Thundery wintry showers

At present, the GFS is showing a shortwave moving eastwards over northern Britain on Thursday- this brings some rain and sleet and introduces milder air from the SW, meaning that any frontal attack from the SW on Friday/Saturday would produce rain for all at low levels: http://cdn.nwstatic..../96/h500slp.png However, the ECMWF and UKMO do not have this feature and as a result the frontal attack from the S and SW would be likely to see rain turning to snow as the belt penetrates north and east. http://cdn.nwstatic..../ecm500.096.png The ECMWF operational run then has near-average temperatures sweeping in from a cyclonic south-westerly flow, but the UKMO keeps the cold continental air hanging on for longer over the north and north-east of Britain, so all of this is well work keeping an eye on. Meanwhile there remains the possibility of significant flooding particularly towards south-western Britain. This morning's model outputs maintain a fair amount of northern blocking into the foreseeable future. A few days ago, I overstated the length and intensity of the upcoming cold snap (on the basis of the strong ensemble support for an east to north-easterly outbreak) but noted that in the long-term, high pressure would stay extensive to the north and probably retrogress towards Greenland, but with the caveat that the UK will end up relatively mild if we get a slow-moving trough in the eastern Atlantic. At present, it looks like this scenario will arise and any future cold snowy weather will rely upon the trough shifting from the position that it is currently shown at for the end of next week. As Gibby noted above some model runs are hinting at the possibility of the trough eventually sliding SE and re-introducing colder conditions, but the hints are quite tentative at present.

The upcoming cold snap now looks like delivering next to nothing on low ground re. North Sea convective snowfalls- there will be wintry showers in the east tomorrow but only cold enough for a sleety mix on low ground, with snow on high ground, then a cold dry sunny day for almost all of us on Tuesday and again on Wednesday. However, I see a lot of uncertainty post-Wednesday. The GFS sends a shortwave eastwards over northern Britain on Thursday which would bring rain and sleet and bring milder air in, such that the widespread frontal precipitation on Friday/Saturday would then fall as rain: http://cdn.nwstatic.co.uk/gfsimages/gfs.20121209/06/96/h500slp.png However, the ECMWF and UKMO do not have this feature and as a result the frontal attack from the S and SW would be likely to see rain turning to snow as the belt penetrates north and east. http://cdn.nwstatic.co.uk/ecmimages/20121209/00/ecm500.096.png The ECMWF operational run then has near-average temperatures sweeping in from a cyclonic south-westerly flow, but the UKMO keeps the cold continental air hanging on for longer over the north and north-east of Britain, so all of this is well work keeping an eye on. Meanwhile there remains the possibility of significant flooding particularly towards south-western Britain. This morning's model outputs maintain a fair amount of northern blocking into the foreseeable future. A few days ago, I overstated the length and intensity of the upcoming cold snap (on the basis of the strong ensemble support for an east to north-easterly outbreak) but noted that in the long-term, high pressure would stay extensive to the north and probably retrogress towards Greenland, but with the caveat that the UK will end up relatively mild if we get a slow-moving trough in the eastern Atlantic. At present, it looks like this scenario will arise and any future cold snowy weather will rely upon the trough shifting from the position that it is currently shown at for the end of next week.

If there's not much of interest going on with the weather, my interest in the weather generally wanes, both re. interest in the current day-to-day weather and the model output projections. However, it rarely depresses me unless it also happens to be persistently cloudy, in which case the combination of boredom and lack of sunlight starts to drag me down. I get despondent for a short while when the forecast models show one of my favourite types of weather for a while, only to ditch it at short notice, but that generally comes and goes within 12-24 hours. I don't find all zonal/mild weather uninteresting, but as a general rule if pressure is persistently high to the S and SE then the first paragraph does apply.

It might seem that way, but it isn't true. I remember that around 22-24 December 2005, the model outputs were mostly showing a dry anticyclonic spell with slightly below-average temperatures and then a return of the Atlantic, but all of a sudden, some GFS runs started showing a cold east to north-easterly outbreak, and then the other models switched. The end result was this: http://www.wetterzen...00120051228.gif I also remember quite a few instances when the models switched abruptly towards a mild outcome only to switch abruptly back to a cold one. Since it is difficult to get synoptics in the UK conducive to widespread snowfall, naturally the switches that result in something widely snowy will be outnumbered by those that don't, but I think abrupt switches from warm to cold outcomes are not unusual.

The snowy weather of February 1991 lasted about a week (7th-14th) although there had been some marginal snow events from weak frontal systems around the beginning of February as well. Frontal disturbances generated some longer outbreaks of snow that penetrated well inland, in addition to the usual periods of sunshine and snow showers generated by convection over the North Sea. In January 1987 the main easterly blast lasted about 4-5 days, with three days of sunshine and snow showers followed by a more organised belt of snow, and some freezing rain for some on the southern flank. However, the persistence of cold meant that many central and eastern areas had 10-15 days of lying snow in both months. I'm not too bothered about having a prolonged cold spell- from a "snow" perspective I would have happily settled for a few days of sunshine and snow showers followed by a snow-to-rain breakdown from the SW, and the main factor putting me off from a fast breakdown was more the potential for widespread flooding. However, the recent change on the model outputs has largely got rid of the North Sea convection as well, so realistically speaking I'm faced with four cold dry days followed by a brief snow-to-rain event and the inevitable flooding- the worst of both worlds really. I had always said, "wait till Saturday- don't trust an easterly till it gets inside T+72", but I can't deny that I was "taken in" a little around Thursday and early Friday when the models were very consistent in having a few days of snow showers near the east coast and possibly further inland at times. However, it's worth trying not to get too worked up about it, because it's something we have no control over.

I've booked my trip to Norwich for next week, but the models have toned down the showery potential to such an extent today that there is no guarantee of wintry showers in Norwich at any point from Tuesday onwards, and it's a similar story for NE England, even near the coast. Monday looks set to have a fair number of showers down the eastern side of England, which will be wintry but it isn't clear if it will be cold enough for widespread snow cover at low levels.

I can understand the frustration at the GFS 06Z outputs, for there are sound reasons why that run would give most of us cold but essentially dry weather- we get a ridge of high pressure over us with virtually no wind, and snow showers would thus be restricted to eastern coasts, then a dry SE'ly follows. There would still be possible frontal snow events afterwards from an attack from the SW though. However I think the GFS is making rather more of the ridge of high pressure than UKMO/ECMWF. Here's the GFS at T+144: http://cdn.nwstatic....144/h500slp.png ...and the ECMWF: http://cdn.nwstatic..../ecm500.144.png UKMO: http://www.wetterzen...cs/Rukm1441.gif The ECMWF and UKMO make rather more of the troughing in the North Sea which would result in snow showers getting somewhat further inland than the GFS 06Z shows. The ECMWF ensemble mean agrees pretty well with its operational run and the UKMO, so unless we see the Euro models make more of the high pressure in the 12Z runs, I think the GFS looks rather out on its own at the moment. We had the "why not take the risk with a frontal breakdown, greatest risk = greater reward" argument regarding the possible super-low of late November 2010 that never materialised. The truth of the matter is, it depends on where you live. For western and south-western Britain, snow showers from the North Sea rarely come to much, and frontal battlegrounds sometimes deliver sizeable falls, so that would be the best option for snow lovers there. But for the east of both Scotland and England, snow showers off the North Sea sometimes give sizeable totals, while in marginal frontal breakdowns the south-easterly winds off the North Sea often result in a rain/sleet mix, especially from Yorkshire northwards, so snow lovers in those areas are better off with a showery north-easterly. I see no reason why a downgrade of the sunshine-and-snow-showers scenario would be a good thing for snow lovers in this case, for even if we get one, we will still have the inevitable attack of frontal systems from the SW afterwards, giving the west and south-west potential for frontal snow.

Some of the posts in this thread, like yesterday, are rather over-egging the "downgrades"- looking over the UKMO and ECMWF charts, the consensus there is for a slack east to north-easterly flow with heavy snow showers near eastern coasts, perhaps penetrating a fair way inland at times in association with disturbances and troughs, while the breakdown doesn't come until at least the 16th December and probably later. The GFS for now is rather out on its own suggesting a weak ridge of high pressure followed by a quick breakdown.

In stark contrast to the previous few days, I prefer the UKMO output over the GFS/ECMWF- we get a fairly straightforward east to north-easterly flow with troughs which would help to bring bands of heavy snow showers a fair way inland despite the relatively light winds. All outputs are looking snowy within 50 miles of the east coast IMHO, and a lot less marginal temperature-wise near the coastal fringes in particular, though areas further inland look less likely to catch significant showery activity than they did on the basis of yesterday's runs.

South Yorkshire will develop its own "snow dome" with snow showers reflected away to the N, W, E and S.

The slack north-easterlies on the current model outputs would favour eastern coastal areas but the GFS is almost certainly underestimating how far inland the showers would penetrate. I would expect some significant snowfalls for east Yorkshire and light snowfalls for west Yorkshire under the current progged setup, with very few places missing out altogether.

The slack east to north-easterly flow shown on the current model outputs would most likely see snow showers restricted to the eastern half of the country, but I doubt that they would be confined to within 5-10 miles of the east coast- we'd be talking more like 40-50 miles at times IMHO. The GFS is pretty good at modelling coastal showers but often underestimates how far inland they penetrate when the airflow is off the North Sea- we've seen this quite a few times with recent northerly outbreaks. My earlier assessment of, "unlikely to turn milder widely before the 19th December", is now looking rather more shaky as we are seeing increasing support for a breakdown from the SW around the 16th December- but I rather think that the Atlantic systems will make slower progress than the ECMWF ensemble mean currently indicates, and the 19th December may well be around the time that we see the (possibly temporary) return to milder conditions. I am still seeing evidence of retrogression of the high towards Greenland- "greens" may indicate a weakening of the northern blocking but the absence of "blues" in the area indicate that it is still there.

Some quite ridiculous posts coming in this evening- granted, the slacker flow showing on the GFS and UKMO could be the precursor to a downgrade into a cold dry anticyclonic interlude followed by an attack from the SW, but then again it could just as easily be the models over-reacting to a new signal, and there will probably be more changes tomorrow. I maintain that the GFS 12Z post T+168 is extremely unlikely to come off. In any case, having seen the latest ECMWF run out to T+120, I'm pretty confident that tonight's ECMWF run will be as epic for snow lovers as the last two, with 850hPa temperatures around or below the -8C mark over wide areas combined with a stronger easterly flow.

Rain here, 3.1C...

Tonight's GFS and UKMO have slackened off the easterly flow and made it more of a N to NE'ly at times in the south- this means colder weather than on previous runs, drier inland, but also snowier within 20 miles of the east coast. The UKMO charts in particular have kinks in the isobars which would be likely to be associated with heavy snowfalls over eastern England, extending from Northumberland to East Anglia, and probably SE Scotland as well. A slacker east to north-east flow was suggested by some of the ECMWF ensembles this morning, so it will be interesting to see if the ECMWF also slackens off the easterly flow tonight. Given the fairly significant changes with this evening's runs so far, I remain cautious regarding the amount of showery activity we'll get off the North Sea next week- a slack anticyclonic dry type is looking like more of a possibility from tonight's runs for instance- but it almost certainly looks like turning very cold. The low-res part of the GFS 12Z run, ushering in a zonal type, is somewhat at odds with the teleconnection and other atmospheric signals for continued northern blocking so I would advise not getting too worked up over it.

UKMO 12Z looks similar to the GFS with a fairly slack north-easterly flow at T+96, T+120 and T+144: http://www.wetterzentrale.de/topkarten/fsukmeur.html Those charts look particularly conducive to heavy slow-moving snow showers from Newcastle/Durham eastwards in my view.

I've realised that I may be going down to Norwich next week for when the easterly hits- Norwich is traditionally a very favourable location for snow in easterly setups (at least as much so as my current part of North Yorkshire) but I remember late Nov/early Dec 2010 all too well, when Norwich sat in a dry cloudy slot for three consecutive days with two inches of solid ice on the ground. The current progged setup shows more of a north to north-easterly over East Anglia though, which would make that scenario less likely. Meanwhile for the north of England the slacker flow shown on the GFS 12Z run would indeed make it less marginal temperature-wise near the coast, but would also reduce the chances of many showers getting inland.

Temperature up to 3.1C, I'll be surprised if I see any advance on sleet from this.

Seems that the band is intensifying but struggling to get over the Pennines. Up at 2.3C here now so it would have to be heavy stuff to provide enough evaporative cooling to give lying snow.

3.3C at Neil Bradshaw's site at South Shields with DP of 0.5, so looks a bit too marginal there, although if any heavy precipitation gets over to the North East then evaporative cooling could make the temperatures more favourable.

I've put in a post in the model output thread detailing my thoughts (it's the closest to a ramp that I've come so far). In brief, I expect some sort of easterly or north-easterly to come off between 10 and 16 December, though with 850hPa temperature and showery activity amounts still prone to changes either way (we could get anything from a slightly warmer version of late Nov/early Dec 2010 to a fairly cloudy ESE'ly with light flurries). Then I expect low pressure systems to start attacking from the SW, pushing us into a less cold SE'ly pattern with some frontal snowfalls initially followed by milder weather, though temperatures are likely to remain rather below average until at least the 19th, I reckon. After that, I think the milder weather will probably be temporary as highest pressure will be in the process of retrogressing to Greenland and this may set us up for northerlies as we approach the Christmas period- the only major barrier that I can see to this is if low pressure becomes slow-moving in the eastern North Atlantic.

My current feeling regarding the long-term outlook is as follows (based on current model outputs and teleconnection signals): 1. A cold continental airflow is highly likely to come off between around 10 and 16 December. There is still scope, in my opinion, for variation either side of the current model outputs regarding the specific direction of the flow from the eastern half of the compass, the 850hPa temperature and the amount of showery activity that we get over the North Sea (broadly speaking the lower the 850hPa temperatures, the colder and the more showery it will be) but continental air and below-average temperatures are looking highly probable. 2. As others have noted, there is a split between models favouring a transition to milder weather with lows attacking from the SW, vs. lows sliding eastwards to our south and keeping us locked in a cold east to south-easterly type, while highest pressure is likely to retrogress from Scandinavia to Greenland. My feeling is that we will maybe get two or three days of "sliders" before the milder air eventually pushes up from the SW as low pressure becomes slow-moving to the W for a time. but I will be surprised if mild weather establishes widely before the 19th December. 3. There is a very strong likelihood, in my view, of any transition to milder weather being temporary, as towards Christmas we are likely to have a strong belt of high pressure around Greenland and Iceland. The only "spoiler" I can foresee as a likely outcome is if low pressure remains slow-moving to the west of the UK leaving us in mild southerlies- otherwise chances are we will get a spell of northerlies. I am reminded of Christmas 1995, when we saw an attack from the SW bring mild weather around 21-23 December only for arctic northerlies to return on 24-25 December. Thus, although it's a long way out, I feel that the probabilities re. likely weather around and just before Christmas are strongly weighted in favour of cold bright weather, especially the further north you are. The NOAA's 8-14 day outlook for 13-19 December clearly shows the tendency for high pressure over Scandinavia to become more focused on the Greenland/Iceland area.

There has been snow or sleet on most days here at Sandhutton, but nothing more than a brief dusting on the ground. Sleet followed by freezing rain during the early hours of the 1st, a spell of snow followed by rain on the 3rd, sleet mixed with rain late on the 4th, and two snow/graupel showers on the 5th.

I agree with the suggested range of 1 to 2.5C. At present I consider it extremely likely that we're going to get a cold spell around the 10th-16th. There is still scope for a toning down of the cold 850hPa temperatures and amount of showery activity over the North Sea in my opinion, but I find it hard to see us not getting an airflow vectored from the eastern half of the compass with well below-average temperatures. From the 17th-21st we will most likely see an attack from Atlantic depressions to the SW which may bring us a mild interlude, but I can see a northerly outbreak setting in around or just before Christmas as highest pressure retrogresses to Greenland. Milder weather may then return towards the month's end. The likelihood of the milder interludes coming off, at least for two or three days, makes the chance of a sub 1C December quite remote (though not outside the realms of possibility) while I doubt that any milder interludes will sustain for long enough to prevent a sub 2.5C December, especially given the cold start to the month.

Indeed, early December 1962 was anticyclonic with some smog, as pollutants got trapped underneath an inversion: http://www.wetterzen...00119621205.gif And a further 10 years beforehand, there was an even more severe outbreak of smog from a near-identical synoptic setup: http://www.wetterzen...00119521205.gif Had it not been for the Clean Air Acts of 1956 and 1958, the December 1962 smog may well have proved at least as deadly as the December 1952 one, but nonetheless it did take some time for the UK to fully adapt to those Acts and thus we still saw approximately 750 smog-related deaths in the London area in early Dec 1962. In early Dec 1952, the overall death toll was at least 4,000. http://www.metoffice.gov.uk/education/teens/case-studies/great-smog