Thermohaline Conveyor

Hi Kris - Good to see you again The 12 GFS guidance has come into line with the overall NAE 00Z and 06Z - as well as the NAE 12Z release. So, finally we have some degree of consistency to work with. Figure 1.1 As attached above, this graphic covers the latest risk based on the trough axis elongation from West Sussex to North Cornwall. Anywhere to the North of this line will effectively be tapping into the 1000-900mb cold/dry air ingestion from the East. The main zone as noted in the 80%/60% zones simulate areas that are running the stalling frontal boundary between +20> theta-ew WBPT and -17. Both of these values are around 4-5 degrees celcius the wrong side of marginal to support lower level snowfall. An isotherm of 400M would be more accurate. 60% areas take into account the main PPN intensity spread forming ahead of the frontal boundary and along it - thus areas that effectively [1] Ingest cold/dry air lowering the WBPT during the coalescence phase [2] Dry air is simulated in vertical sounding forecasts with subsidence at 1000-850mb with marked vertical forcing PVA(Strong) 850-300mb - peaking at 700mb. This supporting the theory of hydrometeor descent being substantial enough to fall through the drier/unsaturated lower level enhancing snow melt in turn increasing latent heat release deduction in the lower boundary. The perfect isothermal melt layer. Like yourself mate, I would favour those areas immediately N of the M4 into the Cotswolds, Oxfordshire, Gloucestershire and South East Wales. The wet surface prior to the onset of a more wintry ilk precipitation will - to a small degree impair overall snowfall accumulations totals. I would say 2 - 5cm is a fair assessment - to lower levels. South East Wales above 300M should see closer to the 10cm margin.

"This kind of set-up relies on advection of cold air at the surface to lower the isotherm. What we look for is a high precipitation loading event from the East to drag down the 0oC isotherm to the surface. Ideally, we need a weaker surface/mid level flow (light winds) in the lower level and a relatively cooler and drier ingestion of air to enhance evaporative cooling induced from melting and release of latent heat. Effectively, this scenario will bring snow to some lower levels if (1) Precipitation loading is moderate/heavy in intensity (2) Relatively prolonged in nature through stalling (3) Cold and dry air is advected at the surface below a relatively marginal mid level (4) A weak airflow or steering flow (wind flow pattern) in the lower levels to limit mixing of warmer air at surface. We call this the isothermal melt layer theory and it will play a role in this scenario. Note marginal theta-ew in the region of theta-e =+16/+18 along the Northern extent of the trough axis and where colder air undercuts from the East. This alone would support a 0oC isotherm to 400M without any evaporative cooling effect. If we had a strong steering flow projected, mix out would occur and evaporative cooling would not be effective. In this instance we have a relatively weak flow which indicates if we can get the PPN to fall moderate/heavy for a short period of time, this could in turn lead to a lowering of the WBPT Theta-ew gradient. Note also the additional CAA undercutting from the East spinning and stalling to the SE dragging in colder/drier continental air - this drier air undercut at the surface becoming our friend giving the ideal ingredients for Isothermal Drag to occur. Note: Theta-e WBPT is projected at +20 NAE 12Z 09Z (9am Wednesday) which is more the wrong side of marginal - so a rain to start with event. Moving into the 12Z (Midday) framework colder air undercuts as gradients drop from +20 to +17 along a long approximately West Country into Oxfordshire/South East Wales into Gloucestershire. NAE PPN guidance indicates at this stage a more substantial risk South of the M4 but I don't by it based on a WBPT gradient of +20. All in all then we have some agreement on theta-e between GFS and NAE despite the disparity on frontal ovemnet and progression East. The +16/+18 by GFS relates to NAE guidance and +17. On this basic guidance alone an isotherm in the region of 400/300M can be attained. We then look to PPN intensity phasing and longevity phasing through stalling position of the front. GFS is really underplaying the progression East, whilst also lowering the overall intensity - which is perhaps the main fundamental reason why it is holding back on breaking out more wintry PPN in its snowfall accumulation and phasing simulations found in the Wetter3 suite. We have to assume that PPN intensity will be stronger than what is being projected. Overall on current guidance an area immediately North of the M4 around Oxon/Gloucestershire running SE Wales would be favoured. Why? Well a strong signal for PPN to fall heavy/moderate in pulses occurs across SE Wales on both NAE and GFS guidance - both leading to accumulations - there-fore we can ascertain some certainty in this area. It also indicates that generally this is where the Northern proportion of the frontal boundary will be. ESE of this areas would mark the wavering/stalling point (approximately) and I do stress approximately. Judging by cold air will be undercutting from the South East/East along the frontal boundary one can assume that this additional injection of colder surface pooling advection will lower the WBPT combined with stronger intensity PPN enhancing the overall evaporative cooling through latest heat release induced by melting of mid level snow. This effectively could drop the colder boundary very close to surface. We await the 12Z NAE and GFS guidance merely for (1) further consistency in the NAE regarding positioning of the frontal boundary and progression East and (2) GFS to come to some sort of agreement in the intensity phasing and trajectory of the frontal boundary" A complex evolution unfolds. We await with great anticipation the 12Z release guidance from both GFS and NAE

Anyhow, enjoy the snow everybody. Might see you during the next cold spell. :lol:

I forecast the Forest of Dean as one of the worst affected areas in and around that SE Wales loop into Wiltshire. Note the pulse of energy will significantly weaken in the next couple of hours with the main energy pulse generally across Gloucestershire way - keeping some intensity up towards North West Wales. Then the band increases in energy as it moves into East Yorkshire with a risk of 2 - 5cm here.

Between 3 - 6cm

A strong energy transfer does move through for you later this evening into the early part of the morning - however dew points on the coast will be marginal for a straight snow event. You likely see rain in light pulses and sleet/snow in heavier. you may well get lucky a see 1 - 2cm more especially if you're a mile or so inland from Bournemouth.

We're just getting within a time-scale where we can use other techniques to decipher energy transfer in bands. The answer to your question is no, not really. Not a lot has changed. An average estimate of 4 - 7cm is likely for the Bristol area - which is significant enough to cause disruption. You will see snow, very little transient in nature.

Yes. It is still in the zone if you like. However analysing the latest mid level vorticity projections it does show weakening of the band that you see on radar. So light/moderate snow in pulses moves through between now and 19Z with weakening there-after before a second pulse moves through in the early hours - again weakening on the central/northern sector of the band and retention of energy in the Southern part moving into Hampshire, Central Wiltshire, Dorset and W Sussex. On latest guidance the Bristol area looks to get between 4 - 7cm (conservative estimate)

South Bristol looks better than N Bristol on the basis that you will always be closest to the main energy along the frontal boundary. The band you see on radar is expected to continue to move ENE weakening significantly around South Bristol - so locally heavier falls to the far South of Bristol between now and 19.00 - slight weakening is then anticipated as the next wave organising across the far SW and intensifies across N Devon/Devon. This will advance towards Bristol again losing energy in the Mid/Northern sector of the band but retaining strength over Dorset/Hampshire and W Sussex from 00Z to 03Z AM. Small pulse of energy moves through Wed 06Z to 09Z South of Bristol ENE into Central Wilts. On this basis I'd say 4cm would be a good average with potential of up-to 7cm.

The band will have a tendency to weaken just before it reaches your part of the world a little later this evening. A pulse of moderate/strong energy moves through between 00Z and 03Z Wednesday AM giving some light snow. Another pulse of energy heads East from N Devon into South Somerset skirting your area from 06Z - 09Z tomorrow morning - this shifting N in the afternoon. So I reckon around 5cm will be possible.

If any of you have any questions on whether you will see snow in your area based on the latest, then please feel free to ask. I will be here for the next 20 minutes.

In relation to the analysis of persistence of the PVA pattern above: "I've done a few manual calculations based on the latest mid level 700hpa vorticity over the next 24/36 hour period. The main zone appears to be locked into Dartmoor/Exmoor and bordering areas of Somerset towards Bridgewater and Tewkesbury. Next I've worked out the T850 wavering boundary and theta-e which unsurprisingly run along the trough axis ... See Moreand the possibility of stalling. Areas E of the Devon border run a -4/-5 boundary layer profile with theta-e average of +17. Moderate intensity PPN will initially fall as sleet but readily turn to snow under the persistence of the VORTMAX. So main areas in terms of persistence of precipitation over time + the right side of cold boundary remain to be higher ground of Dartmoor/Exmoor running into much of South Somerset away from the coast as far North as Bridgewater and Glastonbury. Taunton is in a good spot. The 2nd zone is defined where 2 or more strong vorticity lobes affect the area through the period. These are remaining areas into Bristol and generally W of Bath and N along the M5 into the SE quadrant of Wales including the Forest of Dean. The 3rd zone is an area where 1 or slightly more moderate to strong vorticity lobes cross the area and include Swindon" This essentially indicates that the cold boundary will be most effective around areas of South Somerset with VORT values moving into S Wales away from the coast. Persistence concludes Somerset to be in the CORE ZONE with regards to longevity. Also note though the cold air to the N will mean snow is more in the equation - rather than a transition mix. So Bristol still looks good, based on the VORTMAX projected to move through in the next few hours. I've added a more detailed version of the above:

I know for a fact the cider can be pretty handy in the West Country so I'll go for a pint of the local brew NAE despite showing intensification weakening still retains some moderate pulses in the proximity of Bristol. There has been a remarkably consistent signal for this zone from Somerset N to Bristol - especially W extents of Bristol up the M5 wrapping around Gloucester way into South East Wales. I do note the main VORTMAX lobe is retained around Somerset between 15Z and 18Z following by strong PVA by 00Z to 06Z Wed. Below I have annotated the persistent mid level forcing pattern and where VORTMAX is sustained the longest giving an idea on where PPN is likely to be most persistent and perhaps where some of the high totals are expected on this theory. Note the border of Devon into South Somerset.

I rather suspect that Ian F is being conservative in his estimates - there are signals of a mean of 5 - 8cm across Bristol - the high end value around 11cm I'll take 5 + 11 /2 = 8cm to be the mean spread over Bristol so the 5 - 8cm looks good.

I'm not sure why people are so skeptical about it reaching Bristol? I'll look forward to that pint later Matty

This, as expected has not changed very much at all. Remains the same from a couple of days ago. Sorry about the rough and ready graphic

Precisely the area that I've been harping on about for a while - so far panning out as expected, snow generally away from Cornwall - they've removed the orange here and limited the Eastern extent to Devon - this turning to snow to lower levels before back to rain after clearance associated with weakening PPN. Good falls across Dartmoor/Exmoor and these moving into much of the West Country.

Where is this information coming from? The zone that has been most consistent so far, out of every zone is that South East Corner of Wales running around the periphery into Gloucester area arching into much of Wiltshire and parts of Bristol. That is my take on the evolution thus far and unless I see some change in this bias, that is where I will stick my money.

Highest totals in terms of snowfall potential at the minute look to be in and around South East Wales/Forest of Dean area with some good potential for M5 Northbound from Bristol to Gloucester. Then there is Dartmoor - which does look to be well placed. These are without a doubt the hot spots as we speak.

Bristol is in a good enough spot as well as any areas running the M5 into Gloucester and around South East Wales. I would imagine the first 80% was issued for Devon and Somerset because (A) Devon has higher ground in Exmoor and Dartmoor which is a safer bet and Somerset is on the *real* boundary with regards to the transient nature of PPN, any West of Somerset to lower ground would be at risk of a snow to rain combination. The thinking is that the MO have only issued 80% so far due to the uncertainty on just how far the heavy PPN will make it East and I feel it is only a matter of time before areas of South East Wales, Shropshire, Worcestershire, Gloucestershire and Bristol are bought into the equation - after all this is essentially where the fun and games is most likely. NAE and GFS guidance have been relatively solid in that aspect. As for areas around Dorset, I would suggest it would fall as snow but perhaps be more transient near the coast in these areas. Any further East into Hampshire and Wiltshire there remains a little more uncertainty - although immediate areas around Bath and Swindon should see the risk increase through time I would of thought. This could effectively extend into South Hampshire and Berkshire too - but right now it'll be good to focus on the certainties. The fact that they have kept Cornwall in the 60% reflects my initial concerns on the overall dynamics and whether they are truly cold enough.

I expect that will become more patchy and weaker South of the M4 - the energy gets passed to areas close to Derby

So we know to expect general weakening in the band as it continues to make progress to Bristol - I also expect some energy transfer across Derby as the band to the West spreads East

I am my friend, I'll be heading back to England (Reading) to study meteorology again - hopefully on a path that will take me into the business. It's cold here too at the minute - 2oC and light rain

Bristol will still get some, I very much doubt it will hold form like it is over Swindon though - not bad though.

Gradual weakening of that PPN band will take hold as it exits Swindon and heads in the general direction of Bristol