Nick F

At 9 days out, I would no more trust a chart showing snow than an email in my spam box wanting to pay me money if I do a small favour transferring money into my bank account then back out again but, some organised snow coming down the country on Tues 5th? I'm sure the models will try and ruin our dreams at some point subsequent runs by throwing in some 'pesky shortwaves' from the NW to slow the progress south of the deep cold ... surely?

That's both ECM and GFS ops suggesting a Nly reload around Mon 4th Dec. Hopefully it will tap into some deeper cold, as indicated, than we see this week, thanks to less WAA into Greenland and the arctic area around it prior ...

The jet riding over the top of the mid-N Atlantic block is like a wriggling snake on the 18z. It flattens out next weekend to allow less cold air originating from the NW Atlantic to move toward the UK before the jet buckles north again to allow a northerly to reload to end the weekend and start the following week! No matter how the upper flow tries to supress the block it just bounces back north even stronger! The block is there in that region for the long-run I think ...

certainly not run-of-the-mill standard fare for late November. My interest is not so much in this coming week's weather, which looks cold but mostly dry away from coastal areas. I'm more fascinated in watching the bigger picture unfold for the end of November and ahead for the first few weeks of December. Having seen the extended EPS for the last several runs, blocking looks to remain in place near Greenland for a protracted period, of 10-15 days at least lasting well into early December. That is a big plus with regards to keeping the flow across the UK from a cold direction. Upstream, troughing stretching from Siberia to Alaska above ridging over SE Asia will continue to drive a strong east Pacific jet which extends its strong zonal flow energy over N America, keeping cold air bottled up over N Canada and driving a strong jet over the top of the Greenland block. This energy tending to perhaps flatten the N Atlantic/ S greenland ridge at times to allow some troughing to extend SE towards the UK, bringing surface lows with it. This pattern upstream looks to continue for the next 10 days at least. However, in early December, a pattern change looks to take place upstream over the N Pacific, perhaps in the second week, where the strong east Asian/East Pacific looks to weaken, which will allow a long-wave ridge to form along or off the west coast of N America. We then end of with a three-prong ridge attack towards the pole in the trop = NE Pac/Greenland/W Russia ... ... which will surely put pressure through wave breaking on the polar vortex, certainly potential for cross polar ridging which maybe why the AO forecasts indices are nose-diving in early December. All this while the MJO is forecast to remain weak, with little influence on the northern hemispheric patterns for the foreseeable.

That’s a sea level pressure chart, rather than heights, often the pressure seems extremely high over Greenland, but this is this the model trying to calculate the pressure over the high plateau covered in ice that dominates Greenland rather than representative of sea level pressure. Chart below shows similarly high pressure at the surface over Greenland, while above are low heights (blue) indicative of upper troughing.

Wouldn’t be too despondent over the 12z and 18z GFS op desire to introduce less cold air from the NW and W. The 12z EPS Clusters don’t inspire all out confidence for the idea of the high res runs this evening to lose the ridge influence to the north and bring in troughing from the NW and back the flow to a NWly later next week and weekend. 5 clusters, which indicate we could go down this path or we could see Atlantic ridging build strongly enough NE to block troughing extending down from the NW. No clear signal yet, albeit we are entering a blocky pattern and the models are always keen, often too keen, to break blocks down.

That part of Norway is high elevation / mountains just inland, hence low temperatures shown which aren’t reflective of sea-level. T850s don’t always truly reflect the surface temperatures by rule of thumb of course, though the air on arrival in UK after crossing over the North Sea at the time of your chart will be modified somewhat compared to when it left Scandinavia.

Only just caught up on today's model output, as been busy, but looking at the 12z/18Z GFS they are certainly an improvement on the 00z and 06z GFS. To my eye this because they are evolving a more stout block over Iceland and into Greenland with help of the arctic high. The 00z/06z GFS highlighted my reservations on relying on a mid-Atlantic ridge creating a block while most of the jet energy is going round the top of the ridge, as this inevitably leads to the ridge sinking with little energy/troughing undercutting the block. But both 12z/18z GFS along with 12z ECM high res show the merging of the arctic +ve heights/ridge and the Atlantic ridge building a block over Greenland that prevents the toppling from the NW that GFS was previously indicating. Though the 18z does eventually implode from the west in FI. Some mouthwatering synoptics continually on offer over recent days, especially ECM, but I think we should temper expectations a little, as on face value, the charts are suggesting cold and below average next week but nothing exceptional and certainly no snowmaggedon for many. The cold generally doesn't look too deep on current projections so there'll be snow around for sure, but mostly settling over the hills during the day, maybe lower levels at night, for a time. Where skies clear it will be frosty at night and cold sunshine by day, add in a wind chill from the cold wind, it will certainly feel more seasonal than the 16C I seeing on my thermometer at 11pm last night! We could be entrenched in this cold and blocky pattern for a while given the probable stubborness of the block to our W and NW and that the models, especially GFS, are often too keen to break it down. Which takes into early December. Longer term, some of the MJO forecasts hint at it orbiting, albeit low amplitude for now, towards colder phases 7/8 in December, which combined with possible still weak strat PV via weakening zonal wind forecasted, increased wave 1 activity and warming in the strat could help more blocking in December. So interesting times ahead!

Strong signal from the 00z EPS 500mb mean/anomaly for ridging NE toward Svalbard and Norway by day 10, the extended EPS to day 15 maintaining ridging signal in this area, while deeply -ve anomaly is maintained over mainland Europe with hints of slight -ve anomaly over N Atlantic undercutting +ve anomaly over Greenland and Svalbard. Last night's CPC update to its 6-10 day and 8-14 day 500mb heights/anomalies back up this idea too. So it will be interesting to see if GFS has a change of heart and goes with stronger ridging in the Atlantic rather than the toppling from the NW.

This the downside of not having a proper Greenland block in place next week, but relying on stout Atlantic ridging north instead. The GFS is trying to push the trop Polar Vortex over Canada further east toward Greenland while strengthening in the medium range - which puts pressure on the N Atlantic ridge to the west to flatten out. Certainly the Atlantic ridge slackens in the extended EPS, but still with a Euro trough anomaly, so could be we revert back to NWly flow eventually after another arctic reload next week. The caveat is that the models may not be handling the blocking over the Atlantic and PV over Canada too well, so we may end up seeing more protracted blocking. Indeed 06z GFS FI teases us with further amplification upstream, with ridge building back over the top of troughing sliding SE into Europe and the UK gets an Ely.

Yes, that upper shortwave/secondary low ejecting east over the N Atlantic from the upper trough over northern Canada late weekend and early next week could throw a spanner in the works with regards to how quickly it clears SE - which could scupper a second arctic reload next week after the brief cold NWly blast this weekend. The EPS clusters quite widely spread over how quickly this shortwave trough clears through, some hold the trough back far enough west and long enough to prevent any deep cold from getting far south, though generally the guidance suggests it will get SE eventually to allow a reload form the north. Thankfully no dramas with regards to this feature from 06z GFS, which doesn't hold it back west. If we can be confident the shortwave trough clears SE quickly as per GFS and ECM, more runs needed, we then need the block out west to hold to allow deep cold over northern Norway and Svalbard to get drawn this way next week. As although the synoptic evolution is working with help of blocking to the west, we are not really seeing any deep cold yet in the reliable timeframes. Though that's not to say we can't see some snow maybe overnight at lower levels, certainly high ground in the north and west is looking favourable from Friday.

Clear there that the ensemble mean supports the high res run on the 00z run, be interesting if the trend continues, because although we expect the operational runs to chop and change, even the ensemble guidance has made some seismic shifts over the past week in the extended range. We've gone from a Euro high signal in the medium range to a Euro trough in the space of a few days' runs. Seems that the models may be less reliable when the polar vortex is not driving the show, but high latitude blocking and trop waves are. Though there is the chance that the PV will organise itself eventually as we head into early met winter. Interesting that yesterday's zonal wind forecasts on the Freie Universitat Berlin site are showing a downward slide in 10-30hPa zonal winds after they were showing an uptick - which suggests the strat PV may not be in such a hurry to re-organise and strengthen for now? The MJO doesn't really seem to be a key player in the troposphere patterns evolving, as it has been in COD since the beginning of the month and looks to remain in COD for a bit or perhaps emerging in low amplitude in various phases according to which model you look at. Even the strat looks increasingly amplified on Berlin at day 10: Certainly plenty for coldies to be optimistic about looking at the signals coming from various medium range pointers ... for now.

As John mentioned earlier, the models firming up on a meridional pattern developing downstream over the Atlantic and northern Europe by the end of the week, as a deep trough digs down over eastern N America, a ridge pushes north to Greenland while a deep trough digs south over Europe. Longer term, the 00z EPS extended to day 15 and the EPS clusters suggest -ve height anomaly / troughing over mainland Europe while +ve heights remain to the W and NW - which always a good sign. However, we've seen a lot of chopping and changing over recent days from the ensemble guidance, so I would personally want to see more runs to firm up on this signal for potential for pressure rises to the N and NE above the Euro trough leading to a flow E of N eventually. Although the models are agreeing on the broadscale 500mb pattern, there are some difference from the morning's operational / high res runs with regards to the surface. Both 00z UKMO and GFS high res runs had a low in the west and a low to the NE Thursday night / early Friday, GFS has low to the W of Scotland while UKMO a low to SW of Ireland. Both delay the onset of cold artic flow expected to sweep south this weekend, by this secondary low that develops along the cold front, while the ECMWF high res hasn't got this low to the west and brings in colder air quicker to end the week. However 06z GFS seems to remove this deep low near the west the 00z had early Friday! 00z and 06z GFS for 00z Friday - spot the difference easily west of Scotland! 00z 06z This differences so early on at the surface, with regards to secondary low development, seem to be why the BBC may be a little uncertain with regards to the potential for cold air sweeping south to end the week and through the weekend and its extent and coldness. UKMO has this secondary low (note I don't like using the word 'shortwave') running across southern areas Saturday - delaying the cold - GFS and ECMWF don't have it though. GFS and ECMWF bring another secondary low SE from Greenland / Iceland area early next week, differences in track this far out, which could reload a N or NWly flow from the arctic by Tuesday. Thereafter potential for pressure rises to the north which may allow a more NEly flow to develop - big uncertainties as early as this weekend with regards to arctic blast, so although interesting and nice to see the potential next week, it maybe worth not looking too far ahead for now!

Looks like it's turning into a horror show the 06z GFS operational run, a protracted run of long-draw mild southwesterlies and the dreaded +ve heights across mainland Europe. A glance at the extended 00z EPS 500mb mean/anomaly out day 15 doesn't offer much joy either, with low heights extending south toward the Azores which teleconnects to ridging to our south over mainland Europe right through to D15. Really need to see blues across mainland Europe, not oranges. The only positives I can draw from the morning's outputs is the medium-longer term signal for cross polar +ve heights to continue. Only problem is, for now, is the ridging is too farr away from our part of the northern hemisphere to have much influence. For now, eastern N America and eastern Siberia/Far East will be the main benefactor in terms of deep cold digging way south thanks to high latitude ridging.

Convective snow showers off the North Sea with thunder snow later next week anyone? Let the 18z GFS tease you ...

After several days of poor 18z runs, tonight’s is polar opposite with a cold NEly, only the GFS could make it up! Perhaps too early to throw the towel in on this particular high latitude blocking episode, but equally lots of caution required before getting hopes up for such an evolution, as a lot of complicated jigsaw pieces to fit to get to such a position as the 18z.

Models making the synoptic evolution past t+144 rather complicated as they try to handle a split jet stream over the N Atlantic going around the block next week and how much energy goes into the southern arm and how much into the northern arm. Impressive Greenland block and cold air trying it's hardest to push south across the UK early next week, areas of low pressure spinning up from the SW beneath the block preventing the cold air getting all the way south in a confident manner, though the 00z EC det. does briefly at day 10 - albeit can't trust that would come off at that range. Ultimately, I think the high latitude blocking will be situated too far north for all the UK to tap into the cold air to the north, however, could be some interesting battleground snow events playing out across Scotland next week?

If it was bad enough that Monday morning is looming, the 18z GFS is dishing up the horrors of wet and windy fair from the west to send us to bed on a low! Doesn’t look like it can redeem itself either, hopefully the EC will stick to its guns in the morning.

The 8-14 day 500mb anomalies doesn’t show a NEly though, the coloured lines are anomalies or the deviation (+/-) in heights to the average. Then green lines are more representative of the mean flow in 8-14 day range. Still on track for some blocking over N Atlantic and Greenland area in the medium range as per ensemble and operational guidance. Longer term, the AO shows hints of recovery towards neutral indices on the chart Allseasons posted and the EPS AO and NAO forecasts likewise return to neutral indices towards the end of the month after a dip in neutral over coming 7-10 days. However, these forecasts have been chopping and changing, though there is potential for the trop PV to strengthen as we head towards late month and early Dec, the reason this could well happen is a continued forecast for an upward spike in zonal winds at 10hPa which indicates a strengthening of the stratospheric PV toward the top which would ultimately work down towards the troposphere. http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html but in the meantime, northern hemispheric flow over N Pacific, N America, N Atlantic and Europe looks like becoming rather convoluted and blocky ... so some interesting Synoptics evolving in the days ahead. Whether they produce what many are looking for, i.e. deep cold and snow, seems elusive for now, so not seeing another late Dec 2010 for now.

There is always the danger that high latitude blocks will be undercut by troughing which keeps the real cold at bay from the UK, yes. Certainly it’s more unusual in November for blocks to sustain a cold pattern across the UK for long, than say in Jan or Feb when there is a greater chance because of SSWs propagating down. Exception was late November 2010 of course, though don’t think we are heading that route for now, but I still think there’s some merit in a blocked N Atlantic that will keep the UK locked into a below average spell of temps, until ECM decides to move away from this signal as well.

Too early to declare a zonal rest of month and early Dec based on one GFS op run, a look at the 12z EC ensembles suggests N Atlantic blocking could be a strong possibility. There is still the chance that the trop PV will begin to organise as we head into early winter as the strat cools from top to bottom, as hinted by strengthening of zonal winds in EC forecasts on Berlin site, but for now the trop blockiness and weakish and amplified jet seems to be in control for next few weeks IMO. Below average temps more likely for a fair duration after this coming week’s milder blip, though less certainty of wintry precipitation away from the northern hills.

Yes, 18z GFS with the much flatter, less amplified flow over the Atlantic, with no wave breaking of the trough over eastern Canada to help amplify the flow downstream over the N Atlantic. Still, teleconnections still seem to favour a blocked N Atlantic thanks to the weakened and convoluted jet stream over North America caused by the anomalously strong Aleutian high. 12z EPS clustering at t+240 gives strong support for mid N Atlantic blocking. So plenty to be optimistic about, despite a possibly wayward 18z GFS ... which I think is too quick to rid of the mega Aleutian block and replace it with troughing which then opens up a zonal and stronger jet to take over downstream and prevents wave breaking and blocking over N Atlantic.

Next weekend could be the fourth weekend running that we have a northerly, seems reasonable ensemble support and high res consistency for it too. Quite an usual feat in recent years ... though never really potent looking for now we have to rely on it as our coldest source at the moment until out east gets colder.

Said wave appears to be the remnants of now Tropical Storm Rina currently over the W Atlantic before swinging round the north side of the Azores before dropping SE across Ireland and UK Friday night/early Saturday. EC tracks it further south across SW UK and weakens it quickly. Could bring a lot of rainfall Friday night though, especially Ireland. Chart credit: wxcharts.eu: Looking at the rest of the 06z GFS op coming out, the well-advertised brief N to NWly developing late weekend could bring some snow to the Scottish mountains and tops of northern hills of England. The 500mb vortex over Greenland and Iceland re-organising and the zonal jet returning early next week to turn the flow westerly again, but still no signs of sustained re-organisation of the trop PV - it does appear that part of it will break away southeastwards into Northern Europe, while the bulk of the trop PV retreats over northern Canada - which opens up possibilities of high latitude height rises. Still plenty of promise longer term from the morning runs.

Certainly continues to be encouraging signals from the models for colder blocking patterns to develop mid-late month, with no real signs of the Polar Vortex organising itself yet, a sprawling Siberian High and +ve heights over the Pole too. Early days, but the AO and NAO forecasts look similar from 00z/06z GEFS and 00z EPS with a downward / increasingly negative trend for both indices.