Thundery wintry showers

Potential for some moderate to heavy snow over much of the UK tomorrow, but temperatures will be pretty marginal due to a pool of warmer air with 850hPa temperatures around or above the -5C mark, especially the further south and west you are, so it is doubtful as to how much of it will settle on low ground. The GFS is suggesting that north-east England is most likely to be heaviest affected: http://cdn.nwstatic..../uksnowrisk.png We have seen a significant toning down of the subsequent easterly- many earlier runs would have seen a fair number of snow showers come in off the North Sea, particularly for SE Scotland, NE England, Lincolnshire and Norfolk, but it has now been slackened somewhat. The ECMWF and UKMO versions are a little less slack than the GFS and more in line with what the FAX charts show, and thus would bring snow showers further inland than the GFS precipitation charts are suggesting, but if the GFS version comes off then it will only be coastal fringes that will see any showery activity. The easterly flow is likely to slacken off almost completely by Thursday with snow showers restricted to coastal fringes, but there is considerable potential for classic "frontal battleground" snowfalls as Atlantic weather systems struggle to make much headway against the Scandinavian blocking high and also bring some fairly potent polar maritime incursions in behind the fronts, causing cold air to meet slightly less cold air. A fast breakdown from the west is not implausible as we saw the models overdo Scandinavian blocking as recently as mid-December, but at the other end of the spectrum, nor is something similar to the 5th-7th February 1996 when some parts of the UK had a sizeable dumping.

It's been snowing for a while in Sandhutton, a light dusting on most surfaces but nothing major. Hoping that the pool of >-5C 850hPa air doesn't make too much of a difference tomorrow as with the current temperature of 1C, anything heavy would provide significant accumulations.

Dry and mostly cloudy but there are some well-scattered breaks in the cloud cover, currently about 6/8 oktas cloud. Temp 4.0C, quite steady A light easterly breeze.

The ECMWF so far has made rather less of the easterly over the weekend than the GFS (may still be some marginal light to moderate snowfalls in the south on Saturday, but not many snow showers in eastern areas on Sunday). However, Monday's chart looks like it could bring frontal snow over a fair slice of the country with rain largely confined to the south-western quarter of Britain. As others are noting, with generally high pressure to the north, it's hard to see the rest of the run going similarly to the GFS!

I think it relates to why Pete has been constantly posting links to the Model Mayhem thread- unfortunately the thread has become swamped with 1-2 liners posting value judgements on what the models are showing and not a lot of substance, and that tends to lead to confusion. The reality with the GFS 12Z is that in the short term (out to T+144) it is quite a snowy-looking run- some light to moderate snowfalls are likely in southern England on Saturday (though marginal - possibly sleet/rain near the south coast) and then we get colder brighter weather move in on Sunday/Monday with scattered snow showers for SE Scotland and down the eastern side of England, particularly NE England and Lincolnshire. In the longer-term though the northern arm of the jet powers up and we move into a mild pattern with high pressure to the south and lows moving across to the north- hence the comment about a horror show. This evening's UKMO is less potentially snowy out to T+120, with a shorter-lived easterly on Sunday, but then by T+144 we get a full-on northerly with strong blocking over the Greenland/Iceland area, so the UKMO evolution would be more likely to produce a cold snowy spell starting around mid-January and lasting for upwards of a few days.

In the short term there is growing agreement on some sort of easterly incursion on Sunday and possibly into Monday, which may bring scattered snow showers into SE Scotland, NE England and Lincolnshire, although we may well only be looking at a window lasting between 18 and 36 hours for this before the shortwaves come down from the north-west. There may then be a frontal snow event on Monday/Tuesday as the frontal systems associated with shortwave activity push into the cold air, although the GFS is suggesting a snow-to-rain event (as opposed to this morning's ECMWF which would see rain confined to south-western Britain). I have a feeling that the GFS is overdoing the shortwaves but that the ECMWF is underdoing them- this split quite often arises and I think some sort of middle ground is likely- how cold/snowy this will be is uncertain.

I've mentioned before that in the super-mild second week of February 1998, the warm sunshine struggled to penetrate as far north as the Tyne and Wear area, only really making it on the 13th. Although the satellite images on those forecasts imply a similar story for the January 1998 spell, there was actually a fair amount of hazy sunshine in Tyneside on the afternoon of the 9th, throughout the 10th, and for most of the day on the 11th, and most of the cloud cover was thin high cloud. Perhaps the greater southerly component to the wind direction helped the sunshine penetrate further north to the east of high ground. It was still grey and wet in the NW though.

Sunshine has been pretty variable across the UK during the recent mild settled spell- most regions have been cloudy but there have been exceptions. I remember a couple of days when there was a large hole in the cloud sheet over much of the Midlands and the south and east of Yorkshire, which tallies well with what Bottesford and Blizzards said. However, it's been dull here apart from occasional fleeting glimpses of the sun, and today is set to be the first generally sunny day since New Year's Day.

Essentially, when we get the low pressure sliding down from Greenland/Iceland at some point between Sunday and Tuesday, a build of pressure to the north of the system would result in an east to north-easterly flow over the British Isles once the low slips away to the SE- which is pretty much what the GFS shows. But if we get no build of pressure to the north then we may just end up with a cyclonic type like the ECMWF operational run shows.

Regarding snowfall, there is potential for a frontal snow-to-rain event on Sunday as fronts come down from the NW pushing into an accumulation of cold continental air. At present SE Scotland and NE England are looking most likely to see the white stuff at low levels, though it looks set to be pretty marginal: http://cdn.nwstatic..../uksnowrisk.png The preceding SE'ly looks likely to bring mainly rain or drizzle in off the North Sea during Friday and Saturday with temperatures around 3C. The above event is prone to considerable uncertainty though as UKMO/ECMWF make a bit more of the preceding easterly (perhaps some snow showers near the east coast for a time) and delay the onset of the systems sliding down from the NW, which could mean a snow-to-rain event on Monday or Tuesday instead. After that we have shortwaves sliding down from the NW. I do share many of Ian Brown's concerns, as the subsequent easterly blasts rely upon a build of pressure to our north and this is currently far from guaranteed- the ECMWF operational run does not show one and the ensemble mean is unconvnicning.

At Cleadon on Christmas Day 1999, following a morning of sunshine and rain showers, the last of the fairly heavy showers (at around 1:30pm) unexpectedly turned to snow. Normally I see one snow shower that doesn't lie as a disappointment on a Christmas Day, but in the case of 1999 it was a bonus as I was only expecting rain throughout.

Cleadon in Tyne and Wear somehow contrived to see out December 1999 without a single morning with <50% snow cover, despite most other parts of the region seeing at least one such morning. The snowfalls of the 14th/15th failed to produce significant accumulations within a couple of miles of the coast in South Tyneside, while the snowfalls on the 18th were only light near the coast and heavier further north and inland within the region. In fairness it probably evened things out as Cleadon was one of the snowiest spots in the region during the winter of 1998/99. It's also worth noting that December 1999 was an exceptionally sunny month in the Tyne and Wear area (over 90 hours of sunshine at Sunderland) and also that many parts of the UK had a lot of wet snow that never stuck around on the ground for long (a stark example being the 11 days of sleet/snow falling at Manchester Airport but none with >50% snow cover at 0900). I'm enjoying browsing through these old forecasts- will be interested to see what they made of the snow event on the 16th February 2000, as I seem to remember that one not being very well forecast for the Tyne and Wear area.

Good points about the Azores High- I was thinking more in terms of a persistent east-Atlantic trough but you are right, the high building from the Azores has become more of a substantial feature on recent runs with the jet rather too far north across the Atlantic to give the UK easy shots at significant easterly outbreaks (they can still happen but it is harder to get the low pressure undercutting to the S and SE). I remember that high pressure over the Azores ridging at times into the mid-Atlantic was a recurring feature that scuppered many "nearly easterlies" in February 2005 although of course we got there in the end (albeit not off a particularly cold continent).

Dry and cloudy for the 5th day in a row, currently 7.0C. The cloud has been mainly high cloud since Saturday, and today is no exception- Thursday/Friday had a greater emphasis on low cloud.

From the accounts that I've seen, forecasts in the 1960s/70s for the next day were often inaccurate (e.g. forecasting a quick arrival of mild south-westerlies, and in reality frontal systems stalling against northern blocking and buckling south-eastwards followed by a cold spell) so there's probably no way of knowing! Whenever I've looked through the Wetterzentrale archives, though, it appears that many easterly types, in particular, have been "knife-edge".

I'm seeing greater consistency in the model outputs this morning. A few days of relatively mild westerlies lie in store, with rain set to move slowly SE across the country during tomorrow and Thursday followed by a return of drier conditions. Then, we are set to have troughing in the eastern Atlantic which will give us southerly winds and temperatures near or slightly above average but with accumulating cold pools over the near-continent. Yesterday I posted about "northerly potential" around mid-January but this appears to have receded somewhat on this morning's runs, and our main "shot" at a cold snowy spell involves the main Atlantic trough drifting south and west, associated shortwaves sliding SE to our south, and high pressure ridging across to our north pulling in a cold east to north-easterly flow. The GFS 00Z and ECMWF 00Z runs both get there eventually and the UKMO 00Z looks like it is evolving the same way, but this particular evolution is somewhat flimsy and I have a suspicion that the trough may prove too stubborn in the eastern Atlantic to allow that evolution to come off as shown.

http://www.netweather.tv/index.cgi?action=monthly;sess=

I would suggest that this is one of the poorer sceptic arguments out there (and yes, there are better ones- for example I remember a discussion with Captain_Bobski where he really had me reconsidering some traditional pro-AGW positions). It's riddled with the usual straw men and the "uncertainty over AGW implies certainty that AGW is being seriously overestimated" type of position. Also, there has been global warming over the past 18 years (contrary to his claims)- it has just happened at a slower rate than it did during the 1980s and early 1990s. The "What if I am wrong?" argument ignores a range of factors including rising fuel costs as a result of increasingly scarce oil reserves. As for the following argument: For a laugh, here's what a similarly extreme misrepresentation of scepticism could look like: They say: “We don't care what the truth is. What matters is that free market capitalism is the solution to everything, and that AGW is just a sham intended to overthrow God. For all that matters in the world is to maximise consumption, maximise economic growth, and maximise GDP, all taking a short-termist viewpoint, and screw everything else including what we do when our oil reserves run out. For Free Market Capitalism our Lord, amen. I could go on, but I'd be here all night if I was to sit here picking holes in his analysis. It's a shame to see things like this routinely trotted out, when there are many good reasons for being sceptical about AGW. Most climate scientists are happy to have their science challenged and questioned in a well-reasoned way- the problem arises when politics get in the way and politicians want to make out that the science points towards certainty when in reality it points to uncertainty and probabilities, which are currently seen to favour a warming somewhere within the range of 1 to 6C but most likely between 2 and 4C.

Just replying to this after having been away for a while. I say that because of the issue that while some relationships between atmospheric variables stay constant regardless, others may potentially change if the global climate changes significantly. Due to the latter issue, if climate models are good at reproducing past and present climate, it doesn't mean that they'll do similarly well at predicting future climate under enhanced warming of, say, 2 degrees, and this is a significant problem re. verification of the accuracy of predictions of late-21st century climate. The same sort of issue could potentially affect shorter-range forecast models- though as their accuracy can be verified very quickly by comparing the forecast against the actual outcome, chances are the scientists behind them would soon be able to tweak the parameterisations appropriately to provide a better fit for the "new" climate and so any drop in accuracy would be short-lived (and probably indistinguishable from the short-term drops you get from the likes of stratospheric warming events anyway).

Personally I've largely given up on anything snowy before mid-January but am quite hopeful that the stratospheric warming could bring something more substantial after midmonth- a spell of messy synoptics followed by a cold snowy spell, like around New Year 1985 or mid-December 2009, is far from out of the question. At present, though, anything post T+96 appears to be pretty speculative.

I can potentially see an argument that if the global climate system shifts significantly, the thermodynamic equations that work up to a point today may become less effective in the "new" climate- but I haven't seen any evidence of this happening yet, as the model verification stats have consistently shown a gradual upward trend in accuracy. I feel that the sudden stratospheric warming is probably the main factor that is causing so much variability in the outputs.

Many cold spells have failed due to, for example, the models' tendency to overdo heights between Greenland and the mid-Atlantic and underestimate cyclogenesis around Iceland, resulting in overdoing the length and intensity of northerly outbreaks. There have been countless past examples where the models have suggested a few days of northerly winds, troughs and snow showers getting a fair way inland, only for it to downgrade to a 36-hour northerly "toppler" with wintry showers confined to the usual coastal areas. It is often possible to spot the warning signals, e.g. by noting a gap between the mid-Atlantic and Greenland highs at T+120 prior to the initiation of the northerly and suspiciously low heights. With easterlies, the most common "spoiler" is a northern arm of the jet stream causing high pressure over Scandinavia to sink southwards rather than ridge westwards to the north of the British Isles. The December 2012 failure was relatively unusual as the models overestimated the strength of the link-up between highs over Siberia and Iceland, and as a result shortwaves filled in the gap between the two highs and the Icelandic high settled over the British Isles for a few days. As a result it caught even the most seasoned pros out. I have to agree with Ian Brown about the confusing picture we currently have- no definitive "spoiler" but also no definitive route into a generally cold snowy spell, although I think that if we can get heights establishing over Greenland/Iceland (as keeps happening near the tail end of ECMWF runs) then we will be well on our way towards one.

Indeed, the ECMWF eventually gets to a typical Greenland high/Scandinavian low type setup by T+240. I am now back to feeling that widespread snowfall is unlikely before mid-January, but then again this always looked likely until we had the couple of ECMWF runs showing a potent east to north-easterly blast in the second week. It's hard for most snow lovers to be patient after the letdown in December 2012, but many significant cold snowy spells of the past have come about gradually, with a week or so of increasingly blocked, dry and progressively colder weather before we wander into the freezer. Mid-December 2009 was a good recent example, with a couple of "nearly-easterlies" before the 36-hour NE'ly hit on the 17th/18th followed by generally cold snowy weather until the second week of January. The very cold and, for many, snowy weather starting around 6 January 1985 also evolved gradually. In the near term, it's likely to be pretty wet over much of Scotland and Northern Ireland over the next two to three days as fronts come in around the periphery of our high pressure- this could well create more problems with flooding.

An essentially cold but dry pattern showing on the ECMWF at around T+120-192- the 850hPa temperatures are generally above -5C and this would not support snow showers off the North Sea, in contrast to this morning's offering, but the ECM was always at odds with the UKMO/GFS on that particular easterly outbreak. The amount of divergence between the UKMO/ECMWF/GFS is indeed staggering at T+120 onwards and the models are clearly struggling to deal with the impacts of the sudden stratospheric warming event on the weather in the vicinity of the British Isles. Below-average temperatures look a highly likely bet, but it is impossible to call whether or not it will be cold or moist enough for snow. My long-range forecast issued a few days ago went for a very cold and possibly snowy spell (though with considerable uncertainty attached) in the last third of January and to be honest I've been surprised by how abrupt the shift in synoptic pattern is progged to be at present. It may well be that if we do get a snowy spell, rather than being ushered in initially by an easterly, it might come on the back of a northerly plunge, as the ECMWF has been very consistent recently in projecting a Scandinavian trough and Atlantic/Greenland blocking around mid-January.

Yesterday I posted about how the two stumbling blocks for a snowy spell could be, high pressure building over the UK, and/or persistent troughing in the mid to eastern Atlantic. The GFS 12Z run shows the latter, with low pressure stalling just to the west and feeding in relatively mild southerlies while the continent stays cold. The GFS is all over the place at the moment, so I don't think too much should be read into it at this stage, but I retain a view that snow lovers should not get too excited, particularly as far as snowy easterlies or north-easterlies are concerned, unless they appear inside T+72.