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Weather analogue index

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22/09/2021

What is a weather analogue?

A weather analogue is the term used in meteorology to refer to similar weather situations. The basic approach is to compare today's weather situation with a number of dates in the past. The closest matches are then used as the basis to see how the weather may develop in the coming days, weeks or even months.  

What is being compared?

The current weather situation could potentially be measured in many different ways: for example, comparing patterns which persist for relatively long periods of time such as El Niño/La Niña, Ouasi-Biennial Oscillation (QBO).

Alternatively, a comparison of specific atmospheric parameters can be done: for example, comparing temperature anomalies at different levels.  

With the Weather Analogue Index (WAI) the comparison is made between 500hPa geopotential heights. The 500hPa level is basically half way up in the tropopause (where the weather forms) and it can be used to identify the main troughs and ridges.

Northern hemisphere MSLP and 500hPa height chart example

Mean surface level pressure and 500hPa height example chart

Weather Analogue Index (WAI)

The WAI is generated daily at approximately 06:00 GMT. The process is automated and follows these basic steps:

1) A grid is generated to represent the present 500hPa height profile across the Northern Hemisphere between 20 and 90 degrees north

2) The grid is compared to similar ones which have been created for the same date in every year going back to 1948

3) Differences between the grids are aggregated   

4) An index value for each year going back to 1948 is generated

5) The WAI table lists these in ascending order. A lower value represents a better match

The WAI table can be viewed for each of the last 10 days.

There is also an option to view an aggregate for the last 7 days. This has been included to help smooth out daily fluctuations.

How accurate is the WAI?

Although we hope you find the WAI interesting and useful, absolutely no claims are made for its accuracy as a forecasting tool. Also be aware that changes to it may be made without notice. 

Viewing the latest WAI

The WAI can be accessed from the site menu and the Model inventory. Alternatively the direct link is below.

Weather Analogue Index

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