Thundery wintry showers

It's risen to 1.6C now and the frost is melting. I don't have much hope of snow from this system- my area has avoided getting more than a brief dusting so far this season, and temps look distinctly marginal and the Pennines will tend to take the sting out of most of the precipitation. However if this morning's ECMWF or GFS or UKMO verify then I will be in a very favourable location for significant snowfall next week- still a significant "if" at this range but I'll be pretty confident if the outputs on Saturday morning look similar.

That would be true in a showery situation but we're heading for a frontal situation- temperature inversions were a common issue during the previous two frontal events with temperatures near zero at the surface and a couple of degrees higher at higher altitude. However it does mean that even if the temperature doesn't get far above freezing, we could still end up with rain.

The model output agreement on a "sunshine and snow showers" type of east to north-easterly flow, with the possibility of organised snowfall at times via troughs and southerly tracking depressions, has strengthened further this morning. From T+168 to T+240, the ECMWF ensemble mean suggests an east to north-easterly flow slowly swinging around to the SE, but with continental air established and fronts stalling against the continental block which could well mean frontal snowfalls. The key timeframe is now around T+96, for where we get to at T+96 on the GFS, ECMWF and UKMO there can only really be one outcome. If the model outputs still look like this tomorrow, and more so Saturday, with the key transition remaining on Monday, then you might see me starting to indulge in some ramping. However I have seen these easterlies toned down somewhat between T+72 and T+120 so I'm just holding onto the very top of the "ramp" at the moment. I agree with the posts re. flooding in the South West- some serious rainfall in many places today. There is the chance of some snowfall from this system too, but it will be very marginal and I suspect that most of us won't see significant accumulations. The second low for Sunday/Monday looks like it will produce rather less precipitation but at present it is modelled as the "trigger low" that is followed by the aforementioned east to north-easterly regime.

Currently -1.6C and the sky has a grey snowy look to it here too, although there remains the question of how much the temperatures will rise as the precipitation approaches- I'm still not expecting much of the white stuff from this. Meanwhile the model outputs have strengthened the consensus on a "sunshine and snow showers" type of east to north-easterly flow from about T+120 this morning- if the outputs are still looking like that in two days' time then it might be time to start ramping!

Maybe if we could steepen that ramp up a bit, the chart might roll over from T+168 to T+0? Seriously, it's hard to imagine a better setup for classic "sunshine and snow showers" conditions for this part of the UK.

Looking at Neil Bradshaw's site, the South Shields area must be quite slushy/icy because the temperature crept above 3C around 5-7pm- presumably due to a temporary easterly component to the wind that brought enhanced warming at the coast (this may also have caused the readings of 3C at Boulmer). Meanwhile it was -0.5C here last time I checked, but no snow on the ground. Tomorrow's event looks interesting but I haven't had more than a slight dusting from fronts so far, so my expectations are quite low. If anything like tonight's ECMWF outputs from T+144 onwards get to within T+72 then my expectations will be very different, however!

I'm refusing to get too excited about the "easterly potential" just yet, because the critical point of the evolution happens between T+96 and T+144- there is still room for some movement at that sort of range and we could still see a tone-down to a relatively dry and cloudy type of south-easterly, of the sort that some runs were showing a couple of days ago. The UKMO T+144 output is rather different to the GFS and ECMWF and shows lows engaging the cold air from the SW, although it is rather out on its own with this and even that evolution could potentially be snowy, as we keep pretty strong blocking to the north and potential for battleground snowfalls. Despite the above words of caution, there's no doubt that the model outputs have shifted strongly in favour of an east to north-easterly type setting in after T+96. Most of us would then be looking at a few days of sunshine and snow showers, perhaps a wintry mix near east-facing coastal fringes but cold enough for lying snow elsewhere, and only sheltered western parts largely missing out. In such airflows there is also the chance of organised snowfalls via troughs embedded in the airflow, and frontal systems pushing into southern areas associated with southerly tracking lows. If those outputs get to within T+72 then I might be tempted to start ramping in the north of England regional threads. Before that, we have some serious precipitation coming up tomorrow and into Friday which may well aggravate the flooding problems. Parts of southern Scotland and the north and east of England may also get a marginal snow event out of this with some significant snowfall for a time in some areas, though it will generally turn to rain at low levels. The second depression at around T+96 won't generate much rain but it will almost certainly be followed by the arrival of continental air- what type of continental flow remains less than certain but as noted above the majority of model outputs are suggesting a snowy east to north-easterly type.

If the GFS and ECMWF "easterly" charts come off then I think falling and lying snow to low levels are highly probable upwards of 5-10 miles inland from the east coast, but I agree that areas near the coast would be pretty marginal. During my years at Cleadon an easterly or north-easterly with 850hPa temperatures around -7C would generally bring a barrage of hail and sleet showers off the North Sea at this time of year, with temperatures of around 4C in the clear intervals and falling to around 2C in the showers, while Durham would generally have falling and lying snow. I think the last three frames of the ECMWF operational run could well be conducive to lying snow right out to the east coast though as the 850hPa temps drop to near -10C. Thus I find those synoptics rather more exciting from the perspective of my current location than I would if I was still living in Cleadon, though even if I was in Cleadon I would be excited at the possibility of some significant wintry convection coming in off the North Sea. I'm not allowing myself to get too excited just yet though as the north-easterlies are only just coming into the semi-reliable timeframe and there is still time for them to be replaced with relatively dry cloudy south-easterlies, albeit with recent runs having trended away from those.

I'm pretty sure that it's on the grass, as most Met Office sites are in open fields.

As expected, a light snow flurry with the temp at 1.9C. In all fairness, though, I hadn't expected the snow showers to penetrate more than ten miles inland, so I've had more than I expected, even though it's somewhat less than my old home region of Tyne and Wear has had. For significant snowfalls in this area of North Yorkshire it seems that you need the cloud movement to be vectored from somewhere east of due north, though not as much of an easterly component as I had originally suspected before moving to the area.

The big snow shower over Newcastle and Durham is heading straight for Sandhutton but losing a lot of intensity- I may get a light to moderate flurry from it but it doesn't look promising here for much in the way of snow cover.

Snow can lie no matter how wet the ground is, as long as it is heavy enough and the temperature and humidity are low enough. However, wet ground makes snow less likely to settle in situations with marginal temperatures/humidity and light snowfall- this is true of non-frozen ground also.

A brief shower penetrated to Sandhutton at last- started off as light snow and then turned to moderate graupel. Not heavy enough to cover the ground, but enough to make it a day of snow (and graupel) falling. I'm not confident about getting much tomorrow as the last few frontal events have given no more than a slight dusting here, but you never know, as I remember Yorkshire (especially West Yorkshire) getting sizeable falls from fronts in the past (25th January 1995 and 31st December 2003 spring to mind).

I'm surprised at how widely snow is accumulating around South Shields despite temperatures above 1C- it must be to do with the intensity of the snowfall. Nothing here at Sandhutton, some sleety stuff overnight and snow showers now passing just to the east of here. Days like this were to be expected with having moved further inland, although if we get anything similar to the post T+168 outputs from the GFS and ECMWF then we may well see the other side of the "coastal vs inland" coin with snow falling and lying readily here and struggling to lie for long at South Shields. It's still a big "if" at this stage though which is why I'm refusing to get too excited.

The main issue tomorrow will be the wind direction, from the N/NNW, which will tend to restrict showers to areas close to the east coast when the coldest air arrives. Thus, I'm not expecting snow showers as far inland as Sandhutton tomorrow (though I have been proved wrong with that before, most notably on the 26th/27th October), but South Shields for instance is certainly in with a good chance. According to Neil Bradshaw's site a NE wind has picked up at South Shields which will hold the temperature up for a while, but the warming effects of the sea at the coast will diminish when the wind backs northerly.

Showers are making it as far as Sandhutton but just rain, with a temp of 3.8C.

The earlier "easterly" runs were suggesting an east to north-easterly type at 7+ days out with sunshine and snow showers (perhaps marginal near the east coast) but this morning's runs have shown a clear step towards a dry cloudy south-easterly scenario. It is often fiendishly difficult to get the Scandinavian blocking high to ridge far enough west, with enough low pressure development to the south, to give us an E'ly rather than SE'ly. For this reason I am a much bigger fan of Greenland blocking than Scandinavian blocking- when the winds are from the NW/N/NE, at least if it isn't snowy, chances are it will be sunny and dry rather than dull and dry. Of course nothing is finalised yet- we do still have the ECMWF operational run sending in an ENE flow- but the trend is notable.

The GFS 18Z is overdoing the Atlantic somewhat in my opinion, particularly in the low-res part of the run, but I remain cautious about this easterly potential as it's far from impossible that we could end up with low pressure stalling out to the west and the UK under a moderately cold, dry cloudy type of south-easterly flow with the Scandinavian high too far east to send the sub -6C 850hPa airmasses as far south and west as the British Isles. I've seen that happen many times in the past, although for now that scenario has only very limited support with the CPC charts suggesting an east to north-easterly type with blocking extending from Scandinavia to Iceland.

As a general rule, in an easterly or north-easterly type in Scotland or northern England, I think that the 850hPa temperature ideally needs to be -8C or below to support lying snow within a few miles of the east coast, and -6C or below inland. Most runs have been suggesting 850hPa temperatures between -6 and -8C, hence my assessment of marginal near the coast but snowy inland. This is also supported by the temperature projections on the GFS runs that have gone for an easterly or north-easterly type. However, there is certainly potential for colder airmasses to come in off the continent. For south-east England the shorter track over the North Sea means that the airmass doesn't have to be as cold at upper levels to support snow, but on the other hand convection over the North Sea is less guaranteed there.

According to Weather Log the 1961-90 average maximum temperature for Stockholm in January was indeed -0.7C, although the 1981-2010 average is bound to be at least a degree higher than that.

Indeed- between T+168 and T+192 we see a build of pressure over Greenland and Scandinavia which would render a subsequent evolution to an easterly type quite plausible. The ECMWF 12Z run at around T+120-168 is quite reminiscent of tonight's UKMO at T+144, which on the face of it looked a bad chart for cold/snow lovers- the key to the evolution is the way that the advancing Atlantic low stalls against the rising heights to the NE and ends up filling and drifting south-eastwards. I often use trends in the ECMWF ensemble mean for general guidance and tonight is the first time that I've seen a pronounced east to north-easterly flow showing on the ensemble mean. The one caveat with easterly types is that if the Scandinavian high sinks southwards we could end up drawing in a dry cloudy SE'ly type instead, but so far the longer-term signals have been pretty consistent in suggesting an east to north-easterly type which would be highly likely to bring wintry showers (snow inland, marginal near the east coast) well inland off the North Sea. With it being so far out, though, I'm still stopping short of calling it nailed-on- in particular the trigger low next weekend will be absolutely crucial.

The UKMO wasn't too bad regarding the cold snap over the weekend- it was too progressive pulling the trough down the North Sea and thus introduced a north-easterly that never happened, but the general pattern was pretty close. I can see why the UKMO charts are concerning to a lot of people as from where we get to at T+144, there is a real possibility of the Atlantic low to the west dragging up warm southerlies and leaving us on the warm side of the blocking with high pressure stuck out over eastern Scandinavia, but I'll class that chart as unlikely to come off unless it gets significant support from the ECMWF or its ensembles. After all, it would only take a slight shift from the UKMO T+144 chart to get to where the latest GFS and ECMWF runs lead us. I'm not so sure about that- I've had many experiences in the past of Scandinavian highs sinking southwards and drawing up modified south-easterlies bringing mainly dry cloudy weather. A stark example of this happened in early to mid December 2002. However, the post T+180 area of the GFS run, as it stands, would certainly give widespread snowfall, possibly penetrating to some western areas at times. In the near term, there could be some wintriness tomorrow associated with a weak frontal system that is set to drift southwards, followed by a northerly, rather more potent in terms of low temperatures than last weekend's one, which could mean a general snow cover for eastern Scotland and NE England within 5-10 miles of the coast, perhaps sleety near north-facing coastal fringes.

I rather like this article which explains various factors that contribute to the likelihood of snowfall, in particular examining the relationship with humidity: http://www.sciencebi...owAboveFreezing One broad rule of thumb is that if the average of the air temperature and the dew point is close to or below zero, it will generally snow, while if the dew point is above zero then snow is unlikely. For lying snow (not mentioned in the article) snow will generally settle if the air temperature is less than a degree above freezing, although if the snow is particularly heavy and/or humidity is low, it may accumulate at higher temperatures. However, there are certain cases where the analysis in that article doesn't quite work, and temperature inversions are one of them. If it is 0.5C at the surface, but 2.5C a couple of hundred metres higher up, then you may well end up with rain because the snow ends up melting as it passes through the warmer layer, before it reaches the cold air at the surface. This is a coommon issue in frontal situations where precipitation zones bump into cold stable air and relatively warm moist air rises over the top, and it was particularly prominent with the frontal event last Friday night. Also, if there is a particularly sharp drop in air temperature with height (most common during sunshine-and-showers setups in spring), snow can fall at higher temperatures than the aforementioned article suggests because the snow only has to pass through a thin layer of warmth on its way to the surface, and so you can get wet sleety snow at the surface despite an air temperature of 6 to 8C and relative humidity around 50%. As is usually the case with meteorology, it's a pretty complicated picture.

The forecast ran as follows: [quote]Changeable, near-average temperatures November 2012 will be a changeable month with low pressure set to control the weather for most of the time. The long-range signals point to near-average temperatures and no significant cold spells during the first two-thirds of the month at least. Low pressure centred over northern Scotland will bring a rather cool and showery theme during the first five days of the month, with the majority of the showers in western areas, but some will penetrate into eastern areas too, particularly over north-east England. On the 4th and 5th a secondary low will track over southern England and will give a spell of cloudy weather and prolonged rain from the Midlands southwards with snow on high ground. Further north, a mix of sun and showers will continue but as the wind switches around to a northerly or north-easterly direction, showers on the 5th will mostly be concentrated in eastern areas bordering the North Sea. Between the 6th and 8th the weather is expected to turn milder and cloudier with a moist west to north-westerly airflow in occupation, and there will be some persistent rain over Scotland, Ireland, Wales and northern and western England on the 6th and 7th although south-eastern England will probably stay dry. Temperatures will generally reach highs of 10 to 13C. The 8th/9th will see low pressure become established again to the north and north-west of the country, so a band of rain will sweep south-eastwards across the country, followed by brighter, showery weather. Towards mid-November we can expect a generally showery regime with low pressure sat to the north and north-west of the British Isles. Most of the showers will be concentrated in western areas, while there is also the possibility of some more organised belts of rain at times, particularly over southern and central parts of England. Temperatures will mostly be close to the seasonal average. After midmonth a milder, unsettled regime will take over with more frequent frontal systems and some inputs of warm tropical air, and it will become generally cloudy and wet, particularly in north-western Britain. The last third of November is likely to feature a colder, quieter interlude, most likely during the last week of the month, which may feature some snow showers for northern and eastern counties for a time although confidence on this is low. Broadly speaking mild and unsettled weather around the 20th will give way to high pressure heading in from the west. I am not expecting any exceptional cold snowy weather like most of us saw at the end of November 2010- just an ordinary late-November cold snap. Overview Temperatures during November 2012 will generally be within a degree of the 1981-2010 average, with southern England up to a degree warmer than average and northern Scotland up to a degree colder. A Central England Temperature of around 7.1C is expected. Due to the changeable nature of the weather it will generally be another wet month with rainfall excesses of 30-70% in most regions, and western and southern England plus much of Wales will generally be wettest relative to normal. Eastern Scotland will be the main exception, where rainfall will be near or slightly below average. Sunshine totals will be well above average over much of eastern Scotland and north-east England with excesses of 20-50%, but sunshine will be near or below average elsewhere, with south Wales and south-west England probably dullest relative to the long-term average (shortages of 20-30% are possible). [/quote] After a couple of forecasts which were rather wide of the mark, I thought this one went quite well, particularly during the first and last weeks of the month. The showery north-westerly regime that I had predicted leading up to midmonth lasted just a couple of days though, and instead high pressure was the dominant theme around midmonth, though a rather cloudy high for most of us. I managed to pick out the colder interlude towards the end of the month with high pressure advancing from the west, though not the exceptional rainfall events which led to widespread flooding. Statistically sunshine, temperature and rainfall anomalies were generally similar to what I predicted, although I won't lie- I got a bit lucky with the rainfall stats as the unsettled periods of the 1st-20th were generally less wet than I had envisaged, while the last third of the month was wetter, and the two cancelled each other out resulting in my monthly rainfall projections being near the mark.

Snow melted here after 2am, so nothing on the ground- currently sunny and 1.9C after a foggy start. Temperatures at altitude must have been quite similar to those at the surface for although there were no problems with freezing rain here, it did rain for a while with the temperature below 1C, which would be rare in a showery setup.