Nick F

IMO has been and still looking a slim chance of snow falling away from hills of Wales, northern England and Scotland from that precip band along the cold front as it moves south, just don't think there's enough inertia in the surface flow to undercut sufficiently cold and dry air for snow to fall low down. If it moved through overnight, might be better chance at lower levels. Potential for snow to lower levels across N/E Scotland and perhaps NE England from the wintry showers following in the northerly Sunday night into Monday though.

I'm always uber wary of Atlantic troughing undercut of an Atlantic high, as it's been so rare in winters since 2010 and models, particularly ECM, are keen on the idea out at day 10 and beyond before backing off subsequently. However, with the PFJ undercutting the N Pacific block and tracking way south and strong over the U.S.A with deep cold sinking way south there, thus stronger likelihood that the PFJ energy will transfer under the block over the Atlantic than over. But not guaranteed. Hopefully the 12z suite will firm up on this idea or garden and path spring to mind.

Looking at the GEFS z500 mean/anomalies days 7-11 and 11-15, there seems little impetus to push the mean N Atlantic block SE into Europe as per some of the operational runs yesterday, thanks to cold air / low heights draining down from NE Europe into central Europe Stunning 00z EPS mean days 11-15, can't show them, but picture the troughing beginning to undercut near the Azores at day 10 on chart below spreading further east into SW Europe, with a blue band of -ve anomalies from SE of New Foundland all the way east into southern and central Europe beneath +ve HGHT anomaly over Iceland.

If anyone wanted a taste of how good the 12z EPS control was, Joe B has kindly posted a chart on twitter ... now imagine that trough undercutting the Griceland block and you get the picture

Again the GFS operational (18z) is progressive at pushing the N Atlantic block E/SE into western Europe much faster or contrary to GEFS and EPS guidance - which keep the block to the W and NW. Be interesting where tomorrow's operational rollercoaster takes us ...

Nick Sussex in the Pyrenees will be salivating over the 18z GFS FI, though he's not around recenty Overall, a fairly cold run for the UK though, bar a brief warm up between the two northerlies.

Looking at the NH profile day 10 18z GFS vs 12z EC, you can see the differences upstream with the EC having a negative tilt and MLB undercut to the 500mb trough over the eastern side of N America and far NW Atlantic, where as GFS has positive tilt and much more deep and intense 500mb vortex over Canada. GFS seems rather keen, perhaps too much, on intensifying the trop PV over Canada.

Northerly part 2 end of next week on 18z? Just need to beat down that ridge over France more though! At least GFS and EC are singing from the same hymn sheet now until then.

The cold front does seem to slow as it reaches the south coast, thanks to the frontal boundary being linked to the west with the low edging towards Iberia and also moving into high pressure over the near continent. 0Cdewpoint line does catch up the precip by early Monday.

Yes, had a rush of coldie euphoria when I checked out the 12z EPS control days 11-15, that's the sort of stuff that has forums in meltdown with a block over Iceland and Norwegian Sea and lows crossing the N Atlantic east into SW Europe., not a million miles away from the pattern of the obviously more diluted anomalies on the EPS mean 11-15 days. FWIW the EPS mean for NAO just about stays in negative territory for same period, AO mean trending neutral. Quite a remarkable persistence of the N Pacific and Atlantic ridges on EPS and GEFS, all the way out toward the 13th Jan on GEFS mean. That's proper blocking signal ...

I think there is always a danger of getting too far ahead of ourselves and seeing a not so great signal from an ENS mean of one run beyond day 10 and accepting this may be a downward spiral. Likewise I suppose one run of ENS mean that shows deep cold and snow nirvana. Comparing the EPS and GEFS 500mb mean and anomaly charts for northern hemisphere in 11-15 day period, differences are apparent, as I think as has already been touched upon. The differences I can see are that 11-15 day 00z EPS mean/anom pours deep cold across western N America initially before sloshing it east across the rest of NAM (Joe B bath tub analogy) which although initially beats the SE ridge down there and drives a southerly tracking jet across the N Atlantic, eventually it forces a ridge into SW Europe as low heights spread east across the N Atlantic from N America. On the other hand, 11-15 day GEFS mean/anom keeps the cold vortex over western N America and maintains the +ve height anomalies over far N Atlantic and Greenland with -ve anomaly over N and E Europe. So, in a nutshell, the ENS don't enspire a great deal of confidence past day 10 over the N Atlantic even in the means, though one thing they do agree on is a strong Alaskan ridge persisting through to day 15.

00z GFS in the shorter term looks like an upgrade in terms of depth of cold in the northerly NY day into Monday, with -10c T850s getting south of Devon and Cornwall, though looks dry on precip charts away from far north and east coast when it gets cold enough to snow. Longer term I wouldn't put too much faith in any solution past day 8, though GFS over last few runs seems to flatten out the upper flow and strengthen it over the N Atlantiic as the trop vortex strengthens in the arctic, but with the potential for some cold zonality. Certainly not much scope for mild conditions though.

Pretty impressed by the 12z EPS NAO mean too, first time I've seen it negative for such a protracted period ... a rarer beast in our winters nowadays though seems to be more common in months that are not normally cold in the other seasons

A look at the 12z EC London ens spread suggests the det warmer than the average of the spread by day 10, the average temp maxing at 5C from 5th to 10th Jan - which is not that mild and what appears to be a tight cluster of cold members in there in that day range. Could be a lot worse and looks slighty below norm temp wise, at least, for the SE at least for beginning of Jan

That old chestnut comes round every year! Was waiting for someone to mention that. A lot of the data, probably most of that used and simulated by NWP, is derived from satellites as far as I'm aware, also automated weather stations, radiosondes and from aeroplanes and ships play a crucial role if less in number. So can't see how there would be much less data over Christmas as satellite data makes up the majority of data.

That lack of deep cold threat from the 12z EC and now 18z GFS high res runs a product of what happens when you lose the cross polar ridging, and movement SE of the upper high, though at least the 18z GFS op does look fairly blocked in the longer range, just without the deep cold and snow potential thanks to high pressure over/near the UK. Still chance that the models past day 7 aren't handling well changes upstream, so we may see more favourable runs for deep cold tomorrow. On a more upbeat note, the EC weeklies temp anomalies for 1st and 2nd weeks of Jan look ok, cold in week 1 then average week 2

Morning all, hope everyone has enjoyed Christmas, despite being green and mild. A Boxing Day walk along the coast in the fresh sunshine beckons for me later. Catching up and reading through the MOD I see some glass half empty views yesterday of what for me looks like a very interesting pattern evolving in the New Year and my glass is certainly half full. I guess some have been led to believe the strong strat PV is going to run the show as we enter the New Year, but I agree with a post from Stewart (GP) that there is a disconnect between the tropospheric pattern and that of the stratosphere, as it’s clear to see the last few days the models entertaining the idea of anticyclonic wave breaking over the north Pacific and also the N Atlantic. GFS 330K PVU chart shows the anticyclonic wave breaking working downstream over Pac and N Atlantic. So what’s driving this pattern change? As Ian F has mentioned, the PNA (Pacific North American) is going into and staying in negative territory which may play a role in shortening and amplifying the wavelengths that will benefit us with regards getting ridging north over the Atlantic initially, but eventually perhaps to the NE. The N Pacific wave train allowing the changes upstream is probably being influenced by tropical convective wave activity over the western Pacific atm which is likely enhanced by Rossby and Kelvin waves and also an MJO wave propagating east – despite the models plotting the MJO in COD. GEFS (and also JMAN) is again pushing the MJO into phase 1 in early Jan, while EC and UKMO remains in COD, but perhaps not picking up the MJO signal properly. Like I have said previously, these models may not be filtering out the noise from other variables, such as the kelvin and rossby waves, when plotting the MJO – this is apparent from Hovmollers for vertical velocity (CHI-200) that suggest an active MJO wave. Though the active MJO in tandem with the Kelvin waves are probably driving the wave propagation over the N Pacific. The MJO in phase 1 can force the extratropical pattern, via enhanced wave breaking, to favour MLBs across N Atlantic and perhaps Scandi eventually, and it appears the GFS and GEFS in particular, are factoring this in. However, even with the muted MJO signal, ECMWF is obviously following a similar trend with regards to the amplification of the upper trop pattern – which is probably related to the Pacific wave train/ wave breaking developing downstream across N America affecting the wavelengths. The 00z ens means and anomalies are looking encouraging in supporting the recent blocked ops too, particularly the EPS at day 10 Still chance the wavelengths could position or amplify less than favourably, with the MLBs maybe positioning too far east or SE or maybe not building far enough north, but there is plenty of encouragement from the ensembles to suggest a cold outlook for January, particularly encouraged by the ENS shifting the core of the PV into Russia away from Canada and Greenland. Any snow potential too far away to focus on or discuss for now.

Storm Barbara moves on, then hot on her heels Storm Conor arrives by Boxing Day. But high rapidly pressure builds next week, bringing settled conditions. Models hinting at a cold northerly snap New Year's Day. View the full blog here

Someone asked this question the other day re: why models show a strong surface high over Greenland only to quickly vanish, my reply back then is below which hopefully explains:

Good evening I have posted in here a few times over the last few days, the models perhaps not handling well recently the pattern throughout the global tropics, with multiple competing modes of variability apparent, which could be creating noise to an active MJO and creating MJO plots in COD (Circle of death). A few days ago all models all of a sudden initialized the MJO in phase 6, despite forecasting it to stay in COD over previous days and also there is an increase in Pacific wave breaking forecast in next 7 days - hence IMO the change in direction of models in recent days. Despite the MJO plots continuing in COD, apart from GEFS/GFS and JMA, the Homvoller, OLR and Vertical Velocity (CHI-200hPa) charts are suggesting an active MJO wave over the western Pacific. This enhanced convective / upper divergence signal shifting through this location shows well on from the velocity potential anomalies in this region. The GFS has persistently brought it out into phase 8 and/or 1 in early Jan and also JMA model. A phase 6 brings the MJO over the western Pacific with enhanced upper divergence from the tropical convection here, well-illustrated by the development of a Typhoon Nock-Ten heading for the central Philippines. The wave propagation from this activity is likely why the models are showing a Pacific jet wave train and the enhancement north of a blocking high over far north Pacific / Gulf of Alaska - as per 330K PVU chart below >>>> This amplification of the upper flow over N Pacific eventually working through across N America, with models dropping a lobe of the trop PV down across Canada and northern U.S. again - this amplifying a ridge downstream over N Atlantic and southern Greenland while at the same time the PV retracts from Greenland – sending the NAO negative in the means. The neutral to –ve NAO N Atlantic ridge into southern Greenland is a bit of a wild card given models tend to be too keen to model it in the medium range then drop it short range. However, the empirical probability of a negative phase of the NAO is increased about ten days after the MJO is in phase 6, as per diagram below. But with model initializing phase 6 at an amplitude of <1 and now in COD, is such as weak/negligible MJO signal going to have such an impact on teleconnections to bring this change to the upper patterns? Or maybe the models have been underestimating the strength of the MJO? This may be why the models are showing a temporary ridging over southern Greenland/Iceland and –NAO rather than a sustained block for now. The forecast poleward N Pacific ridge well-advertised since early week and trop PV dropping south over N America – will help kick off a major anticyclonic wave break in the far north Atlantic that may eventually allow a Greenland ridge as advertised by the models, albeit it with the caveat it maybe transitory, during the first week of January. Something worth watching over the coming days...

12z EPS NAO mean dipping slightly south of neutral (between 0 to -1) days 11-15, control more so to -2 NAO. The models have definitely seemed to have backed away from zonality in the medium to long range this week. Not so long ago we saw the endless zonality plugged by GFS and EC into the longer range, then they picked up on height rises over Europe and Scandi as they picked up on the signal for the flow to amplify out west, now we are seeing the retrogression of Euro +ve heights into the Altantic in the ENS with the possibility of a Nly in early Jan. However, I think we now have a level of consistency from GEFS and EPS guidance re: ridge north over Gulf of Alaska which is likely a product of a Pacific wave train in action, maybe bolstered by tropical convection / Typhoon amplifying the flow of which the MJO is an influence too. The medium/long range output also suggesting +ve height anomaly developing over N Atlantic too. Still have this niggling doubt with NWP modelling the MJO correctly though, as the neutral/weak MJO plots circulating in COD do not tie in with the hovemollers/200 CHI/OLR charts which show an active progression of MJO. It’s clear the models are factoring in the Pacific wave train which is forecast to amplify the flow over NE Pacific and now they have initialized and plotted MJO in 6 yesterday, we will perhaps see the models progress with more amplified patterns as a result, despite the MJO plots going into COD apart from GFS/GEFS. Interesting times ahead as we enter January.

There was some 00z EC ens cluster support for a ridge to the W of UK / SW of Iceland days 8-10, so the 12z EC det at day 10 may not be such a wild card.

While it's quiet in between runs, interesting how the models are handling the MJO at the moment. Yesterday they initiallized it in the Circle Of Death (COD) and had it generally wandering around in COD, apart from GFS heading for 8/1 like previous days. Today all the model initialize MJO in phase 6 before heading back into COD - while GFS tries to sneek out in 1. The fact it is being initialized in 6 is an important milestone in that it shows, as has been suspected by some of us, that the MJO is active and that the models will start to factor this in over coming runs. So we could see some changes to the MJO forecasts which will bring out of COD probably in the phases favourable for HLB and cold 7/8/1 with the model runs adjusting to this over coming days. Yesterday Today With the ENSO weak/neutral and not the driving force to the pattern this winter, plus the strengthening strat PV unlikely to be shot to bits by wave propagation and a SSW unlilkely, MJO will probably play an important role if we are to see changes in the trop pattern. The suggestions from various quarters and the evidence from Velocity Potential (CHI) charts at 200hPa over the tropics hints at an increasingly active MJO. So there is certainly potential but not a definite promise of a pattern change in early Jan from MJO forcing. Although last winter we usually saw the MJO orbit a few times or more at decent amplitude through favourable 7/8/1 phases, the super El Nino pretty much drowned it out. This year, the weak ENSO will mean the MJO may have more say.

Basically the Gulf of Alaska ridge may turn the Arctic Oscillation negative if it ridges to the pole and thus push cold air equatorward across N America and perhaps NE Europe. Though it may be a temporary feature, given the forecast strength of the polar vortex. Also note in the EC ens map the ridge over the N Atlantic which would suggest a NWly flow - as Ian F suggested earlier.

Hard to be sure for sure, though we can only go on what the ensembles are hinting at. GEFS and EPS suggesting a return of the Gulf of Alaska ridge in early Jan, which would perhaps ridge far enough to the pole to temporarily turn the AO -ve. Also both signal the 'euro slug' ridge retrograding into the N Atlantic. 06z GEFS 500mb mean in early Jan 00z EC ensemble clusters likewise: