Glastonbury takes place between June 22nd and 26th and it will be the first since 2019 due to the COVID-19 pandemic. It is now within range of some of the computer models, but often forecast details remain very uncertain until a few days before the start.
June has started on a cool and changeable note. However, there are indications of it turning much warmer during the middle part of the month.
By Domharrison (Own work) [Public domain], via Wikimedia Commons
The postage stamp chart below shows forecast temperatures on the afternoon of Saturday 25th June. Each stamp represents the forecast from one GEFS model run.
Most of them suggest close to average conditions, although a minority are very warm. Therefore, at the present time the most probable scenario is for fairly average temperatures but there is a significant chance of some very warm days occurring during the festival period.
The GEFS postage stamp plot below shows the rain forecast from all of the runs in the model on the afternoon of Saturday 25th June.
They currently paint a very mixed picture. Quite a few are pointing towards it being dry at that time. However, there are plenty showing rain and on a few there appear to be downpours.
There isn't a clear signal yet, but on balance a changeable period of weather is possibly favoured. That would mean the risk of rain at times, but also dry periods. The possibility of heavy and thundery rain is not discounted.
One important thing to remember is that showery rain in the summer months can be very localised. The upshot is that computer models struggle to accurately pinpoint the locations of showers.
The early part of the summer is sometimes referred to as the "European Monsoon". It refers to the tendency for the westerly flow to strengthen during June and July after weakening in late spring. The impact is to increase the risk of wet weather.
The European Monsoon is unpredictable and often weak, but it is most likely to occur during late June or early July. It could be the reason why some people associate Glastonbury with rain and mud! Nonetheless the records show that despite some mudbath festivals they haven't all coincided with poor weather.
To summarise at the present time:
1) Fairly average temperatures are favoured, but there is a chance of some very warm days.
2) There is a risk of rain and thunderstorms or downpours are not discounted.
The prospects could be described as mixed.
A number of forecasts for Glastonbury are provided. The best one to use depends on the range.
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See the Model inventory for the full list of model charts and data