A real model-stretcher for Fri-Sat this coming week as we have both a trough merging with a secondary low and a shallow 'heat low' over Europe coming into play at the same time.
The first of those causes the greatest overall uncertainty, as the precise path of the secondary low and timing of merger with the parent trough will affect the position of the trough by hundreds of miles. We are, though, seeing a slight north-westward trend in the trough position once the variability on top is filtered out, which may have significant ramifications if it continues much longer.
The second troublemaker, that 'heat low', has been shown on numerous recent GFS runs to 'shake hands' with the front associated with the Atlantic trough on Friday, injecting some extra moisture and instability to help kick off widespread thunderstorms by late morning to mid-afternoon (latest in east). In the process, the 'heat low' dissipates as the associated heat is diminished, allowing high pressure ridging in from the southwest in the wake of the glancing blow from the Atlantic trough to take full control of proceedings for the weekend.
UKMO makes less of that low but has the trough closer-by so perhaps a similar outcome overall with, say, a bit less in the way of thundery activity in the far southeast.
ECM, however, has continued to be the more exploratory of the models, asking the question 'what if the trough is so far northwest that the 'heat low' doesn't interact with it much and remains present through the weekend?'. In my opinion, the trough movement away from the UK and encroachment of high pressure from Scandinavia brings a good chance of keeping that low from having much influence, but the ECM 00z did not agree, introducing a mix of thunderstorms and thundery rain across a wide swathe of the UK by late Sunday, after some more fleeting activity on Saturday evening.
In that last paragraph I've hinted at a suspicion of mine - that the westward reach of the Scandinavian High for Friday onward is being underestimated.
This line of thinking stems from the fact that while the models are certainly seeing that the forcing pattern emanating from the tropical Pacific is increasingly shaping up to be one that favours high pressure from the UK to Scandinavia, the observed pace of this coming together during the past 5 days has exceeded the modelled pace.
So those signs of a more stable form of easterly heat past day 10 might actually be more relevant for next weekend onward.
I must of course stress the might in that statement, as tropical forcing theory only enables one to see what's more probable, as opposed to what's definitely coming our way, and that 'more probable' is typically only a chance of around 60 to 70%.
Regardless of the timing, having such a vast and intense pool of heat coming together ahead of a period when a UK-Scandinavian high pressure pattern looks likely to come together for at least a few days, is plenty of reason to be concerned about dangerous levels of heat reaching the UK for at least a short time at some point within the next few weeks.
That being said, it's not sufficient at this range to start waving your arms around and shouting across the street, though it may be wise to have a bit of a think about what you can do to help the more vulnerable of our population should such conditions come around. I've already had a long think over how best to help out my 96 year old grandfather who, thankfully, lives down in Poole which is unlikely to see the peak of whatever level of heat we see.
Originally Posted by: Stormchaser