Weather updates from site founder Brian Gaze that keep you in the know.
Posted Wed 18th July 09:08
Ex-hurricane Chris didn't act as the great disruptor but it could have led to subtle changes in weather patterns across the North Atlantic region. One consequences is there now may be a greater chance of extreme heat than at any time this summer.
The flow across the North Atlantic seems to have become slightly more choppy in the last week. The link between high pressure cells in the Azores and Scandinavia has broken at times and shallow troughs have pushed eastwards over the northern half of the UK. That could eventually lead to a cooler and more changeable weather, but it also has the potential to trigger extreme heat.
This morning's GEFS 00z London plot shows temperatures rising during the coming days. By this time next week the most likely outcome is for it to be very warm or hot in southern and central regions with maximum temperatures of around 30C (86F). That's happened several times this summer before temperatures have eased. This time the 30C (86F) mark could turnout to be a base camp for an assault on the summit!
The plot goes on to show a greater divergence in temperature forecasts during the last few days of July and the start of August. However, note that a number of the individuals rungs are now breaching the 30C mark and several are heading towards or into the mid 30Cs. Usually the GEFS under cooks temperatures or a given scenario, so if you add 1C to 3C to those values you start to get some serious levels of heat.
The serious heat would be the result of low pressure becoming slow moving to the west of the UK and high pressure centring over the near continent. That would lead to a Spanish plume scenario with very warm air moving northwards from southern Europe or North Africa. Given the dryness of the ground and the time of year the chance of record breaking heat couldn't be discounted. At the moment the computer models don't show that outcome, but I'm not entirely ruling it out.
The ECM 00z run shows the sort of set-up I'm describing, although it never really gets there in full. It positions the Atlantic trough a little too far east and consequently the warmest air ends-up pushing northwards over France, Benelux and western Germany. In the UK it would be hot but not what I would categorise as extreme.
If the Atlantic trough ends up slightly further west than ECM 00z shows the thermostat gets turned up. As I said this is a possibility and nothing more at the moment. GEFS 00z Perturbation 2 shows this outcome with maximum temperatures on 31st July reaching 35C (95F).
I'm using the chart below for illustrative purposes only and if we do end-up with the furnace scenario it could happen any time during the last week of July or the first week of August. Remember also what I said about adding up to 3C to GEFS temperature forecasts.
To cover all bases I'll reiterate: the hot or very hot scenario is not certain. Some computer models bring the Atlantic trough eastwards across the UK quite quickly. That would lead to cooler and showery conditions returning by late July. I'm not discounting that outcome.
The last week of July and / or the first week of August could bring the hottest weather of summer 2018. This would be likely if the UK becomes sandwiched between low pressure to the east and high pressure to the east for several days. Temperatures would probably top out close to 33C but possibly head on upwards into the mid 30Cs. There would also be a slim chance of the all time record coming under threat. Nonetheless uncertainty remains and there is a chance of it turning cooler and more changeable if the Atlantic trough pushes farther east.
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