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Summer 2021 Update 2

Forecast indicators


11/05/2021

What does summer have in store? This is the second and final update taking a look at what the latest data is showing. The first update, Summer 2021 weather, is still available to read and provides more background information about recent years. TWO summer forecast covering June, July and August will be issued in late May. 

Fair summer weather. Berkhamsted, Hertfordshire

Summer cloudscape close to Berkhamsted, Hertfordshire

Seasonal model overview

A range of seasonal models are available. However it is important to remember that their skill level (accuracy) for the UK is low.

Model Temperature Rain
GloSea (UKM) Average to above average favoured No strong signal apparent
ECM Above average in the south and east Below average weakly favoured
Meteo France Above average Below average in the south and east
DWD Above average in the south and east Below average in the south and east. 
CS3 Above average No signal
JMA Mostly no signal Mostly no signal. Above average in the far north west
CMCC Mostly no signal Mostly no signal
CFSv2 Above average Mostly above average
Table 1: Forecast temperature and precipitation anomalies, June, July and August

 

The notable things at this stage are:

i) The models either favour average to above average temperatures or offer no clear signal. (Taken over the 3 month summer period as a whole)

ii) There is a signal for lower than average levels of rain

It is often the case that the rain signal is more difficult to identify and prone to greater error. 

Anomalies masking the outcome

The anomalies in the table may not be telling the complete story. They are for the meteorological summer as a whole and there is a chance that extreme swings are cancelling each other out to an extent. For example, in recent summers there have been several intense bursts of heat despite rather mixed conditions dominating. 

ENSO conditions

The state of the El Niño Southern Oscillation (ENSO) has significant impact on temperature and rainfall patterns around the globe. Nonetheless its link to the UK summer is thought to be weak.

Latest forecasts suggest there is an 80% chance of ENSO neutral conditions through June and July. Therefore ENSO is not expected to favour a particular outcome.

Atlantic Hurricane season

Several forecasts for the 2021 Hurricane Season have been issued. They are suggesting above average levels of activity. 

Colorado State University is predicting 17 named storms of which 8 become hurricanes.

Accuweather is predicting 16 to 20 named storms of which 7 to 10 become hurricanes. 

Tropical Storm Risk (TSR) is predicting the possibility of activity levels being 30% above the long term average.

The predictions possibly favour an increased chance of settled weather in the UK during the late summer.

Recent summers

See Summer 2021 Update 1 for more information. 

Spring 2021 so far

March was a milder than average month with the Central England Temperature (CET) finishing at 7.2C which is 1.5C above the 1961-1990 period average. However, that anomaly was offset by April which recorded a CET of 6.4C. That is 1.5C below the long term norm. It was also an exceptionally dry month in most of the UK.

The first third of May has brought more unsettled weather. Nonetheless, it has remained cool and provisionally to the 10th May the CET is 1.7C below the norm, running at 8.4C. 

Summary

1) Seasonal models on balance suggest above average temperatures taken over the June, July and August period as a whole. There is a signal for below average rainfall. 

2) ENSO neutral conditions continue to be expected. They do not favour a particular outcome.

3) Updates to the 2021 Atlantic Hurricane season forecasts perhaps point to an increased chance of settled weather relative to the average in the late summer.

4) Recent climatology favours above average temperatures and an increased chance of hot spells. However, the spring has brought long cool or cold periods with slow moving patterns becoming established across the North Atlantic region.

At this stage a warmer than average summer is still favoured, but due to cool conditions dominating in recent weeks the confidence in that outcome is lower than it was despite updates from the seasonal models. Uncertainty about rainfall levels is high, although the latest updates to the seasonal models have increased the probability of it being drier than the average.

The summer forecast covering June, July and August will be issued at the end of May.

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