Geordiesnow

That shortwave has always been there, maybe it was not picked up on the 00Z run hence the clean northerly. Apart from a couple of the minor models hinting at the clean northerly flow, a less cold sector was always predicted by the other models including UKMO. It's a reason why the initial northerly collapses, always forecast as far I have seen.

Maybe 24 hours I would say, all depends how fast any reload occurs. It's why we needed the initial northerly to hit as far west as possible to give more margin for error but sadly it was further east on this run so the milder sector was more prevalent. Give it another 24 hours to fully confirm it I would say either way, Tuesday is still another 4 days away and in terms of detail that is a long time.

Yeah UKMO has dropped it's idea of a clean northerly all the way through, think a couple of the minor models did hint at that but it was always the least likely. Let's just hope if there is a reload the air will be cold enough and that will all depends on where the pieces end up. I will say from the outlook I have seen, there is a 80% chance we will see a less cold sector coming into play for Tuesday.

Only if the air is cold enough I'm afraid and clearly on that chart, it's not. Sadly the GFS 12Z run has gone the wrong way aswell so I'll suspect the milder sector will come into play more on this run.

It actually could work out like that to be fair! I think it's harsh saying negative when it's just an analysis of the runs I'm seeing. I think in all honesty I rather just stay in the cold air and see if the showers form, it removes any milder sector. If I'm living in Cumbria I be hoping the feature develops and just hope the WSWly ahead of it does not bring in too much less cold air.

Looks to me the UKMO has gone for the total clean option whilst ECM and GFS is still going for a potential feature to run in I have to say whilst that feature may give a more widespread snow risk, most runs I have suggests it will be too marginal and probably wrong side of marginal at lower levels so the UKMO being right might mean more hit and miss showers(and I might miss out being too far north) but it keeps any milder sector largely away.

Because as the initial northerly collapses we develop a WSW'ly flow which brings in less cold in from the west, the question is, how much of a WSW'ly will we see, if we only see a slight one as per the 18Z follow quickly by the cold trough from the north then the less cold sector will brief but if it develops and doesn't break down quickly by colder air from the north we risk it becoming more extensive. Quite fine margins admittedly. There's always a chance if the initial northerly keeps upgrading in terms of bringing the lower thicknesses further south and west, it means the less cold air has more colder air to battle through before it reaches us therefore making the milder sector smaller. Be interesting how it develops because slight changes could make a huge difference either way.

I think the GFS sort of has it there also but fizzles so definately one to keep an eye on. Whilst it may develop slightly less cold air around it but because it comes from a cold source and thicknesses are low, it shouldn't be marginal.

And like how we seen charts go from zonal to blocked fairly quickly, it of course can do the same in reverse but at least all that stuff is well past the medium term, it's the cold stuff first and for some some snowy weather. And yes the upgrades to the initial northerly is pleasing, makes a nice change really, hopefully any milder sector can be squeezed out before a potential reload. For Scotland from what I'm seeing, the cold arrives on Saturday and looks more or less guaranteed for at least 5 days. The window for beefy convective snow showers in northern and eastern Scotland has also improved for Sunday daytime into Monday so could be some quite deep snow here by Monday night.

I never understand the hypocrisy where people can post about an upcoming cold spell at 192-240 hours range yet when people post charts showing a potential return to Atlantic weather, it's frowned upon. Lots of weather to get through of course but you can't rule out the possibility as 2 different models have picked this up now but of course a subtle change could have impacts down the line also.

Whilst I don't think you can rule out the Atlantic coming back in(as it's hinted at by 2 different operational models), it is quite incredible just how quickly the outlook turns, quite interesting really. However for a nice change, it's cold first with another slight upgrade to the initial northerly with -12hpa hitting Scotland, really can't be underestimated in a warming world how impressive that is(of course it still has to happen but even the more conservative ECM is showing those uppers).

Must be noted in terms of short term surprises, this GFS run picks up what looks like a Cheshire streamer at 72 hours Of course maybe the air is not quite cold enough but it's quite close. I actually don't think it's a Cheshire streamer because of the wind flow so is it the cold front developing a wave? Might be gone on the next run but an example how features in the short term could pop up.

I thought it was a fair analysis tbh, what do you have an issue with? Anyways the GFS is kicking and screaming to push the instability more into the UK with the first northerly, I've been looking at the pressure numbers and compared it to the euros and the lowest pressure slightly further eastwards than the UKMO and ECM. Might be only a few 100 miles in it but of course it can lead to consequences down the line. Also to note the 18Z run is pushing the troughing further southwards quicker so coupled with again a slight upgrade on the strength of the northerly means the less cold sector is less pronounced than the 12Z run. Good run so far.

Just look at the ECM run in the 6 hour frames and the coldest air holds on for a little longer than the GFS/UKMO runs but perhaps predictably that means the PPN is not there either until later on in the day where the air is less cold and occurs at the worse time(just after the warmest part of the day) I don't think it would take much to alter things but most runs have gone for this milder sector to be quite wide. Nevertheless it's still over 100 hours away and in model run terms that enough runs for things to look different either better or worse.

Sadly that was my fear that the UKMO was a bit too mild. We really need as little of a westerly influence as possible otherwise things will be the wrong side of marginal at lower levels. Still time for things to change of course and the PPN might never arrive(hope it doesn't if it's just going to be rain or sleet). Will be interesting too see how things end up developing.

Yeah alot of subjective posts and I can see both sides to be fair but I think if people want more chance of a prolonged spell of cold then if the frontal snow is further south then the cold trough comes into play which means we could potentially stay in the cold air longer although even as the ECM shows it's not a guarantee. Still some interesting days coming up from Sunday onwards.

I'll be getting excited if I was living in Scotland especially northern and eastern Scotland but the central belt is looking tasty also at times. As others have said, there has been some upgrades to the initial northerly, it means it's colder and the area of instability is also a bit further west, okay for most it won't change the weather type but it's a start for sure. Yep it's ashame it's not a more straight line northerly edging towards a NNE'ly but it what it is and we can post all the current PPN charts as much as we like, at the end of the day it's down to the radar at the end of the day.

At least it shows the possibilities are there at least. ECM marginally better than the GFS and UKMO in terms of less mixing of the cold air but this morning's runs were somewhat better. Still the details will definately alter and I have to say models that have been going for the cleaner northerly might be right because the main OP models have definately trended that way hence the slightly colder uppers that are being forecast. For us, we just got to hope any frontal snow stays well away otherwise those systems may stay in Scotland, still all to play for.

Hmm I thought the UKMO might of been slightly better than the GFS regarding the less air cold but it looks similar too me unfortunately. The PPN might not even reach out region anyways and we could still see some short term changes. As a general rule of thumb, for snow right down at low levels without any influence from the sea, uppers have to be at least -6 imo and we see pockets of hpa of around -4 if not slightly higher. Still of course some way out so slight changes could still make a difference but we really want that less cold sector as small as possible.

GFS and UKMO you both could say are fantastic runs but for different reasons, the UKMO possibly colder but the GFS does produce a snow event for the midlands, would like those uppers a tad lower though just to be on the safe side but it's all how the models deal with that shortwave coming down from Iceland. Whilst the GFS12Z run did make it more shallower it still introduced too much less cold for my likening. Scotland goes from uppers of -12 to around -3 -4hpa in the space of 12 hours! So overall some more slight upgrades of the initial northerly in terms of the cold and still the threat even if we get some less cold air a reload is possible and if not, then a frontal snow event in the south is possible.

It did but the GFS still brings too much less cold air in aloft so it looks marginal. The UKMO on face value looks a bit better but still touch and go though. The upgrades to the initial northerly is impressive though, -12hpa(if it's right and if it happens) is nothing to be sniffed at for a northerly where the main instability is just clipping the far NE of the UK.

I must say I think the UKMO and ECM has somewhat upgraded the initial northerly too my eyes, consequence of that is the milder sector is less pronounced than previous runs and of course what happens after that on the ECM is an upgrade for sure. I like to think the UKMO is better than it looks 850 wise as that low comes in. Even the GFS looks to of upgraded the initial northerly slightly but then makes the shortwave feature more stronger in terms of wind which mixes the air out quicker. Still all to play for but a little bit more upbeat this morning compared to last night.

You can add your post to that list aswell. It's a complex outlook for next week in terms of details so of course the posts will be highly varied and subjective so I do have some sympathy for those less in the know but it's been like that every year during an upcoming cold/snowy period. I think the summery you can take from today's op runs is there is going to be a cold blast with limited snowfall followed by less cold air before a potential reload of the cold from the north and at the same time there is a chance of a frontal snow event but there is strong hints beyond the medium term the blocking signal is weakening and there is a chance of the Atlantic returning but that is still some way off and far from certain.

I can say with confidence there won't be any disturbances in that initial northerly flow. Only Northern and Eastern Scotland will see anything convective from that. Thicknesses are too high and pressure is to close to the west which will topple the northerly pretty quickly. My hope is the second surge can force it way more southwards before any threats from the SW come into play. Some runs have hinted at this including the UKMO run but others have the frontal stuff winning out which like I say I would take it but only if the models firm up the Atlantic weather is set to return.

To be fair with the initial northerly, it was only that one UKMO run that went with such a straight line northerly(hence the fax chart), the GFS in particular wasn't having anything of it. ECM a bit more half way house but has sadly fallen more in line with the GFS. The northerly was always forecast to collapse regardless what model you looked at but it seems that collapse is happening about 12 hours quicker now. The cold air does get in still though and there is just enough instability to still produce heavy snow showers for Northern and Eastern Scotland but the window looks even more brief now. It's ashame really because it still is a potent northerly but a relatively stable one for the vast majority which then gets cut off with less cold air coming in from the WNW'ly direction, if too much less cold air comes in then forget about shortwave giving snow at lower levels. Then you got the potential reload of cold from the North and then perhaps an attempted push of milder air from the south giving a frontal snow event somewhere, it probably would be better to keep the cold going for longer if that frontal stuff stayed well away but if the Atlantic is going to come back into play then let somewhere in the UK get potentially a major snow event.