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Autumn 2021 update 2

Seasonal forecast indicators


19/08/2021

Autumn outlook

Please note: This is an update to the first Autumn 2021 UK weather article which was published on July 14th, 2021. 

Before looking ahead it is worth quickly reviewing the weather so far this year as well as Autumn 2020.

2021 to date

[Updated to include July and provisional August data]

Central England Temperature (CET) has shown more variability than in 2020 when the first 6 months were all warmer than 30 year average. This year the 3 months have been colder than the norm.

However, July was 1.7C above the average and the fifth warmest on record in the UK. The maximum temperature recorded was 32.2C (89.96F) on Tuesday 20th. 

Month CET Anomaly
January 3.1 -0.7
February 5.1 1.4 
March 7.2 1.5 
April 6.4 -1.5 
May 10.1 -1.0 
June 15.5 1.4 
July 17.7 1.7
August (Provisional to 17th) 16.3 0.3

Autumn 2020

The season was 0.5C warmer than average with a UK mean temperature of 9.9C. This contrasted with the previous autumn which was about 0.3C colder than the 1981-2000 average. 

For more information see Autumn 2021 UK weather, first discussion

 

Autumn mist, Berkhamsted, Herts

Autumn mist in Berkhamsted

Autumn seasonal model overview

The table below summarises output from seasonal forecast models for September, October and November 2021. It  represents the data available at time of publication and subsequent updates may favour a different outcome. 

Model Temperature Precipitation
UK Met Office GloSea (UK) Above average Near average
Meteo France (France) Mostly above average No anomaly in Scotland. Below average elsewhere
DWD (Germany) Above average Below or no anomaly
CMCC (Europe) Above average in the north Above in the east. No anomaly elsewhere
ECCC (Canada) Mostly above average Below in much of the north, west and south. No anomaly in other areas
ECMWF No anomaly

Above average in the far north. No anomaly elsewhere

JMA (Japan) Below in the far north. No anomaly elsewhere Above in the north
C3S multi system (European combi) Above average No anomaly. Below to the south and west of the UK
CFS v2 (USA) Above in the north west. No anomaly elsewhere

Mostly no anomaly

C3S multi system incorporates date from ECMWF, UK Met Office, Meteo France, CMCC, DWD, ECCC, JMA and NCEP.

There is a signal for temperatures to be above the average. The precipitation outlook is not clearly defined, but is possibly trending towards below the average.

The relationship between autumn and winter has been discussed before. There are suggestions of a correlation between warm autumns and mild winters. 

Background signals

In addition to the seasonal models a number of key points are worth considering:

1) The autumn is expected to start with ENSO-neutral, but there is a possibility of La Nina returning. The current forecast from the National Weather Service in the USA gives a 70% chance of La Nina conditions during the November to January period. 

There is a correlation between La Nina and an increased incidence of cold weather in the UK through late autumn and early winter. 

2) A more active than normal hurricane season. An active hurricane season possibly increases the chance of high pressure areas forming over Europe as storm systems track northeastwards.

3) Current high levels of global warmth.

4) Warmer than average seasons have been common in the UK during recent years. However, the temperature anomaly during the first eight months of 2021 has been closer to the average than has been seen in some recent years.

The above pointers do not provide a strong signal for the coming autumn. 

Summary

At this stage aggregate temperatures for the meteorological autumn (September, October and November) are considered more likely to be above average than below. Nonetheless, signals are weak and the chance of below average temperatures may increase towards the end of the season.

The precipitation outlook remains very uncertain. However, the chance of if being drier than average seems to have increased since the first discussion published on July 14th 2021.

There autumn forecast will be released at the end of August.  

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