There’s nothing more frustrating than making plans around a weather forecast and then having that forecast change. A rained-out party or a day at the beach where you’d been better off with a jacket as opposed to the sunscreen can make one feel like there’s no point in even paying attention to the weather.

Recently, some organizations are touting the ability to forecast as far out as 30 or even 45 days. The question might be: Should you even bother with a forecast that far into the future. Just how accurate is a forecast, anyway? There’s a lot of subjectivity to the answer, but it’s worth exploring and it may help set expectations for what to expect from your local meteorologist.

The short answer to the question is that a 30-day specific forecast is almost worthless. However, there’s much more to the story.

I’ll show you two images in this blog which really illustrate this point powerfully.

Each image shows the correlation between the forecast of the upper-level patter at about 18,000 feet and the actual observed winds. If the forecast is correct, the score is a 1. For less accurate models, the score falls. A zero would be no better than a completely random guess. The first image is a five-day forecast, while the second image is a 10-day forecast. Notice how the accuracy fluctuated dramatically and how much less accurate a 10-day forecast can be.

You could argue that you’d be better just using the average temperatures and sky condition for a given day once you get beyond day 5.

PATTERNS MATTER

Sometimes the forecast seems to change on a dime. One day you hear it’s going to be sunny in three days, then the following day the forecast changes to cool with rain. How can this happen so fast? Certain weather phenomena, like a small fluctuation in the jet stream, can be tough for the computer models to resolve. Think of it like trying to see a type of tree in a forest from 100 yards away. You can tell there are trees, but whether it’s an oak or maple tree in the middle can be challenging to discern.

As you walk closer to the forest, the tree becomes clearer, you begin to narrow down the type and, finally, standing just a few feet away, you are sure what it is. This is similar to how some weather patterns work. We know the basic features of the weather days in advance, but where exactly those features will end up is often unknown three or four days into the future.

In patterns where there is a huge temperature gradient over a short distance the forecast can change dramatically. A high temperature of 90 on the fifth day of a long-range image could lower by 20 degrees three days later.

WHAT CAN YOU RELY ON?

About 80% of the time a three-day forecast should be accurate enough for planning purposes. The accuracy will increase two and one day before the target day. Days four and five of a medium-range forecast are likely to change, and forecasts further into the future can have dramatic shifts.

Here in New England we have mountains, the ocean, a huge cold landmass to the north and warm and humid air sitting to our south much of the year. All of these factors and more can make forecasting not only a challenge, but frustrating.  It’s a good idea always to recheck the weather forecast each day and not rely on older forecasts, as they can quickly become obsolete.

 

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