An 'omega' blocking high pressure system brings settled conditions this week, this blog looks in more depth at these stubborn areas of high pressure.
High pressure is dominating our weather this week, bringing mostly dry and settled conditions. The area of high pressure moved out of Scandinavia to start the week and will slowly drift west and eventually centre itself over Scotland by the end of the week.
The high pressure system is part of a slow-moving ‘omega’ blocking pattern, where there is a combination of two cut-off lows (in this week's case - one over the Atlantic, and one over Europe) with one blocking high (over UK) that form the Greek letter omega (Ω). The westerly upper-level flow abruptly splits (or disrupts) equatorward and poleward as it encounters the vertically stacked upper level and surface high, creating trough disruption on the western side of the Omega high to form the low circulation (L1). The other area of trough disruption that forms with this block, due to the highly convoluted upper flow, is east of the blocking high and manifests as the second cyclonic circulation (L2).
These omega blocks can last for some time before breaking down, blocking the usual eastward flow of the jet stream, with low pressure systems either stalling either side of the high or moving over the top or beneath, throwing storms systems far off their usual tracks. The current block looks like holding on until later this coming weekend, when the models suggest the block declining southwestward, allowing a cold, north or northwesterly flow to develop across the UK.
The polar jet stream which normally brings us wind and rain in from the Atlantic is being split, with one arm of the jet moving well to the north over Iceland and Greenland, while southern arm moves south over Spain, Portugal and the Mediterranean region getting spells of rain and heavy showers here.
Relentless blocking high pressure can bring extremes of weather. When it sets up over Scandinavia or the NW over Iceland and Greenland in winter time for long periods, it has brought the UK very cold and snowy conditions. While parts of the arctic see record mildness. Stubborn high pressure over Russia led to unprecedented summer heat in 2010, leading to 1000s of deaths either directly or indirectly. While on the southern side of the Russian block, Pakistan saw devastating floods from an unusually strong and persistent monsoonal flow that brought vast amounts of moisture from the Indian Ocean.
The weather we receive in the UK when under the influence of these highs, depends on which part of the Omega Block we are stuck under.
This week, high pressure will be centred close to or over Scotland and only slowly drifting westward. This means light winds over northern Britain under higher pressure, leading to sunnier skies and higher temperatures across the northwest. Further southeast across central, southern and eastern England, tighter isobars between the high further north and low pressure over SW Europe, has brought and will continue to bring a brisk easterly or northeasterly wind. This stronger flow has dragged in areas of moisture in around the eastern flank of the high, so yesterday was quite cloudy across a large chunk of England and also eastern Scotland – with haar spilling off the North Sea along eastern coasts of Scotland and NE England. This has made a big difference to temperatures. Dunstaffnage in western Scotland saw the temperature reach just shy of 16C, while across eastern England, temperatures failed to get into double figures.
Satellite image over NW Europe at 1300hrs on Tuesday 15th March
The 12 noon radiosonde ascent at Nottingham yesterday showed a saturated profile near the surface (dew point line on the left and temperature line on the right close together) – which was representative of the surface airmass across much of England which supported stubborn low cloud and low temperatures
Tomorrow (Thursday), somewhat drier air off the near continent to our east which will tend to support much less cloud, the forecast skew-t shows the dew point and temperature line further apart near the surface, suggesting the air drier.
By Saturday, the cloud returns from the North Sea, suggested by the forecast skew-t, which shows the dew point and temperature lines close together (saturated airmass).
At the moment, it looks like the Omega Block will stick around in one shape or form into next week but probably break down and move away southwestward mid-week onwards, allowing the jet stream to move back down from the north again, though the jet aligned NW to SE, keeping a cool theme, but increasingly unsettled. A change that high pressure close to the SW may ridge back in again at times for the rest of the month.
