BBC monthly outlook
Often unsettled into mid-July but dry spells too
Saturday 22 June – Sunday 30 June
Turning wetter before heating up for the week
After a fine and dry Saturday, things will turn unsettled from Sunday with rain spreading across the country from the southwest in the afternoon and overnight. Thunderstorms and heavy downpours are expected overnight and through Monday for most, but the far Southeast will likely escape with mostly dry weather. Heading into the working week, low pressure is expected to drift southwest of the UK and linger through the week just west of the Bay of Biscay. This will bring in some very warm or hot air from Spain, through France and up into the UK in an event known as a 'Spanish Plume' which typically brings wet, humid, and hot conditions for most of the UK.
Temperatures are expected to climb for most from our rather cool June and will feel very warm or even hot for most of the country, with the warmest temperatures likely being in the South. The heat will come paired with high humidity, making things feel muggy and even hotter. However, along the East and Southeast coasts, a fresher easterly wind will help provide some relief to the afternoon heat, at least through Wednesday. There is still some uncertainty on the exact position of the low pressure centre to the southwest, which will determine how, if anyone, sees heavy rain and thunderstorms through the week. Currently the heaviest rain looks likely to hit Ireland and extreme W Wales but may at times move through Western and Central Britain. The best of the dryness will be in the north and east. The end of the Spanish Plume will come some time into the last weekend of June, but there is low confidence on the exact end date. It does look highly likely that the majority of the working week will be very warm.
Monday 1 July – Sunday 7 July
Cooling off to near normal, but wet again
By the start of July, the Spanish Plume is expected to be over and the heat wave will come to an end. This is due to the jet stream - a current of fast-moving air in the upper atmosphere that drives weather systems - becoming more active and bringing the low pressure system to the southwest into the UK. This means that the weather will turn unsettled and cooler again, similar to what we have seen for most of June. The weather pattern is expected to become westerly with progressive low pressure systems in the North Atlantic bringing some fronts into the UK along with spells of cooler air.
There will likely be a few drier and warmer interludes to break things up, however, especially in the South. Scotland, particularly the northern half, will tend to be cloudier and wetter with fronts tending to linger in the area. High pressure will likely retreat to the southwest near the Azores, and into the Mediterranean, taking the hot and dry weather with them. However, there is a slight risk (around 20%) that near the end of the week and into the weekend a strong ridge of the Azores high pressure will extend northeast into Scandinavia, crossing the UK and bringing in warmer and dryer weather while pushing low pressure systems off towards Iceland.
Monday 8 July – Sunday 21 July
Staying unsettled at first but improving later
The unsettled weather is likely to continue through much of July, and there are indications that it is unlikely we will see any long-lived and sustained high pressure keeping things hot and dry like the previous summer or 2018. However, it does look like we may see a brief respite from the cool and wet summer towards the middle or latter half of the month. Low pressure tracks along the jet stream are expected to remain progressive across the North Atlantic at least through the middle of July. This means the second full week of July will likely still be quite unsettled, with some fronts bringing rain to many, but also some dry and sunny gaps in between. Temperatures will generally be near, or perhaps a touch below, average.
Heading into the third full week of July, the jet stream may begin to wander north and south, which will allow some of the warmer tropical air under the Azores high pressure system to reach into the UK at times when the jet stream heads north. This will bring with it sunnier and drier weather, with temperatures climbing above average. However, we do not expect high pressure to last, and as the jet stream dips southwards high pressure will return to sub-tropical latitudes. This sort of weather pattern is inherently highly unpredictable in the long-range, so day-to-day and even week-to-week details become tricky to pin down with any certainty. There is a risk (20%) that low pressure will remain the dominant weather feature through the entire month, keeping things wetter and cooler than summer normally is. However, there is also a lower risk (~15%) that high pressure may become more influential, especially later into July.
We will be able to pin down with more confidence the end of the Spanish Plume heat wave and see how summer-like July will be.