Thermohaline Conveyor

Rain has moved through here, showers following on behind. We do have some heavy showers to contend with tomorrow, especially across Devon, Cornwall, Somerset, Dorset and Hampshire. Risk of hail and thunder in the strongest cells, and rapid accumulation of rainfall is likely to lead to some localised flooding. Strongest EL signal is towards the far South West and immediately along the coast with -30/-40oC, thus here a slightly higher risk of lightning associated with convection.

On an additional aside, NAE18Z and 00Z has picked up on this frontal wave for Sat 15:00>, has to be said the 00Z was better developed with a cyclonic circulation.

Any thoughts on the Saturday PM wave? MO don't appear to be picking up on the potential of cylcogenisis with this feature. Looking at the Bracknell manually produced FAX, I can see the presence of a trough, but that's about it.

Especially if its associated with a squall type forced line, likely to be even less than a few hours.

NCEP (GFS) Global comparison with the ECM (European Framework) is interesting. The main issue between the two models around 23rd to 25th December appears to be timing and developments of two lows, with ECM running a low around 23rd into the 24th and GFS a little later. The second low is vastly different in terms of intensity at around 11m/s and Christmas Eve for ECM and 15m/s for GFS later into Christmas. Over 4 runs GFS was average 988mb minimum across the Midlands, much more inline with the lower wind-speed perturbations aligned to the ECM. ECM wins at the moment, unlikely to be the exact synoptic showing on both ECM/GFS but not too dissimilar with a wet and windy picture. Looking at the 2mT ensembles ECM/GFS comparative mean for London, the overall trend is a cooling one from after Christmas Day.

Christmas Day - Wet & Windy/Wild? MSLP pressure perturbations for the period from approx 24th until the 26th make interesting viewing. To give a much clearer picture of the overall ensembles and the variability involved at this time frame, I've provided you all with the ensembles for based on the last 4 NCEP/GFS runs. One thing that is very noticeable is the dip in MSLP around Christmas Eve and during Christmas Day. Although the mass of perturbations across the 4 runs, when combined into one super-ensemble, look extremely messy, the overall correlation is one of decreasing mean sea level pressure as we approach the big day. The mean drops from 1005mb to 988mb between 24th 18Z to 25th 18Z. Other perturbations within the super-ensemble drive the central pressure even lower towards the 975mb mark. Not a lot can be deduced from the super-ensemble in terms of specific tracking or general evolution of any particular low, but one thing that is clear is the presence of low pressure around the Christmas period. ECM/GFS both indicate that low pressure will be in control during the big day, with varying degrees of spatial coverage and severity. As for the latest 06Z GM (Global Model) GFS, it's clear by looking at the operational run that it is has a 'low' pressure bias against the rest of the perturbation within the ensemble pack. For example, the operational indicates a low pressure system tracking across England/Wales on Christmas Eve/Day around 972mb, whereas a majority of the perturbations/members within the ensemble suite would prefer an average of 988mb with the general spread highly deviant and messy. One can conclude that although low pressure is likely during Christmas, an extreme storm system is unlikely, at least across Central England, where the ensembles were projected for. It could be that after viewing the Scotland ensembles, MSLP perturbations are better correlated with a more dramatic lowering than further South, suggestive of a more Northerly tracking low or no secondary low. One would therefore prefer the evolution of the ECM00Z, whereby significantly low pressure sits to the West of Northern Scotland during festivities, but the whole country under the influence of low pressure associated with this powerful baroclinic region. Unless the ensembles dramatically change (which lets face it is possible), Southern areas would be windy and possibly wet and Northern areas are at quite a high risk of seeing severe gales. We'll see! One thing is for sure, pressure will be low, and it will be very unsettled.

Absolutely, and with the geopotential thickness falling well below DAM520<, you can bet your bottom dollar that the temporal coverage of convective driven wintriness increases in scope too. It's a very volatile synoptic set-up right now. More akin to the Christmas/New Year synoptic's I grew up with, before the strange ones of recent years ie. Reverse zonality etc. I won't be surprised to see a few more forced linear squall lines either, especially with the jet-streak intensity, the massive cooling aloft and the very strong 850mb flow. Interesting times indeed!

The frontal wave/bulge undergoing development (Wed/Thu) storm system is currently located between Newfoundland and the Southern tip of Greenland. What's noticeable is the extremely powerful jet-streak ejecting out of the Eastern US Seaboard, giving an immense amount of fuel to this wave in infancy. Tracking from the 06Z GFS this morning is more inline with the ECM with a 'left' bias due to the rate of explosive cyclogenisis and the phasing. All in all a very intense system, that will bring a spell of violent winds to the Western Isles especially (Isle of Lewis/Uist) etc. The squeeze in the isobars around the South Western flanks of the developing storm at T+42 (Thu 00Z) across the Western Isles is cause for concern with Storm Force 10 or Violent Storm 11 highly likely, gusting would be in the region of 80mph+ As for the run-up to Christmas, further bands of heavy rain and gales are expected, with very low geopotential beginning to assert further South towards England and Wales for Christmas Day and beyond. As the thickness at 500mb decreases and cooling aloft becomes apparent, so to will the thunder shower coverage increase exponentially, expect some lively weather around Christmas. The big day itself is throwing up quite a few possibilities, it's a case of wet and windy or wet and stormy. Plenty to monitor!

Warnings have been issued in the past with 24 hours prior to a wind event like this, and we still see fatalities due to trees being uprooted and falling onto motorists, and structural damage related fatalities. I don't think you will ever prevent these things from happening, but I would say it's about increasing overall awareness on a national level. You're much more likely to make a larger % of the general public aware of the risk with 4 days of national media coverage than you are with 1 day.

Still very early developmental stages in the cyclones formation, I'm beginning to wonder how much of the latest 'slower cyclogenisis' is down to the absorption of tropical depression Lorenzo and whether this has invigorated the jet-stream as earlier model runs had suggested.

AMBER has been issued on MM, sadly I can't post a link as it doesn't allow me

ECM12Z shows main emphasis generally South of the M4 still, strongest PPN signal across Devon, coastal Dorset, Hampshire and parts of W Sussex. The ECM12Z has less emphasis on East Sussex and Kent. Strongest snow signal around Dorset area towards Southampton. Strong signal also across Dartmoor.

ECM has the system slightly South but generally in line with the GFS06Z guidance, the two models show some similarities. PPN is likely as far N as the M4 and perhaps extending into S Wales, but should be lighter and more patchy. Strongest signal is geared towards Sussex and Hampshire, closest to the coast and 5 or so miles inland. GFS has shown greater consistency in the last 4-runs to run the system up too and around the M4 corridor with stronger snow signals more inland away from the coast. 06ZGFS adjustment has shown a slight shift South, with a continuity of emphasis on Sussex, Kent, Essex and the London area for snow. Heaviest snow reserved for coastal areas especially. On a side-note, it's important to understand that ground temperatures are relatively high and although PPN signals are strong and suggest more than 10cm, these have likely been modified to account for ground warming due to recent mild spell and the fact that the cold entrainment hasn't had any time to become established at the surface.

This is because ECM has things further South I expect, UKMO and GM both project a more N trajectory. It's a case of monitoring for consistencies, and then applying probability.

Latest from MM on Tonight's snow risk: Probabilities of disruption due to snow.

Changed to 00Z, typo

Still a strong PPN signal on the 00Z ECM for SW England, Central S inline with the NAE 00Z and 06Z. Mean around a diagonal of Chelmsford Essex to Gloucester N for snowline

Generally the W COUNTRY looks to sit just outside the main risk, which is further to the North and East. NE Wilts, Cotswolds has the best chance - but even this is minimal. We'll see how it unfolds. It doesn't help that the overall dynamics are so 'marginal'. Always a chance of some sleet and snow on Monday as the frontal system retreats and weakens, by then frontolysis would of done its job and PPN would of become some-what fragmented. As we all know, even with the colder undercut of air from the E by Monday afternoon, with weaker PPN in general it will be hard to achieve any real accumulations of note - especially on a saturated ground.

Snow risk for Sunday

Absolutely, and it's virtually impossible for us to pin-point with precision any cold air entrainment phasing, PPN intensity and an accurate depiction of the theta-ew spread etc. Ian touched upon a very interesting point, in that nocturnal cooling of the BL needs to be accounted for in assisting PPN to fall as snow, even with the marginal T850 profiles that ECM and the GM raw data output is throwing our way, surface cold pooling induced by nocturnal boundary and planetary layer cooling and of course evaporative melting (isothermal melt layer induced) PPN. When you consider the amount of variables and complexities that are in a forecast like this, its no wonder that we have to use a degree of manual manipulation to the data-set. A blend of GM and ECM-ens-mean looks good right now, and of course UKMO. Once we have the higher resolution NAE guidance, consistencies and similarities in this guidance suite will be key.

Don't take the M4 corridor analogy with any seriousness just yet, Ian stresses the uncertainties and complexities of this event. What looks quite certain is that quite a wide area of the country will be at risk of snowfall falling or/and accumulating by Tuesday.

NAE should be released in the next hour or so

For areas of Somerset, Dorset and Hampshire it's still very much in line, the Northward progression or lack of is still a major issue for the GFS. ECM00Z did favour a similar PPN progression and intensity to the this 12ZGFS evolution, so to date I feel it's closer to the mark. Averaging it out a little the GFS12Z is a lot closer to how things are likely to pan out I feel, it's certainly fairly close to ECM00Z, all major parts of South East Wales are covered, as the previous NAE guidance excluding 12Z of course.

PPN more aggressive to the South West so far, GFS12Z rolling

The most multi-model agreement we had was a few days back when UKMO, GFS and NAE had a similar evolution to the one being projected on the latest NAE12Z, the problem being 'all the inconsistencies' in between. Vast variations in the PPN spread and totals and little to no comprehension. A stronger PPN signal on the GFS12Z and some general support in the occlusions evolution would be a good place to start, then the positioning of the front on the ECM12Z, which has also struggled.