The weather has been stuck in a rut over the past week, with the jet stream which is the driving force for change displaced well to the north of the UK, leaving the UK and much of Europe in a slack pressure regime, which means plenty of dry and warm weather but also some showers in places. An easterly breeze has brought in low cloud off a still cool North Sea at night and in the mornings before burning back to the coast in the strong June sunshine, but despite the odd shower, it’s been generally dry and sunny at times for many this week.
Jet stream is currently displaced way north from its normal track
The weekend will see only subtle changes in the weather. We will still be under a slack pressure / weak flow pattern. An upper low and shallow surface low pressure system over the Bay of Biscay and Iberia will pull increasingly warm and humid air north across the UK. A weak upper low over northern Britain will provide sufficiently cold air aloft atop of this increasingly warm and moist air to steepen lapse rates, which combined with surface breeze convergence / upslope lift and day-time heating, will perhaps trigger more widespread heavy and thundery downpours across Scotland, Ireland, Northern Ireland over the weekend during the afternoons as temperatures rise and on Sunday perhaps northern and western areas of England and Wales too.
Little change into early next week, weak upper trough / cool air aloft over the UK combined with daytime surface heating will sufficiently steepen lapse rates to allow further afternoon thundery showers to develop, these mostly across northern and western Britain, where upslope flow over hillier areas combined with sea breeze convergence inland is more likely to trigger them. We could also see some torrential thundery downpours developing over France drift north over the far south of England too, as a thundery low pressure system over the near continent drifts a little further north, though for now, some models keep this activity the other side of the English Channel. Otherwise, for many it will remain dry, sunny and warm, even those areas with showers – some may escape them and stay dry.
From mid-week, we will start to see changes from the Atlantic, as the jet stream that’s been way to the north starts to drift further south in track toward the UK and also strengthens – thanks to an increased thermal gradient over North America between warm air pushing up from the south and colder arctic air dropping down from the north. This will mean it will turn less warm and humid from the west from mid-week. The jet stream dropping south over the Atlantic later in the week will start to bring Atlantic low pressure systems northeast between Iceland and Scotland, bringing cooler, windier and cloudier conditions with spells of rain or showers to northern areas mostly. Parts of Scotland have been very dry over recent weeks, so the rain will be welcome for some. Weakening bands of rain or showers may briefly visit the south, but probably not amounting to much, with much of the south staying dry, bright or sunny and fairly warm as a weak ridge of high pressure holds on here.
Jet stream dropping south towards the UK and strengthening late next week, bringing more unsettled, windier and cooler weather - especially in the north
Looking further ahead into late June, longer range - forecasters will generally revert to ensemble guidance and currently GEFS and EPS guidance points towards Atlantic trough close to northern and/or NW Britain out to day 15 and beyond.
ECMWF/EPS clusters for 20th and 22nd June indicate confidence around upper trough close to N and W Britain
Temperature and rainfall anomalies for later in the month
This indicates that the UK and Ireland may be prone to bouts of unsettled weather brought by Atlantic low pressure systems at times, with temperatures close to average, wettest in the north and west, drier and warmer towards the southeast – where high pressure may hold at times.