UK meteorology

After commenting that the FAX chart from the Met Office that Malcolm included in his analysis was messy with fronts close together, a member who is a retired Met Office forecaster responded. As I don't have permission from him to quote him directly, I'll say that he was very septical that all the fronts shown ought to have been included, as a front should only be marked where there is a substantial change in the airmass, as defined by the "thicknesses" I touched-on a while back. Lower heights (or thickness values measured in tens of metres of decametres, shortened to DAM, between lower and upper geopotential levels) equal colder airmasses, while higher heights or greater thickness values show warmer airmasses. If that doesn't make sense, consider the fact that warm air rises - this is because its density decreases, and this, in turn, means that the same number of air molecules take-up more volume.
 
Today's analysis:
As a general overview today sees the start of a few days that will be dryer, with less wind and not as cold. This does not rule out weak fronts being forced east around, or even through, the burgeoning high pressure from the south west.The 500mb GPH and and wind barbs for Saturday 00 illustrates this quite well with the trough to the NE, high to the SW and the energy flow along the interface that does this forcing. Having said that we are not talking anything dramatic.



Anyway a little more detail.A much better day today for most of England Wales with less wind and plenty of sunshine. Maybe the odd shower in the southwest and west Wales. But the western regions of N. Ireland, Scotland and north west England will still be quite windy with frequent wintry showers of rain, sleet, hail and snow with the odd blizzard in the mountains and feeling very cold here after an icy start to the day but temps rising to 8 or 9c further south. This will continue overnight but generally easing by morning where it will be another cold start to the day.





So sunny with little wind for most areas on Friday as the high pressure is in charge but still fairly windy in the north with some wintry showers but much less frequent but by evening an occlusion and patchy rain will have reached N. Ireland and western Scotland. This patchy rain will continue to affect the north west over night and into Saturday.



The rest of the country will remain dry with light winds but by 1800 (keeping in mind the opening remarks) a small wave that has formed to the west has arrived to the north west of Scotland with a weakening front and this front will track south east overnight and into Sunday so although no longer very active will bring cloud to most areas, albeit it will be quite mild.







Thus we arrive at Monday more or less where we came in but some rapid transitions are afoot and there we must leave it.

 
Malcolm's thoughts on the weekend and early next week (my annotations in bold):
This morning sees the continuation of the battle between the air, a combination of Pm (Polar Maritime) and Tm (Tropical Maritime), and systems from the west driven by the twin energy flows leaving N. America and the Pc (Polar Continental, i.e. from the north east) air to the east, with the UK tending to be the battle zone. This has been see sawing of late and this morning is no exception with the gfs siding with the former in the short and medium term. Nothing definitive here.

Today after a widespread frost will be dry, sunny and with little wind in most places the exception being N. Ireland and Scotland where it will be cloudier and breezier. Max temps in the 7-8C range. But during the day cloud and rain associated with an occlusion will affect N. Ireland and Scotland and track south east continuing overnight which could give some light snow on high ground.

Saturday will be mostly dry and sunny but cloudier in the west with perhaps some patchy drizzle thanks to the weakening occlusion and some wintry showers in NW Scotland. And Saturday is not a bad illustration of the two energy flows and different airmasses to the west.



As can be seen from the last chart another frontal system is on it's way from the west and duly crosses the UK on Sunday bringing with it cloud and patchy drizzle albeit quite mild with temps reaching the giddy heights of 12C in places.



This cloudy, mild air with the odd patch of drizzle continues though Monday and Tuesday as the high cell dominates but things are stirring away to the south west in the vicinity of the cut off upper low but there we must leave it.




The latest fax update for 1200 Sunday has the warm front down NW England, Wales and Cornwall which fits in okay with the above and the usual Exeter convolutions for Scotland and N.Ireland.




Interesting that the ecm has a wave on the warm front at 1200 Monday not a million miles away from the where the latest fax chart has it And at T96 has it over central France but crucially the upper trough and cold front west of Ireland has split the ridge . The two actions in combination allow the high cell to the north east along with the low pressure to the south to initiate an easterly flow over the southern UK with the ridge to the west curtailing any east bound energy. This then establishes the dominance of the high cell over southern Scandinavia and is not a long way from it's position yesterday evening.

 
Saturday thoughts from Malcolm:
Today a weak front will track south east across England and wales accompanied by cloud and patchy rain/ drizzle and perhaps some light snow on the high ground in the north, but dry with sunny intervals elsewhere but showers in N. Ireland and western Scotland, tending wintry in the latter, Temps in the 8-10C range so quite mild.



Overnight most places will be clear with frost in some places but by Sunday 00 frontal systems will have arrived in the south west bringing with them cloud and drizzle which will move slowly east during Sunday morning thus extending the murky conditions, Simultaneously in the western Atlantic explosive cyclogenenesis is taking place, courtesy of the favourable environment off the eastern seaboard, as a rapidly deepening low associated with the aforementioned frontal system tracks NNE to be south east of Greenland.





The fronts will continue to track east overnight into Monday which will be in the warm sector so generally cloudy with intermittent drizzle will be the order of the day with max temps in the 10-12C range.





Once the cold front has cleared east the Azores ridges north east and forms a high cell over the Uk whilst away to the south west a deep low that started life off the coast of the Carolina's is part of a mid Atlantic upper trough. I mention this because the trough has a role to play later.



Thus a fairly rapid transition into the next installment of the current evolution even if things look more complicated out west.
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The fax chart for Tuesday 00 with England and Wales in the warm sector and the ecm still making more of the little wave and placing southern England in a weak easterly.

 
Malcolm on Sunday (less contentious than Andrew Marr):
Pretty much a two way split this morning well signified by the morning temps where it is clearer and brighter in central areas, the east and north with frost in places whilst and much milder in the west which has cloud and patchy rain. The cloud, and then the patchy rain/drizzle, will spread east as the latest frontal systems do the same affecting virtually all areas by evening but particularly Scotland where the rain will be heavier.



Cloudy with patchy rain/drizzle with hill fog remains the general overall picture through tonight and tomorrow as the warm front continues east and the UK is in the warm sector with temps in the 10C range so quite mild with temps in the 11-12C range.



The one complication here is that by Tuesday 00 a wave has formed on the warm front over Kent which over the next 24 hours tracks south through eastern France en route to the Mediterranean where it is absorbed by the upper low which is part of Eurasian/ east European upper trough complex which will be a major player in future events. This movement allows the Azores high pressure to edge north east and form a high cell over the UK by Wednesday 00.



Meanwhile in the western Atlantic a lobe of the Canadian vortex has detached and initiated some intense amplification with a deep trough in mid Atlantic with simultaneously the high pressure ridging north into the eastern Arctic with the aforementioned upper trough channel to the east. This establishes the surface high cell further north and initiates the easterly flow into the UK and the general lowering of the temps.





But what's this lurking in the woodshed? A little intense Polar Low
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