UK meteorology

Thanks Colum. For various reasons I hadn't seen my Aunt for a couple or three years, and it hasn't really hit me yet; no doubt it will next week. She had been seriously ill for many months, sadly, so her passing was a release from pain and suffering, though obviously not so for those left behind.

Getting back on-topic, the usual member I quote hasn't been posting recently, and, in the model discussions, various members are referencing different timeframes; trying to get a feel for the likely progression is, therefore, not easy at the moment. Our cold front associated with the storm is clearing the far south-east, and, after a wet, windy and gloomy morning here, this afternoon has been brighter with some glimpses of sun. As a result, most of the country is now in a cooler north-westerly flow, with showers (snow over higher ground) in the north and north-west, with a likely "Cheshire Gap Streamer" tonight and tomorrow. This is an interesting setup wherein a cold, unsettles north-westerly sends a stream of showers south-east through the Cheshire Gap to affect the north- and north-west Midlands. Such bands can extend as far as Birmingham or Oxford, and this might happen in this case. Sunday's "slider", as described in what effectively became this week's lesson post the other day (when I pasted a post from Netweather which I had found illuminating on the mechanics of this weather forum-named phenomenon) is still likely, though its expected path (and therefore the areas that can expect to see snow and rain) is as yet uncertain. Beyond that, it seems that much of the country will see a few colder days, with the cold lingering over the northern half of the UK even after milder, unsettled westerlies return to the south. At present, the longer-range situation is also uncertain, with a trend for higher Angular Atmospheric Momemtum out of the tropics to maintain a meridional, meandering jetstream on the one hand, and other outputs suggesting a period of more zonal Atlantic-driven conditions during the second half of the month, possibly including the Christmas period.

I'm going to quote a knowledgeable Netweather member who specialises in the global teleconnnections and oscillations which underpin the likely evolution over the next month or so. She (like me) has a rather wordy style, and I don't understand all of the theory behind her posts, but I'm going to quote her as I think that frequent readers of this thread will understand enough by now to get a feel of what she's describing (my explanantions in bold):

Always worth taking a lead from the GSDM (Global Synoptic Dynamical Model) at all times in my opinion, but in the context of current discussions then NWP (Numerical Weather Prediction, i.e. the shorter-range deterministic models such as GFS, ECM and UKMO) may be especially likely to oscillate in the period ahead, both with the tropospheric pattern, but also across the polar field in terms of the AO (Arctic Oscillation) benchmark.

Each post for quite some time from my own point of view has, in anticipation of possible outcomes, focussed on the upsides and then the downsides in terms of where atmospheric angular momentum is to be led during the upcoming transition of the MJO (Madden-Julian Oscillation) (tropical convection) from the current wave to the start of the next one.

As stated yesterday, definitely coming off the fence with this one, though still stressing the need, as expressed in each post, to monitor how far AAM (Angular Atmospheric Momentum, the tendency of tropical convection to cause the jet to become meridional as tropical winds interact with the earth's surface and the system tries to conserve overall angular momentum) falls from the maximum disconnect with the La Nina base state currently happening - to the cyclical ebb as the tropics re-set patterns through the mid period of this month.

The Phase 4 Global Wind Oscillation (at present) orbiting to Phase 8 and then in turn back to Phase 4 (and not heading back into Nina Phases 1,2 and 3) will underpin the bullish notions that the atmospheric circulation wants to continue to pave the way towards an even colder trend heading towards and into the New Year as the North Pacific ridge and Canadian vortex configurations relax sufficiently to allow height rises to the NE but without too much northern branch of the Jetstream to spoil deeper sources of upper cold air advection to head south-westwards across the North Sea.

We could espouse the easterly of late December 2005 as an example of this sort of thing, but not at face value in terms of "intensity" (or otherwise )of cold air and snow distribution necessarily, or indeed replica precise timing to take too literally. It still has good purpose for illustration use though of how things could conceivably evolve in principle.

I expressed slowly growing confidence in this sort of evolutionary possibility to NE heights yesterday. However the UK initially fares, the jist of it is that a deep source of cold air would be in place in this scenario that is highly unlikely to move very far in these circumstances and sets up a very very interesting January scenario

Still with the attached caution caveat that prospects of a fall back in AAM sufficient to take the GWO (Global Wind Oscillation) back to Phases 1,2 and 3 is becoming less likely with each passing day of positive data - *should* this occur then the more traditional La Nina pattern would re-surface which implies the North Pacific ridge and Canadian vortex combo not retrogressing and in this way pressurising the polar jet and neutralising an -AO profile. At the same time, the Azores/Atlantic ridge would be ascendant at the expense of greater influence of heights to the NE and too much energy transport to the N to allow the pattern to back deeper cold air advection westwards

Its highly possible that the models will show especially erratic trends on the way to the final destination, notwithstanding that as posted earlier on this thread, the GEFS (GFS model ensemble output) suite is pointing a way towards the upside scenario in the extended period.

But as a precaution anyway, in terms of keeping a sanguine open mind to keep enjoying the ride, I would suggest not allowing any model vicissitude to correspondingly engulf human emotions into equal vicissitude. On that basis best also to take AO/NAO forecasts with an equal pinch of salt - these will simply reflect the swings of NWP, and equally, ensemble suites

The theme is the same old for me - the signals lead the models, the models don't lead the signals.

Highly interesting start to the winter (for a change), if you don't let every model suite overwhelm you, whichever way
 
Indeed. I think I'll quote all the warnings that are currently issued by the Met Office.

Yellow warning details

Snow & ice

Between 09:35 Fri 8th and 18:00 Sat 9th
Snow showers will continue to affect parts of Scotland, Northern Ireland, Wales, northern England and parts of the Midlands. 2-5 cm of snow is likely in places over the warning area. 10-20 cm is possible for some locations, mainly in northern Scotland, Northern Ireland, north Wales and perhaps the northwest Midlands. Icy surfaces are also likely to be an additional hazard, especially overnight. The heaviest and most frequent snow showers will progressively become confined to northeast Scotland during Saturday. Impacts include possible travel delays on roads stranding some vehicles and passengers with possible delays and cancellations to rail and air travel. Some rural communities could be cut off and power supplies interrupted. Probably some icy stretches on some untreated roads, pavements and cycle paths and also a chance of injuries from slips and falls on icy and snowy surfaces.

Yellow warning details
Snow & ice

Between 09:35 Fri 8th and 18:00 Sat 9th
Snow showers will continue to affect parts of Scotland, Northern Ireland, Wales, northern England and parts of the Midlands. 2-5 cm of snow is likely in places over the warning area. 10-20 cm is possible for some locations, mainly in northern Scotland, Northern Ireland, north Wales and perhaps the northwest Midlands. Icy surfaces are also likely to be an additional hazard, especially overnight. The heaviest and most frequent snow showers will progressively become confined to northeast Scotland during Saturday. Impacts include possible travel delays on roads stranding some vehicles and passengers with possible delays and cancellations to rail and air travel. Some rural communities could be cut off and power supplies interrupted. Probably some icy stretches on some untreated roads, pavements and cycle paths and also a chance of injuries from slips and falls on icy and snowy surfaces.

Amber warning details

Snow

Between 04:00 Sun 10th and 18:00 Sun 10th
A spell of heavy snow is likely over parts of Wales, the Midlands and parts of Northern England on Sunday. Road, rail and air travel delays are likely, as well as stranding of vehicles and public transport cancellations. There is a good chance that some rural communities could become cut off.

Yellow warning details

Wind

Between 04:00 Sun 10th and 19:00 Sun 10th
A spell of strong winds is expected to move east across the far south of England and Wales on Sunday morning. Some short term loss of power and other services is possible, as well as some delays for high-sided vehicles on exposed routes and bridges. It is likely that some coastal routes, sea front and coastal communities affected by spray and/or large waves.

Yellow warning details

Snow

Between 04:00 Sun 10th and 23:55 Sun 10th
A spell of heavy snow is possible over some central parts of the UK during Sunday. This could lead to road, rail and air travel delays, with the potential for vehicles to become stranded or public transport to be cancelled. Rural communities with limited access routes could become cut off.
 
Based on the short-range, high-resolution models, the above warnings seem pretty accurate. I'd add that Monday could be quite wet and windy, with snow possible over higher ground. As I type, it seems probable that milder, zonal Atlantic conditions will move in again, at least for the south after midweek. How long these conditions last, or if they ever become established in Scotland, is open for debate; there are indications that easterly winds may enter the equation from the end of the year and into January 2018, though this is obviously highly conjectural this far out.
 
Another interesting forum post that discusses the possibility of mild zonality versus continued cold:

A lot of attention obviously on Sunday and Mondays events, though I'm also looking to see whether we can sustain the cold.
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Admittedly it will be quite tricky but there seems to be a Scandi high popping up in the operationals again. I'll highlight the key to look out for. First to stop a zonal onslaught, it's vital we prevent the polar vortex from creeping into the Atlantic. Here is the 12z GFS operational later on in the week:



The Polar vortex is held back and heights are allowed to build northwards from the Azores. Whereas the control....



Is poor and we see a return to more familar territory. Thankfully the ECM supports the operational instead. Next step to hold on to the cold can be achieved in two ways; either trough disruption (ECM) or a held back polar vortex over NE Canada (GFS).

ECM GFS operational


For good trough disruption cold air over the UK is a big help (as shown by this weekend) so we want to see as much cold pooling as possible on Thursday. The 06z GFS operational was actually very cold with sub zero maxima at times in central and northern areas but that was because of a snow event on Thursday. It will be difficult to get more snow on Thursday though there is still uncertainty about the incoming low.

Overall the 12z's lack the stronger cold later on and would probably result in a CET of around 4.0C by the 18th. They are ok though but the threat of a return to zonality is there later on. Lets hope the Scandi high builds or we get more trough disruption in around 10 days time!
 
I thought I'd pass this information from Netweather's chief forecaster on, as it clarified my understanding of the Greenland Hiigh and, as it's a mainly seasonal matter, I thought it made an interestingly relevant 'lesson':
A surface high is usually always present over Greenland, as much of the interior landmass is an ice sheet which creates cold dense air on the surface (resulting in high pressure) while above the surface in the mid-upper levels the airmass is often cold shown by the low heights/ upper low (blues on 500mb charts) - this cold air circulation aloft over Greenland, often part of the tropospheric Polar Vortex, usually drives a strong westerly jet further south over the Atlantic, even if there is a surface high present over Greenland.

However, when there is warm air advection aloft from the south over Greenland, it causes geopotential heights to rise in the mid-upper levels (greens and yellows on 500mb charts), which combined with a surface high, creates a vertically-stacked high or ridge that contains very stable air with descending air aloft so cause blocking, which diverts low pressure systems around it. These blocking highs of course can happen anywhere, so long as there is high pressure at the surface and warm air aloft, Azores high is a semi-permanent version of this.
 
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Who has snow, then? It's trying to lay here but not having much success!
Rather briefer post from the usual chap this morning; he's had internet problems apparently, and he's mainly referencing the anomaly outputs:
A quick look at last nights NOAA, GEFS and EPS anomalies, not forgetting previous runs, to see whether there are any indications of the forthcoming evolution. I need to keep the waffle short as I'm on my Tablet as my main website connection is still on the blink.

Overy the next 14 days there are some upstream changes perhaps indicative of the PNA going negative. An Aleutian trough develops with a stronger Alaskan ridge and more importantly a reorientation of the Canadian vortex lobe and associated troughs, one of which is aligned east south of Greenland. Quite importantly this affects the orientation and intensity of the Atlantic Ridge and with a strong upper flow still leaving the eastern seaboard a tendency for the flow over the Atlantic to become more zonal

Having said that the flow in the eastern Atlantic does abate somewhat and splits under the influence of the aforementioned Atlantic Ridge, positive anomalies NW of Norway connected to the east European Ridge and the weakening trough to the south East of the UK.

Ergo the changeable and quite cool weather could quite possible become more settled with Temps creeping up to be near average but more than likely a return to the proverbial north south split.

 
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