Q: Just why has April been so dry & sunny?

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.

Still on course for a change in type

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.

Figure 1

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.

Figure 2

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.

Figure 3

 

March 1957 – warmest in over 357 years

Figure 1

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).

Figure 3

Figure 4

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).

Figure 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.

How does a low cross a mountain range?

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.

Nursery rhyme change mooted due to climate change…

It’s no wonder that there are moves afoot* to update the nursery rhyme the “north wind doth blow”

The North wind doth blow and we shall may possibly have snow over high ground in the north,
And what will poor robin do then, poor thing?
He’ll sit in a barn and keep himself warm
and hide his head under his wing, poor thing.

Take a look at this morning’s 06 UTC synoptic chart (fig 1) in what is a classic northerly just to see what I mean.

Figure 1

Have a look at the plotted observations from Lerwick (fig 2) for the last 24 hours or so, and even with a partial 1000-500 hPa thickness of 5209 M in the midnight ascent there, the wintry showers have not managed so far to put down a covering there. And yes, I do know that there are multiple severely contorted occlusions in the UKMO analysis, but then again when aren’t there?

Figure 2

The main reason why a northerly is not as cold as it used to be apart from the fact that global temperatures are that bit warmer than they were 40 years ago, is all down to the temperature of the ocean between here and the pole. At the moment if you trace the trajectory back north and then northeast across Svalbard (fig 3) you’ll see how the SST anomalies en route are all positive and as high as +3°C or more.

Figure 3 courtesy of NCOF

Not only that the sea ice at this time of year has not reformed for a huge section of the Barents sea east of Svalbard (fig 4), as it hasn’t done for the last few winters in that area. That in itself allows the surface temperatures to be around 20°C warmer than they would have been over pack ice.

Figure 4 Courtesy of NSIDC

Classic northerlies are not that common, and are usually very fleeting affairs, but to whet your appetite I did find one from the 15th of January 1981 in my archives (fig 5), although it’s a little bit west of north, it did have a bit more bite than the one that just ushered in 2017. It would be interesting to see some research done in this area, just to see how cold northerlies may have been modified in the last 40 years, unfortunately I don’t have access to the kind of observational and climate data any research like that would require.

Figure 5

*As I’m sure you realised I made that line up in the first sentence for the sake of a punchy title!

 

It’s the winter solstice – but where’s winter?

According to Wikipedia the winter solstice occurred at 10:44 UTC on the 21st of December 2016. So now that it’s officially winter, the big question is when can we expect some winter like weather in the UK? I’ve been looking at the latest circulation patterns across the British Isles by means of the LWT data from the CRU. Interestingly the last month (19 November to the 18th December) has been highly anticyclonic (57% by type) across the British Isles, in fact it’s the 4th most anticyclonic period (19 November – 18th December) since the record started in 1871.

Data courtesy of the CRU at the UEA

In fact since the start of Autumn the circulation across the British Isles has been a series of longer anticyclonic periods interspersed with short very cyclonic spells, even when the type is not purely anticyclonic it’s often a hybrid anticyclonic type.

Data courtesy of the CRU at the UEA

You could say this is how most winters play out, but the stats for the month show that there is something unusual going on in 2016, and a high preponderance for anticyclonicity. Angus and the low that followed it is a perfect example of how the last few months have gone (17-20th November), they disrupted an anticyclonic spell, and then just as quickly as they came the departed and allowed high pressure to return. Similarly, the next named storm Barbara seems to be one of a pair, and just as quickly as they arrive they’re gone, according to the latest NWP, as high pressure quickly reasserts itself after Christmas (and by the way that’s one intense anticyclone of 1047 hPa if this forecast comes off).

Courtesy of OGIMET

Don’t ask me about the significance of the last months highly anticyclonic nature, perhaps it will persist and the rest of the winter will be highly mobile in the north with pressure remaining high over southern areas and the near continent, I don’t know. Here’s what the CET did in the following winters that were also very anticyclonic from the 19 November to the – 18th December in case you believe in analogs.

What about the rest of the winter?

Most pundits are looking for an anticyclonic block to distort the upper flow and make it more meridional and maybe end up with cold northerlies. But maybe we already have a block of sorts, and instead of the block sitting well to the west of the meridian and producing a northerly or over Scandinavia and producing an easterly, it’s maybe been sat over or just to the southeast of the British Isles for much of the time in the form of a rather cloudy high producing mainly dry and mild weather, occasionally though a cold front will clear the cloud out of it , and introduce clear skies and we end up with sharp frosts and fog, in fact I’ve never seen as much fog as we’ve had this Autumn (and I mean astronomical) here in the southwest of England.

Here’s the latest medium term thinking from the boys in Exeter. They mention the return of the anticyclone next week after this brief cyclonic hiatus. They also hint that the high pressure may retrogress westward during January allowing northerlies to permeate southwards across the country. It’s strange that even when they freely admit that there’s very little confidence about this happening, in a period where there is a good deal of uncertainty anyway, why they even bother to mention it in the first place? Perhaps they are longing for a bit of snow like a lot of us are!

Courtesy of the Met Office