Thermohaline Conveyor

One of the most complex to forecast in a long time

If you're in Devizes the latest graphics I've done probably puts you in 40%, the first 60%. This suggests that the general trend has been for the shift of more significant snowfall to move away from the far East of the warning areas and everything to shift slightly South and West. I wouldn't put Devizes in 60% on latest guidance.

Taking into account a blend of the latest NAE, UKMO and ECM guidance the strongest PPN signal for potentially disruptive snowfall is across North/South Somerset, North Dorset, South East Wales, potentially the Forest of Dean, all these regions now fall within my 60% probability catchment areas. There still remains a considerable amount of uncertainty over PPN totals and snowfall amounts due to a wide disparity in data, however ECM00Z projects a strengthening PPN signal, along with NAE with weaker signals from GFS. All guidance concludes that the greatest PPN spread, -4/-5oC T850 frontal deviation sits within the counties of Somerset and North Dorset and South Wales. Those areas that have greatest potential to be upgraded are surrounding counties, the remainder of much of Dorset away from the coast initially, the remainder of Somerset and down into parts of Northern, Western and Eastern Devon, especially to higher ground. Other counties included in the 40% probability are South East Wales, Herefordshire and much of Gloucestershire. There remains some uncertainty on the Eastern extent across West Wilts and currently we have removed the 40% and have issued a 20% here, subject to change should a slight deviation exist in the model output today. Areas in the 20% are less certain, either due to the extent of the PPN North and Eastwards across North Wales, Central Wales, Wilts, East Glos and Hampshire or due to the rain/snow line undulating across parts of West Devon and Eastern Cornwall. There remains potential in a slight Easterly focus to include West Wilts and East Glos and perhaps parts of Hampshire. As does the snowline to the South across Devon and Cornwall, this could be extended, but on current guidance a mix of rain, sleet and perhaps patchy snow would seem the most probable outcome. Slight downgrade from 40% to 20% for parts of Wilts Slight upgrade from 40% to 60% for parts of N/S Somerset Slight upgrade from 20% to 40% for parts of East and South Devon away from the coast initially W extremities of Glos remain in 60% although very marginal between 60% and 40% Included is the original risk map issued yesterday: The latest risk map issued today: Conclusion Confidence remains low to partially moderate for snowfall distribution and intensity of PPN ie. snowfall amounts. The risk of significant snowfall is still probable. The biggest disparity in guidance is with the GM GFS with wild fluctuations and a significantly weaker PPN signal, especially for the Northern most extent of the warning area. We will be looking for some consistencies and growing confidence from the GFS12Z in particular, and a stronger comparative ECM12Z to the ECM00Z which showed a strengthening PPN signal. Should ECM12Z and NAE continue to project stronger signals, an amber could be issued, especially if UKMO displays greater willing. A strong uniform set of runs, with broader and more comprehensive agreement is needed before a higher confidence than 60% is issued for any part of the region.

Good to see some consistency between NAE runs, however ECM is fluctuating and has the front further West and the low pressure further South. The PPN signal is inconsistent in both terms of intensity and regional coverage. Generally the lack of support denotes just how complex this synoptic evolution is and if we don't get any model concensus soon, we'll be looking at a now-cast. There's a slight PPN signal for some light snow to develop across Hampshire extending into Berkshire and into the London area. For the West Country, highest chance of some light snow will be to the North of the region across Gloucestershire as the low continues to migrate and a slight signal across coastal regions of Dorset and Hampshire. Tuesday, is becoming more and more confusing and fresh eyes will be needed on this one tomorrow. Taking a mean out of the guidance, one would forecast snow primarily Bristol area into parts of Gloucestershire, N Dorset, N Somerset and into Hampshire. The northern extent and intensity of the latest 18Z NAE goes against the grain and unless support by other models evolves or NAE drops back to a more Westerly or Southerly low, things will continue to remain uncertain. You can certainly see why the MO have been broad in the assignation of the yellow zone.

That is absolutely wonderful, cheers mate, massive help!

This is true Paul, on the GFS12Z update. UKMO, NAE and ECM continues to support the more N trend. Right now unless I see continuity of the more Southerly bias from the GFS and other models coming on board, I would stick with my original prognosis. Very complex and a lot to be resolved.

No problem Ian, I know if you could you would. I'll do with what I've got.

Thank you for this Ian, close to my thoughts. It is all very diffuse, and models are chopping and changing rather erratically. Do you have access to ECM PPN models? It's quite limited on the ECM front, online. I usually try to calculate the absolute vorticity by positioning of a low pressure system and the ECM ENS Mean projected positioning of the -4oC frontal boundary, attempting to calculate PPN intensity is rather more challenging using this methodology.

Hi Chris, I remember that! Fascinating looking back on past snow events. I'm not too far away from you now, I'm based in Tibberton North Gloucestershire.

All mine. I have been forecasting and analysing the weather for many a year.

My analysis techniques are rather similar to those of the MO, use a lot of experience and a blend of the different models adopting super ensembles where possible. Confidence is growing on this coming event, it'll be interesting what Ian Fergusson's latest thoughts on this are. Could be quite a bit of disruption. UKMO, ECM, GFS, BOM and NAE all on board in terms of the low progression and to a certain extent the alignment of the -4/-5oC T850 frontal boundary. Could get very interesting!

Sorry to cause confusion, this is from myself. It's likely the MO will be issuing a yellow warning initially with an amber once clarity sets in. The amber some-where along the lines of the 60% or perhaps 40% most likely.

There is increasing confidence that an area of low pressure will bring its attendant weather fronts in across the South West during the course of Tuesday bringing another period of potentially significant snowfall to parts of South West and Southern England. We have issued a % probability of disruption due to snowfall on Tuesday with the most emphasis placed on areas immediately around and South of the M4 corridor with some areas to the North of the M4 also included. Main counties of concern for the risk of 5-10cm or perhaps more are currently North Somerset, North Dorset, Bristol, Gloucestershire, Wilts and Hampshire. There remains some uncertainty on snow totals and the extent of the snowfall to the South and close to coastal areas, it's likely that precipitation will fall as rain and sleet initially across Devon, South Somerset, South Dorset and Coastal Hampshire, but as the cold air undercuts later it's likely to turn increasingly to snow - giving accumulations. The main snow line is generally in line with the M4 corridor with strongest and most persistent signals for severe disruption around the Forest of Dean, Bristol, Bath and Central/Southern Wilts into North Somerset and North Dorset. The Northern extent of the band could also be extended and progression East across the South Coast to affect West and East Sussex also remains a possibility. We'll have further updates through the course of today and tomorrow.

Certainly has, the severe and exciting weather tends to bring me back - you can be rest assured of that

Big hello to everyone in the West Country, all eyes on this very complex baroclinic feature on Monday.

These watch areas will be increased in scope and coverage as we get nearer to the event

Thanks Weather Dude. I have a vested interest in the forecast for the Bristol area in the coming days, as I have a BBQ planned on Satuday for my Grandads 80th birthday. Chances are, this encroaching low pressure system (vortex), that seems to be stubbornly persisting in on/off cycles for the past month or so will likely play a big part in our plans too. The low pressure anchored to the West of Ireland is forecast to retrogress ever so slightly East to be centred across Ireland tomorrow. This added vorticity, lowering geopotential thickness, cooling uppers and the added bonus of a relatively modest warm air advection regime from the South - combined with diurnal heating will all work together in creating an ideal kinematic environment for thunderstorms to develop. Wednesday is the first interesting day as slab of cooler air tries to intrude from the West (in vicinity) of the encroaching and troublesome vortex. Focus will be along the leading edge of higher theta-ew airmass just ahead of the undulating cold front boundary and of course within the proximity of the low circulation. It's essentially complex, but Wednesday promotes quite a plentiful risk of heavy showers - most likely becoming thunderstorms across Northern England, East Midlands, Lincolnshire and parts of Yorkshire and Cumbria. Note areas further North are forecast to have the strongest portion of the relatively weak jet-streak overlap with strongest vorticity lobes, thus more organisation will be possible in these areas and therefore spatially and temporally speaking a greater density of lightning strike possibilities. CEQ projections (GFS) are around an average of sub -30 in areas further East and North of the Midlands - but the odd heavy shower could well become electrified across the West Midlands, Wales and South West England late in the Morning on Wednesday. Slack cyclonic flow exists, so many showers becoming slow moving. Updraught/Downdraught separation will be more likely in Northern/Eastern areas around the Northern periphery of the vortex across Northern England, thus showers spatially become more sparse across Southern England and Wales tomorrow. Friday is without a doubt a day that would favour a quite widespread initiation of pulse storms - energy propagating from one cell to another - peaking and back-building. I'll work hard these next few days to issue my thoughts on the coming days with regards to specifics. Deep and Shallow layer wind-shear looks on the whole, insignificant. So, organisation to anything more severe is inprobable. That is not to say that convergence induced tornadoes will not be a possibility. I would say that many areas have a pretty good chance of seeing a heavy shower or storm (depending on storm mode/phasing) these next few days from Wednesday right through into Saturday. Of course, boundaries will tend to shift daily as the vortex shifts and wavers.

Hi Liam, many parts are looking good these next 3-4 days. I'm Keithy Joe from null by the way - nice to meet you

In a sense yes

Friday looks good right now, plenty of deviation regarding (CAPE), but a slack cyclonic flow will mean plenty of PVA and VORTMAX lobes oscillating

Really disappointing!

Just an update. All guidance now favours this pattern of increased activity from Wednesday onwards now, with the end of this coming week into next weekend looking increasingly active across E TX and E OK. Through Wed/Sun period the dryline undulates nicely across C TX and C OK which is of course the focus for more severe storms firing that have the potential to become tornadic. The risk prior to Wednesday remains very minimal as high pressure remains stubborn with cooler/drier air remaining intact. The risk across TX could be some-what mitigated due to a strong subsidence inversion - so less certainty here - but in proximity of the vortex moving from the W across C/SE OK things look better with more widespread initiation. As for specific kinematic parameters and of course wind-shear/energy levels these will need to be covered in more detailed closer to T+24 mesoscale.

A quiet period on the cards in the next few days across the Plains as a ridge of high pressure extends. ECM00Zand GFS18Z and 00Z multi-model guidance suites are beginning to suggest the possibility of a vortex developing to the W of TX and OK during the middle of this coming week which could increase WAA from the deep S and move E to affect many parts of TX and OK in the form of severe thunderstorms, greatly enhancing the risk of a few tornadoes. This pattern has only recently evolved in the last few runs on GFS and a single run on ECM, so caution advised. Areas of focus initially W TX/W OK and perhaps more widepsread and to the E of these regions for the end of next week extending into more E states too. Otherwise the weekend and the early part of next week looks generally quiet in nature.

Looking good and right in the vicinity of the triple point too.

It's funny that you should mention the 'South of Spain', down here it is actually decidedly more green than it usually is. The South of Spain had a very wet and abnormally cold winter in 2008 proceeded by a further wetter and cooler than average winter of 2009. All this excessive rainfall and 'Gota Fria' conditions has led to a lot of the landscape looking very much like the United Kingdom.