If you examine the forecast chart for tomorrow and next Sunday, you might be forgiven for thinking there is something of a weekly cycle going on here (fig 1), both days are northerly, in fact next Sunday is a cyclonic northerly worthy of any cold spell in winter worth it’s salt. And if you look back over the last two weeks (fig 2), you’ll notice that both the 29th of October and the 5th of November had northerly tendencies too.
Figure 2 – Courtesy of the Met Office
Of course it’s all just chance, but just out of scientific curiosity, I’ll be keeping a close eye on the weather pattern for next Sunday, just to see if we can manage four successive northerly Sunday’s.
- 29th October [N]
- 5th November [N/NW]
- 12th November [N] (forecast T+24 but odds on)
- 19th November [CN] (forecast T+192)
Figure 1 – Data courtesy of the CRU/UEA
November is not renown for being an anticyclonic month in the British Isles. As you can see (fig 1), November is one of the few times in a year when cyclonicity is on a par with anticyclonicity. So looking at a ranked list of anticyclonic November’s since 1871 (fig 1), the number of any November’s that are over 50% anticyclonic (when using the Objective Lamb Weather Type as a crude measure) is quite small.
Figure 2 – Data courtesy of the CRU/UEA
As you can see, joint top of the list of most anticyclonic November’s along with 1942, is the year 1988, when 15 of the 30 days were pure anticyclonic.
Figure 3 – Data courtesy of the CRU/UEA
An anticyclonic Autumn month in November can only spell mean one thing as regards weather is concerned – frost and fog – and 1988 did not disappoint in that regard. The headline in the Monthly Weather Report for November 1988 read:
“Generally sunny and dry; cold at night with frequent fog”
Figure 4 – Data courtesy of Wetterzentrale
Looking at the daily charts it’s easy to see why it was so foggy, and it was also the 10th driest November in the UK since 1910, and fourth sunniest since 1929. In the Review of Autumn 1988 in the Weather Magazine, R.A.S. Ratcliffe said when explaining about the upper air pattern across the northern hemisphere for November 1988:
On the 500 hPa monthly mean chart the circumpolar vortex was split, with part over Novaya Zemlya, and part over northern Canada. The jet stream was unusually strong in the eastern Pacific, but the most unusual anomalies of 500 hPa height were -140 m over western USSR, and +100 m over the UK.
I don’t have that particular 500 hPa level chart to hand, but I can generate the mean surface pressure anomaly chart for the month (fig 5).
Figure 5 – Data courtesy of NCEP reanalysis
Figure 1 – data courtesy of NCEP Reanalysis.
July continues it a mood of being mobile and often quite cyclonic at times, in fact that’s how it’s been since the end of May across the British Isles, as this graph of zonality shows (fig 2). The simple answer of why has it been so mobile is that mean pressure in the first three weeks of July 2017, has been 8 hPa below average across Baffin Island, and there has been a band of lower than average mean pressure extending westward across Iceland, before arching around Scandinavia and into eastern Russia. South of that the mean pressure across much of the central Atlantic has been higher than average (+3 hPa), and between the two the W’SW gradient has been tightened.
Figure 2 – data courtesy of NCEP Reanalysis.
It’s very likely that this is all driven by temperature of the atmosphere at all levels. The SST is well below average around Baffin Island (-4°C), and the central mid Atlantic remains generally cooler than average (-1.5°C), although there is now a band of warmer water (+1.5°C) extending westward from America at ~38° north. It’s interesting to see the SST down the east coast of Greenland being colder than average, probably from fresh water from the summer melt of the glaciers.
This may help explain the very intense lows for July’s that we are seeing. Here’s the forecast for this Wednesday (26 July) from the Met Office, which shows an intense low of 973 hPa low at 19° west, throwing a frontal system across the British Isles. This low in turn, will spawn a series of secondary lows that will dumbbell around it before the week is out maintaining the mobility.
Finally, I thought that I would look at a virtual barograph that I would sit at 57.5° north and 22.5° west (where hopefully it won’t get too wet). As you can see (fig 4) the anomalies there have been mainly negative there for long periods, and this is in an area of low pressure anyway and generally close to where the Icelandic low is found on mean pressure charts.
I noticed that the warmest spring* in the daily CET record back to 1772 in Central England was 1893. I don’t make a habit of looking for exceptional warm springs in the Victorian era, it was just that the spring of 1893 was even warmer than the spring of 2017 which has just ended (fig 1). The other thing that caught my eye was how exceptionally high the mean maximum was (anomaly +3.82°C), and how comparatively normal the mean minimum (anomaly +0.37°). This obviously points to a very anticyclonic regime back in the spring of 1893 to produce very warm days and comparatively cold nights, the graph below (fig 2) shows the contrasting anomalies during that spring perfectly.
So just how anticyclonic was it? A quick scan of the reanalysis charts for that spring reveals it was very anticyclonic.
Figure 3 – Courtesy of Wetterzentrale
Here are the headlines from the monthly weather reports compiled by the Met Office back then:
- March 1893 Exceedingly fine and dry in all but the extreme north and northwest where showers were more frequent.
- April 1893 Remarkably fine, warm and dry, especially over southern England where the severe drought continued with scarcely any intermission.
- May 1893 Mostly fine and dry, especially in the south and east till mid-month, then unsettled with rain and thunderstorms in places.
- June 1893 Generally fine and dry first half with local thunderstorms, the second half saw frequent showers and thunderstorms.
As you can see from the LWT analysis (fig 4), spring 1893 is easily the most anticyclonic in the series that started in 1871, with 58% of LWT being either anticyclonic or anticyclonic hybrid. It wasn’t cold, because the predominant flow was generally southeasterly or southerly rather that east or northeasterly. Spring 2017 is currently in 22nd position with a couple more days of records to go.
Figure 4 – Data courtesy of the CRU/UEA
It was also the second driest spring [MAM] since 1766 in England and Wales (fig 5).
Figure 5 – Data courtesy of the Met Office
* I’m old-fashioned, and because of this I prefer to use seasons that start and end (approximately) at the times of the various equinoxes and solstices, so most of these stats are based on so-called ‘astronomical’ rather than ‘meteorological’ seasons.
Figure 1 – Data courtesy of NCEP/NOAA 20th Century Reanalysis
You can understand why May ended up being the second mildest on record since 1910, when you see the mean pressure chart for the month (fig 1). There was a large negative anomaly (-13 hPa) in central Atlantic, which distorted the flow around a large elongated low of 1006 hPa (49N 30W), which pushed the Azores high further south than usual, producing a SSW flow over western Britain. Pressure over central Greenland was much higher than average (+15 hPa) pushing the normal Icelandic low south. The Greenland high ridged southeastward into Scandinavia and central Europe, with a mean northerly flow over much of that region.
Another product that I can generate from my reanalysis application is a 4×3 grid of charts for monthly mean pressure and anomalies. Here are the circulation patterns for the last 12 years of April’s (2006-2017). If I ever do acquire a monitor that is larger than my Dell 24″, I may be able to pack more into a single screenshot! There is a broad similarity between 2015 and this April. April 2015 was the sunniest on record in many regions across the UK, and a comparison between it and the incomplete chart for this year show that although the mean pressure anomalies were not as large, the centre of the positive anomaly was further east at around 1° west (+7 hPa) rather than 10° west (+11 hPa) as it is this year (fig 1).
I’ve added extra functionality to the application to allow the selection of any area at any zoom level from around the world, as this example shows for Australia in 2016 (fig 2).
I’ve had a quick look at some of the results that I have generated and compared them with those from the IRICS, which is part of the Earth Institute, at the University of Columbia, and they seem to be in reasonable agreement, where they might not agree so well is when comparing monthly anomalies, because I use some extra long long-term averages when I calculate anomalies for my charts.
You may ask why I bother to write a bespoke application to do this when you can access anomaly charts from this great site, and my answer is, I have the time and resources, I love climate and weather maps, and because that’s what I do.
Q: Just why has it been so dry and sunny and dry this April?
Figure 1 – Data courtesy of NCEP/NOAA reanalysis
A: Because for the first 18 days of the month there has been a large (+11 hPa) positive MSLP anomaly sat just to the west of Ireland (fig 1). As I reported earlier this month (never thinking that the first half of April would turn out as anticyclonic as it has), 1938 was the most anticyclonic in records that started in 1871 (fig 2). The two April’s are indeed very similar, but the anomaly chart for 1938 was for the entire month, and not just the first 18 days, and were larger and even more pronounced. That’s not to say that the second half of April 2017 won’t continue to be just as anticyclonic as was the first.
Figure 2 – Data courtesy of NCEP/NOAA reanalysis
I’ve just put quite a lot of programming effort into the program that I use to download, parse and visualise reanalysis MSLP data from NOAA, so hopefully I’ve got things right. The LTA that I have used to calculate the anomalies for years 2012 or earlier is for the whole of the 20th Century i.e 1901-2000. For the years after 2012 the LTA is for the 66 year period 1948-2013. This is because the older reanalysis data uses a 2 x 2° grid, whilst the data after 2012 is from the 20th Century reanalysis on a 2.5 x 2.5° grid.
We’re still on course for a change in type during by the weekend as the low pressure complex slips away south and high pressure builds across the north of the country, and an easterly flow becomes established. This change may not last too long according to the GFS, but it will make a change to the cloudy, mild southwesterlies that we’ve had for most of March. The weather next week always looks more settled over the north, especially the northwest of Scotland than it does in the south, with strong easterly winds across the south.
I noticed that John Hammond mentioned warmer weather by the weekend on the 1 PM BBC forecast, this must be somehow connected to a pulse of warmer/less cold air being caught up by the easterly flow from the continent on Saturday.
I always thought that high pressure was synonymous with fine weather – well not according to the Met Office and their forecast chart for Friday at any rate, which has four separate occlusions draped across the anticyclone (1033 hPa) that’s centred over the British Isles.
March 1957 stands out as being the mildest in the CET series which started in 1659, and as mild as this current March is, the mean anomaly of which stands at +2.48°C on the 12th, it will have to do a lot more to surpass the mean temperature of 9.2°C of March 1957, which was +3.53°C above the 1960-1991 long-term average (fig 1). Here are the daily surface charts for the month courtesy of Wetterzentrale (fig 2). In fact even 60 years later, the month still holds claim to three of the warmest March days in the CET series, the 3rd, 11th and 12th. The MWR explains how a pulse of warm dry air from Spain on the 10th, produced a period of Summer like weather that lasted to the 13th. Temperatures at several places in England and Scotland reached 70°F on the 12th, at Haydon Bridge (Northumberland) they reached 74°F (23.3°C), and unusually 69°F was recorded as far north as Cape Wrath.
Figure 2 – Courtesy of Wetterzentrale
As you can probably see from the daily charts (fig 2) the country was flooded by mild south or southwesterly for much of the month, the mean pressure chart for the month shows a negative pressure anomaly of -17 hPa at around 30° west and 50° north (fig 3 & fig 4), with higher than average pressure over Greenland (+11 hPa) and Scandinavia (+6 hPa).
This of course resulted in unseasonably warm temperatures, not just affecting the British Isles, but also across much of western Europe as well, with a positive warm anomaly (+4°C) over eastern France and Switzerland (fig 5).
So March 1957 ended up with very few stations reporting an air frost, and ended up warmer than the following April, quite an exceptional month.
Courtesy of freeworldmaps.net
The low pressure system that most of Europe know as low Axel thanks to the Institute for Meteorology in Berlin, demonstrated perfectly what a low does when faced with a mountain range in its path. Does it go round it or over it? Or is it maybe deflected in someway or maybe even blocked? Well in this particular case Axel seems to have almost jumped almost skipped over the Scandinavian Mountains or the Scandes straight from the Norwegian Sea into Southern Sweden. As the Wikipedia article says about the mountains of Southern Norway, they are not particular high (~8,100 feet) but they are very steep on their western flank and must present a formidable hurdle for a low-level vortex to surmount, and low pressure as a rule generally avoids centring itself over high ground. Of course Atlantic depressions coming up against the mountains of Norway happens all the time, but I think the one I captured yesterday was a classic example.
Of course if the question and been why and not how does a low cross a mountain range, the answer would have been to get to the other side.