Thoughts from the MWIS forecasting team:
Whilst the weather conditions are fairly benign and rather cool at the moment, things are expected to change dramatically in the coming days as a plume of intense heat spreads in from the continent.
This is clearly going to be a very rare or unprecedented event affecting most of the UK, rather than some of the flash in the pan bursts of heat that have largely been restricted to southeast England in recent years.
In recent times, we have seen a persistent small 'cold pool' in the upper air patterns west of Iberia, this associated with low pressure in this part of the world. To the east of this, lies a ridge of higher pressure and warmer air. There is essentially a fairly large 'meander' in the upper wind patterns across southwest Europe. We call this a 'meridional' weather pattern, where wind patterns blow south-to-north, rather than a more common west-to-east as we have at the moment. It's a key factor in displacing air masses and bringing abnormal temperatures into higher latitudes.
Tuesday next week
Repeatedly over the last few weeks, forecasting models have at some stage in their forecast cycle allowed this cold pool to be absorbed into the mid latitude westerly flow over the north Atlantic. This transition thus allowing hot continental air from Spain and even north Africa to become drawn northwards towards our shores.
This sort of scenario is a complex story in the atmosphere, and fairly difficult for the numerical models to understand. However, within the last 24 hours, the definition of this process has been resolved, although there will no doubt continue to be a little oscillation in its forecast track.
We now have considerable consistency in the outcome on Monday and Tuesday, that exceptionally high temperatures are very likely across Britain.
There are still some subtle factors which may yet come in to play and dictate the final peak temperature values early next week. High level cloud is one element which can make or break the story, impacting on the duration of solar heating of the land. Also, the pressure pattern over the coming days will be fairly slack, so the exact isobaric pattern remains uncertain - this will affect the easterly component of wind into western Britain.
Dew point and therefore humidity is also an interesting part of the current weather story. Conventionally, plumes of warm air extending north from Spain arrive with rising dew points. Going way back to 1976, in what was a long anticyclonic summer of extreme dryness, a key feature was almost day-on-day the dew point; frequently near zero - as the air was often recycled Arctic air having had various convoluted routes to the UK - most typically via or east of Scandinavia before making for the UK directly from the east. The current situation is similar, in that the air in the lowest layers of the atmosphere reaching the UK will have arrived having passed north of the Alps, and without having been moistened by rain, will indeed arrive with very low dew points. This together with the fact that at least over southern Britain, there is considerable soil moisture deficit, suggests briefly a ‘1976 look alike’ with very large diurnal temperature variation.
Thus we may have widely more than 20 degrees rise from dawn minimum temperatures. At least until in cities buildings ‘ingest’ massive amounts of heat, retained in the buildings overnight, this suggests the night minimums leading up to Tuesday will be nothing special.
1976 had constantly very low rainfall, even on ‘convergence days’ (sea breezes and perhaps weak dynamic uplift), thunderstorms were rare. So looking ahead further into next week, without in depth analysis, an initial forecast would tend to include severe thunderstorms. They may well occur, but due to the extent of dry low level air, there will not be the moisture drawn in to produce widespread storms, except perhaps where there is dynamic uplift as Atlantic air replaces the very warm air. The process of storm formation will depend on subtleties of high cloud and low level wind convergence.
It is worth noting that the dry air will also exacerbate the already high fire risk on the moorlands and heathlands.
Finally, looking at the mountain forecast, it is important to stress the extremity of the heat at high elevations, especially by Tuesday. Dehydration (perhaps sunburn depending on the upper cloud) will be a serious issue.
Monday sees summit temperatures over England and Wales reach 20 to 25C, whilst valleys will be 30 to 35C, hottest for Wales and the Peak District.
Tuesday sees the heat potentially intensify and extend further north - extremely warm even from dawn. By then, summit temperatures in excess of 25C for England and Wales are currently indicated by models, whilst in Scotland, Munro temperatures would top 20C, possibly nudging 25C.
Forecasting models tend to smooth out the topography across the Highlands and therefore often underestimate maximum temperatures in the glens. Looking at current indications, values of 34 to perhaps 36C are not out of the question for glens in the middle and east Highlands on Tuesday.
Whilst media focus will no doubt be on the heat around cities, particularly London, there may be a few surprises when it comes to the highest temperatures reported. With the right weather 'ingredients', places such as the Vale of York, maybe even the Lancashire coast not being far behind the urban 'heat islands'.
Take care and be fully prepared if you are thinking of heading for the hills in the days ahead.