Met4Cast

I think the hope of wedges remaining to the north with colder air across the UK beyond the 20th can be discounted now, this is a very strong signal from the EPS for a return to milder, wetter weather. The Canadian PV lobe being pulled towards Siberia (likely thanks to the recent warming) is the dominating signal and will override any potential blocking. Late January into early February however does still hold potential for a return to more blocked conditions with this next momentum surge. Before all of that though - A week of cold weather across the whole of the UK with the potential for snowfall just about anywhere.

If we look at the GFS strat forecasts I think we can find an answer to where this zonal push within the modelling is coming from. The Canadian lobe of the sPV appears to get sucked towards Siberia by the stronger SIberian lobe, this occurs 17th-20th, around the time we're seeing modelling flatten the Atlantic pattern out and return the tropical jet northwards. This could perhaps be a response to the minor SSW we've recently seen, perhaps a case of the worst timing ever? (I did muse that a SSW minor or otherwise could potentially disrupt any cold spell a while back). Once that transition has occurred & based on the current background forcing I see no reason why we wont see high pressure beginning to re-establish itself in a more favourable place for further cold weather outbreaks, Tamara has outlined this far better than I can in a few of her recent posts on this thread.

Seems unlikely now. The vast majority of the EPS, MOGREPS and GEFS keep this system south and the envelope is shrinking rather quickly. I think for us in the south we need to start looking for smaller disturbances within the flow coming down from the north next week.

Not quite..

Because of a single run? I think we need a little perspective here. We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. We go into next week with much colder air moving south across the UK with increasing snow risks. Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5009289

Because of a single run? I think we need a little perspective here. We currently have below average/cold conditions across the UK and by the end of next week we would have had (apart from a couple of days) 2 weeks of colder, blocked conditions. Many southern areas saw snow (albeit not a lot) last Monday. We go into next week with much colder air moving south across the UK with increasing snow risks. Background forcing has done it’s job, the broadscale pattern is as advertised & expected, high latitude blocking is becoming established. The UK is a small island though so despite everything being favourable, blocking & cold conditions in place snow can still be elusive, but I don’t think it will be. Next week looks very unstable and open to small-scale features developing and bringing widespread snow risks, these may not be picked up until 12hrs before they arrive.

Although worth noting the increase in ridging on the GFS, it's certainly a step towards the UKMO even it it hasn't quite managed to go the whole 10 yards.

Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards. What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling. I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens. No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008741

Sounds like what you’re expecting is something similar to the 00z GEM, i.e a more active jet into the Atlantic meeting residual wedges of high pressure to the north causing the jet & subsequent low pressure systems to disrupt SEwards. What you advertise is appearing within model outputs albeit as a lower probability outcome currently within the modelling. I’m not sure models will be handling this particularly well at extended ranges, resolving split flows & disrupting energy is something modelling always struggles to handle so what initially looks like a flat, zonal-esq pattern often corrects as the lead time shortens. No doubt that less cold air will push in as blocking wanes, but that doesn’t necessarily mean a mild zonal onslaught is on the way.

There was no blocking at day 5 either on a few recent GFS runs.. that's sortof my point. We've just seen a huge shift in the broadscale pattern from the 18z, there's no saying that wont happen again as we approach the current day 10. Wedges are extremely difficult for models to resolve and get a handle on.

If the models aren't getting blocking right at days 3/4, why do people expect them to be right at days 9/10?

I meant I can't work out why the Met would have the system tracking south as the lowest probability.

The GEFS mean is also an improvement. Not quite as large a shift as the det but certainly a shift.

Unfortunately since then it's continued to track southwards across all modelling & ensemble suites. the Met said it was a lower probability (still cannot work out why, these systems generally do trend south) but todays model trends have placed that in the "highest probability" risk.

This is a substantial shift in modelling from the 18z so I'd very much like to wait until the ensembles before judging but this extends the blocking far further than the last few runs have managed to. In terms of snow detail, honestly couldn't care less at this stage, that will chop, change & new features will pop up seemingly from no where. The low in the middle of next week was always expected to go into France and miss the UK, been saying this for days now.

Erm.. Okay GFS 18z.. wow?

The GFS det whilst not technically an outlier is completely out of wack with the vast majority of the GEFS.. as per usual really. Many of the ensembles opt for a more southerly tracking low next week & keeps the colder air on place across the south.

UKMO very very good and probably the hallmark for what we want to look for. GFS is an improvement but looks like it's turning rather messy now. Colder 850hPa's continue to shift southwards.

Not for me it hasn’t..

Mentioning 1947 is going to cause a riot..

The 20th-25th January still looks like the broader period for less cold weather to begin moving in across the UK, albeit with a lot of uncertainty. This in itself could bring significant snowfall risks but before that, cold next week with the risk of snow for some. Ensemble spread for the above mentioned period is large with some members keeping the cold until later in that period, it may take a few attempts but there's certainly a signal there now for less cold air to push into the UK as blocking to the NW begins to relax.

I'm not expecting a change the Met extended today but possibly tomorrow. The extended forecast is written from a longer, more detailed piece of internal guidance that is typically written on Monday & Thursday nights. If there's no change today it's tomorrows update that could be an important one & begin to suggest a change in the outlook. Currently though, things are progressing largely as expected. Anything beyond next week remains open to doubt & significant changes within outputs, only takes a small wedge to completely change expectations across the UK.

Ha, yes I appreciate it's not the cleanest of charts to read. Essentially each red line shows where an ECM ensemble member has the warm front.

Here's yesterdays 12z run and todays 00z run.

The low goes south, the south stays cold but we miss out on a potentially significant snowfall event. The 00z spaghetti frontal plot shows a substantial shift south on the last two runs but with still a large amount of spread across France & the English channel, the exact track of this low still hasn't been resolved within the modelling. Going forward I suspect models are being too progressive with removing the cold. Wedges will be key for cold remaining in situ across the UK and there's certainly some support within the ensembles for cold weather to hang on, more so across northern counties. A very difficult and volatile forecasting period coming up. Don't be surprised to see modelling little by little extending the cold..