If you’ve followed my blogs and Twitter posts over the past decades you may already understand this, hopefully remember all the times I’ve explained it. This is after all a forecasting site and a learning blog not one for those looking just for easy spoon-fed bumper sticker answers to a complex world.
Computer models use mathematical formulas of physics translated into an algorithm starting with an approximation of the current atmosphere and then projecting how it will evolve into the future.
It’s called Numerical Weather Prediction (NWP) using equations such as:
Those models and equations generate charts and maps like these that need a meteorologist to translate into the weather forecast:
The analog method (sometimes analogue) looks at past weather history and tries to find matches from the past to the recent and present, an historical analogy if you will. The matches will unlikely be perfect but will be close/similar even though not exact.
We can seek these analogs by looking at anything: jet stream patterns, ocean temperature patterns around the world, snow cover, sea ice, temperatures and rainfall, the stratosphere etc.
All similar years from the past are then merged together for the mean of the weather that followed. Those are the analog years.
Once the near matches are found, those years are then merged to reveal the composite or mean (average) of that history to make a prediction about the future:
HERE IS A GREAT REAL WORLD RECENT EXAMPLE.
We are in a La Nina (-ENSO) condition in the Pacific Ocean. It’s is the second year of this condition. So we look back at past Aprils in a second year La Nina to check on what temperatures have looked like in past LaNina2 situations and find:
So if one had used those ANALOGS to predict last month it would have done a good job of showing the general pattern of what actually did occur in April 2021:
Not too shabby. Of course it’s not going to be perfect that’s to be expected. As I said we use a composite of the past based on closest matches or analogs, close does not mean perfect match. The past years and the future being forecast will be similar but similar is not the same thing as THE EXACT SAME. Like history the weather may not repeat precisely but it will rhyme.
If you notice those financial ads on Radio and TV or if you invest in stocks directly or through a 401K/mutual funds then you know the disclaimer: “Past results do not guarantee future results”.
That goes for the analog method as well.
Similar and same are two different words with two different meanings, never confuse the definitions.
REMEMBER, as I mentioned at the top anything can be looked at to find possible analogs, this version looked only at recent second year La Ninas. When making a forecast for a month or season ahead we use multiple analogs and computer models in a blend method to make a long-range forecast out weeks or months.
And as always these outlooks show the average or mean for the month or season as a whole, not every day or every week.
So if you didn’t know now you know.
For more weather and climate info follow me on Twitter @MellishMeterWSB.