The heat wave in California that has been going on for some time now, may have escaped many people’s attention because of the heavy rains and flooding of Hurricane Harvey. The air temperatures at 1400 PDT on Saturday the 2nd of September in San Francisco was 38.3°C (100.9°F). At Redding in the north of California it was already at 45°C (113°F) at this time. It look like many all time maximum temperature records have already been broken or will be broken in the course of today. It’s a shame they can’t somehow use the cool waters of the Pacific just to power their air conditioners in some way, but knowing the many entrepreneurs out there, I’m sure it won’t be long before they can.
Temperatures in the summer of 2017 in the UK crashed on the 20th of July and have never recovered in the almost three weeks since it’s been since then. You can see clearly how the temperature has almost flat lined in the NCEP reanalysis data for the grid point 52.5° north and 2.5° west (just to the west of Birmingham), with almost all the 6 hour anomalies negative since then, negative (fig 1). I remarked in a blog only yesterday about how unusually flat the daily CET values had been since the 20th of July. Its probably all tied up with that ‘ribbon of high wind speed high in the atmosphere‘ that we like to call the jet stream.
Meanwhile in stark contrast to the UK, just to the northeast of Rome in Italy, at the 42.5° north 12.5° east grid point, things have been slightly different. A part from three short cold spells, the temperature anomalies there have all been well above average since early June (and before), with the recent heatwave this month clearly evident.
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).
The baking hot weather over southern Europe continues for another day with little sign in the short-term of any relief, here are the 12 UTC temperatures for that part of the world (fig 1), and as you can see that there are a number of stations already reporting 40°C or higher on it.
Alghero in Sardinia is one of the hottest places on the chart, with a temperature of 41°C at 12 UTC. This is a coastal site but these observations are from the airport which is a short distance inland, here are the temperature stats for Alghero for the last month (fig 2).
So today makes five days in a row that the temperature has exceeded 40°C. Here are this week’s plotted 3 hourly observations for Alghero (fig 3).
You would have thought than a 12 knot wind from 200° would have brought a cooling sea breeze into that part of Sardinia and cooled things down at the airport (fig 4), but of course there is very little in the way of gradient over the Mediterranean, so I suppose the low-level flow could be from more of an easterly point.
It’s not till this time next week before the hot air looks like it loses its grip in that part of the world, according to the GFS model (fig 5), and this hot weather will only exacerbate the severe and very serious drought conditions that are affecting a large part of Italy at the moment.
The heatwave continued yesterday over much of central and eastern Europe, and was particularly intense over parts of SE France, Italy and the Balkans, with temperatures in excess of 40°C in places (fig 1). In the above chart of maximum temperatures for yesterday the temperature at Alghero in Sardinia reached 41.9°C. Yesterdays overnight minimum at Capo Caccia never fell below 30.4°C (fig 2).
I didn’t quite believe this one, because of course Capo Caccia is a cape that’s stuck out in the sea, but on closer scrutiny it does seem to agrees well with the hourly temperatures (fig 3). The station is at 204 M, so it’s not exactly on the beach, and may well explain why it stayed so warm overnight. Then it all came back to me that I’ve actually been to Capo Caccia, because at the bottom of a series of concrete stairs built into the cliff face, is a series of sea caves called Neptune’s Grotto, that my wife and I visited on our first holiday abroad in 1985, over 30 years ago!
Here’s a picture of Capo Caccia with its lighthouse that I never thought to capture in a picture back in 1985, perhaps because Kodachrome 25 was very expensive back then (fig 4). All I can imagine is that somewhere on the site is an AWS, it’s very similar to Berry Head in Devon, but a lot more dramatic.
I notice that there’s some talk of a ridge of high pressure nudging up from the Azores in the medium term, that might bring a return to summer across the British Isles, so I thought I would have a look at the NWP. In my experience, NWP forecasts are normally very good out to T+96, after that it’s more like science fiction than science fact. At first glance, the forecast chart from the UKMO (fig 1) look quite appealing for this coming weekend, apart that is from the trough over Scotland, there’s no front for miles which is unusual, and there’s none of those pesky lows in the immediate vicinity.
Take another look, and you may notice the cold trough of sub 546 dm air that’s across western parts of the country, that’s really quite cold air for early August, and at this time of year cold air means instability and showers. The GFS model for the same time shows this showery regime very well on Saturday (fig 2). Even without this cold trough the mobility looks likely to continue, and the next low in a cyclonic series that started on around the 20th of July, has already started to form at 40° west.
If you want the real summer it’s still alive and kicking this week over central and eastern Europe, with heatwave conditions across Corsica and Italy. It was well forecast by the models and has been going on all week. In this chart for Friday (fig 3), you can see the 582 dm 1000-500 hPa partial thickness line meandering over central Italy, which marks some exceptionally hot air.
We’re having a heatwave,
A tropical heatwave,
The temperature’s rising, it isn’t surprising
She certainly can, can can!
That’s how the lyrics go in first verse of Irving Berlin’s 1933 song ‘Heatwave’, but by next week hot conditions look likely to have extended as far north as the Baltic, across much of southern and eastern Europe, if the latest run of the GFS is correct. I can’t remember seeing 1000-500 hPa partial thicknesses of 582 decametres extend as far north as Corsica, as they are in this forecast chart for midnight next Wednesday the 2nd August (fig 1).
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.
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).
I was just looking at the recent hot spell, how warm it got, and how long it lasted in the daily CET series using the latest provisional data from the Met Office. If you classify a heat wave as a spell of five consecutive days or more, with mean maximum temperatures above 24°C then 2017 just qualifies. 24°C might not look particularly high, but you have to remember that the CET is a composite temperature from three separate sites. It may have been the warmest since 1976 because spells of this length are still rather uncommon in the month of June (fig 1), but looking at all summers since 1878, it still pales when compared to the hot spells in the years 1947, 1955, 1983, 1995, 2003, 2006 and of course 1976.