gravesend

In conclusion all I can say given its location is that Gravesend seems a most unlikely place for a hot spot. Why higher temperatures were not recorded at places relatively nearby like Heathrow, Northolt or St James Park on a day when there’s a light flow from the east and sitting as it does jutting out into the Thames with the open sea no more than twenty miles to the east beats me, perhaps it’s all down to its proximity to mainland Europe. The 13th of September was not a one-off, Gravesend is a well-known hot spot and has done it before and come under scrutiny, and now it ‘s done it yet again.

13 September 2016 – Forecast

img_2760

I’m forever being left in a position where I could have sworn (and often do) that the BBC forecast for that day is markedly different to the one that I remember from watching on TV from the previous day. This of course is due in part (maybe a large part) to increasing senility, so to get round the problem and lack of evidence (because if you try to find some the next day, you’ll realise that the BBC are remarkably adept at removing any) I always record the weather for the week ahead. The program itself is not easy to catch, but my PVR never seems to miss it.

Anyway the purpose of this article was to examine the BBC forecast for yesterday and see how close the presenters using the best Met Office NWP got to reality. I reckon that the BBC record the program in the evening even though the program doesn’t sometimes go out till the early hours, this particular one aired at 0020 BST. So I reckon that the graphics are from the 1800 UTC model run, and the forecast steps they use to cover the next days weather the T+12 to T+30. So how well did they do on the 13th?

Temperature

img_2763

This chart is a clever get out because the BBC forecast never actually display a maximum temperature in a forecast as they did in the old days of magnetic symbols, and which to my mind is just as valuable as any spot value. The graphic above is displaying forecast temperatures for the specific time of 1500 UTC, and if I were to claim that they were underestimating the maximum temperatures in the parts of the southeast by more than 3°C,  the first line of their defence would be is that they’re not the maximum temperatures. But to be quite honest, Nick Miller had started the whole forecast with an introduction about advection and insolation standing in front of a graphic with 31 emblazoned on it, so having become adept and how they employ social media (and a TV forecast is just another form of it) the maximum temperature they were going for was most certainly 31°C. Small point to some, but in my opinion they got caught out in many places in the east and southeast of the country, but they would never admit to it but quickly move on and talk about how tropical storm “Ian” is coming our way.

maximum-temperature-c-0600-1800-utc-on-tuesday-13-september-2016-in-wmo-block-03-highest-maximum-temperature-c

Thunderstorms

Not quite getting the maximum temperature correct was quite minor to how they mishandled the thunderstorms in some central and western areas of the country. Here is the picture at 1000 UTC and then the weather radar reality:

img_2761

rainfall-intensity-0900-tue-13-september-2016

All the activity further east and aligned differently because the activity had nothing to do with old cold front like the NWP seemed to suggest. Let’s move onto 1500 UTC.

img_2762

rainfall-intensity-1500-tue-13-september-2016

The model has again failed to pick up on all the severe thunderstorms that were tracking north into the west Midlands and heading for Manchester in the early evening. What’s most noticeable from either of these two BBC forecast graphics is the total lack of lightning symbols, but I suppose if the graphics just had scattered light showers there wouldn’t be any. I did hear Nick Miller say at this point in the forecast that there could be a thunderstorms over northeast Wales but that didn’t tally with the graphics.

The day seems to end with the 1500 UTC frame in the ‘Forecast for the week ahead’ forecast, as if nothing ever really happens in the evening, but it did that evening and the severe thunderstorms continued to develop and play havoc with parts of the northwest.

rainfall-intensity-1700-tue-13-september-2016

Severe Weather Warnings

I did check the warnings page at the Met Office last night and to be fair there was an alert one out from mid morning for heavy rain over northern England. But I’m sure there wasn’t an alert out for the thunderstorms and the even heavier rain that affected the west of Cornwall around 1800 and again at 0300 UTC  the next morning. I could be wrong about this, but because there is no archive of alerts and when they are issued (as far as I know) I can’t be 100% certain.

Conclusion

I don’t think the forecast for the next day was accurate either in regard to the exceptional temperatures in the east and southeast, or from missing the thunderstorms and heavy rain in parts of the west and northwest. I watched some of the output for the next day through Twitter, and if that was anything to go by the press team at the BBC seemed to be more focussed on the rising temperatures in the southeast than any thunderstorms that were already occurring in the southwest of the country.

13 September 2016 – Thunderstorms

Here is my best guess rainfall accumulations for yesterday between 0900 UTC and midnight. I now use 5 minute data and this has increased the accuracy of my estimates.  You can see the south north track of the thunderstorms from when they initially made landfall in Devon and Dorset in the morning before moving up through the west Midlands and curling into northwest England during the evening. The wettest places (09-00) where around the Manchester area with an area of lime green pixels indicating totals of between 32 and 40 mm.

estimated-rainfall-accumulations-0900-0810-utc-on-wed-14-september-2016

Estimated rainfall accumulations 0900-0000 UTC on 13 September 2016

This is a table of 24 hour rainfall totals (06-06) which show even larger totals than those of northwest England occurring at Culdrose and Camborne in the west of Cornwall with over 47 mm at both stations from thunderstorms. Due to Met Office policy, there are currently no SYNOP stations in the Manchester area (population 2.55 million) so you won’t see any totals around the 34.8 mm mark from around there – oh dear, how sad, never mind.

daily-rainfall-mm-13-september-wednesday-14-september-2016-in-wmo-block-03-highest-daily-rainfall-mm

And finally here are the 40,929 lightning flashes that occurred between 0900 and 2300 UTC yesterday over the British Isles courtesy of Blitzortung.

archive_6_image

13 September 2016 – Heat

The 13th of September 2016 apart from being our wedding anniversary, was quite an exceptional day temperature wise across a lot of  western Europe and eastern England. I always apply my own home grown rules when looking at maximum temperature anomalies.

  • -1 to +1 Near average
  • +1 to +4 Warm (or mild depending on the season)
  • +4 to +8 Very Warm (or exceptionally mild)
  • +8 – +12 Hot
  • >+12 Very Hot

The 13th was hot in a large area from the southeast way up to the northeast of England. There were some coastal exceptions but generally inland anomalies were above +10°C and in the top five SYNOP stations maximum anomalies exceeded +12°C.

maximum-temperature-anomaly-06-18-utc-on-tuesday-13-september-2016

Maximum temperature anomalies 06-18 UTC on Tuesday 13 September 2016

Here is the ranked list of hottest places. The highest 34.4°C occurred at Gravesend and made it the warmest September day since the infamous 35.6°C at Bawtry in 1906, there is some talk regarding its validity, but I’ll have a more detailed look at the observations from their in another article.  I remember Bawtry well because it was there I had my initial interview with the Met Office in June 1970.

maximum-temperature-c-0600-1800-utc-on-tuesday-13-september-2016-in-wmo-block-03-highest-maximum-temperature-c

Meanwhile in the more western parts thundery showers were tacking south north producing some severe thunderstorms and heavy rain, but more about that in another article – so much to do and so little time.