Weather updates from site founder Brian Gaze that keep you in the know.
The weather is in quite a changeable mood at the moment with a combination of hefty showers or thunderstorms and warm sunny periods. In the next few days things look quite mixed. In the longer term there are indications of it turning very warm reports discussing a heatwave are beginning to appear in the media.
What constitutes a heatwave? According to the UK Met Office definition:
When a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by county. In the north and west of the UK it is as low as 25C. In the Greater London area it is 28C.
In the media the term is often used more loosely to describe a period of very warm or hot weather.
At this stage there isn't yet cross model agreement on the weather developments next week and beyond. That is of course not unusual. Once the models look more than a few days ahead they generally start to diverge markedly. Nonetheless, a lot of recent runs have shown very warm 950hPa level air coming into play for the UK during the last third of June.
The chart below shows forecast 850hPa temperatures and thickness values for the UK on Tuesday 23rd June. The key things to note are:
1) The 564 dam thickness line, often considered a heatwave marker, is north of much of southern and eastern England.
2) 850hPa temperatures are very high in southern and eastern England as indicated by the orange shading. It's a lot cooler in the north west.
The ECM chart suggests the possibility of temperatures reaching 30C in the south and east during the first half of next week. Another model, the GEFS ensemble, doesn't support the idea of such high temperatures during the same period.
The plot below is for London and south eastern England. It does show warm or very warm conditions early next week but the chance of real heat is reserved for the period between June 24th and 28th. Some of the runs show upper air temperatures reaching 15C for several days, but others either show them dropping back quite quickly or staying lower throughout.
I've annotated the chart below to make things clearer. Also note the rain forecasts on the lower half. They point to drier but not necessarily dry conditions for much of next week. To me it suggests there could still be showers or thunderstorms to look out for. A pain if you're planning a barbecue.
The plot below shows the forecast 2m temperature spread from the GEFS. These are the temperatures we experience down at the ground level, rather than approximately 1500m above sea level as on the plot above.
Notice how between June 25th and 29th some of the individual runs shows temperatures reaching 30C. Remember too that it is often possible to add a couple of degrees to the raw values generated. However, also see how a number of the runs show lower maximum values during this period. Therefore, 30C is not yet nailed on in my books.
I think so far this century there have been seven Junes when 30C has not been reached. At the moment I'd say 2020 is slightly more likely NOT to make it eight. Nonetheless, the margins are fine and it wouldn't surprise me to be wrong on this one.
There are signs of it turning very warm or hot for a time during the final third of June. At this stage computer models are showing discrepancies and have not yet nailed down the details. Nonetheless, temperatures should reach the high 20Cs. In the southern half of the UK I think it's slightly more likely they'll reach 30C this month than not. The chance of very warm or hot weather decreases the farther north and west you go. Heatwave or not? We'll see and perhaps it will end up depending on whether the strict definition is applied.
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