Why Weather Forecasts Are Only Accurate Two Weeks Out

LifeHacker ^ | September 5, 2023 | Daniel Oropeza

[DoodleBob]

No matter how advanced science gets, we’ll always reach a wall on forecasting the weather into the future—and that threshold, on average, is two weeks. That is how long an unaccounted-for event in our environment takes to have a significant impact on the weather. If you’re planning on inviting friends for a picnic or thinking of going on a hike in two weeks, don’t bother looking at whether it’s expected to rain or not: The chances of it changing are very likely. Here’s why.

Why can’t we forecast the weather two weeks ahead?

As you can probably guess, two weeks isn’t some arbitrary number—it’s based on science. Dr. Kerry Emanuel, an atmospheric scientist at MIT, found that two weeks is our limit to predict the day-to-day weather, even with the best forecast models in the most desirable conditions.

Falko Judt, a research meteorologist at the National Center for Atmospheric Research, tells Mashable the reason behind this can simply be attributed to chaos. The main reason is that there is no way to know exactly what every atom in space is doing at every given moment, a law called the uncertainty principle. A simple atom is all it takes to kickstart a profound chain reaction in the atmosphere. After all, the last ingredient for a thunderstorm to start is a simple motion in the atmosphere.

How the butterfly effect has an impact on weather forecasting

Climate scientists use sophisticated climate models to forecast what the weather will look like for your birthday weekend. They put in as much data as they can to make these models as close to reality as possible. But there are limits. “We reach a point where there’s nothing that we can do in order to improve the simulations,” says Dr. Paul Ullrich, professor of regional and global climate modeling at the University of California, Davis. “Eventually, we just get to the point where we would need to observe basically every single molecule of air in order to do these long-term forecasts. And even then, if we knew where all those molecules were, there are external factors that can grow over time and influence the weather forecast.”

Say there is a hypothetical model that perfectly simulates our reality down to the last atom in a droplet of water in the atmosphere. You run the model to forecast the weather, but moments later, in the real world, a tree falls down that is not accounted for in the model. The more time that passes, the further away the model and the reality with the fallen tree will be from each other since the fall will undoubtedly have a ripple effect that will deviate more from the model with time. You might know this as “the butterfly effect.” So the further you look into a forecast on your weather app, the higher the chance it is to be wrong because of the impossibility of accounting for future events.

How accurate are longterm weather forecasts?

Ullrich explains that essentially unobservable effects that are impossible to predict occur all the time at small scales and can grow over time to become important for the weather forecast. “It’s a fundamental limitation, both of our observational network and just the inherently chaotic nature of the atmosphere,” Ullrich says.

This is not to say this two-week average is accurate in all areas around the country, much less around the world. The atmosphere over Florida is way more unpredictable than it is in most parts of California. So a two-week prediction in California will likely be more accurate more often than a two-week prediction in Florida’s more dynamic and less stable tropical weather.

Ullrich explains that the best way to look at forecasts is to determine whether the event is a small or large-scale event. Small-scale events like rain and thunderstorms are the hardest to predict accurately since they can be easily triggered or altered by many environmental factors. Even tropical cyclones, which are bigger-scale events, are hard to predict accurately three to five days out. On the other hand, big-scale events like heat waves or cold snaps are much more accurately predicted 10 days to two weeks out. “They’re coupled with these kinds of global wave patterns that propagate around the earth. And so they occur over very large areas, so being able to get large-scale temperatures correct is pretty easy.”

So while you might not know if it’ll rain or not on your birthday weekend, looking at bigger-scale events like the temperature, or an impeding hurricane, will give you a more trustworthy forecast.

[drop 50 and fire for effect]

There are a lot of comments here about failures in weather forecasting. My perspective is this- Weather is inherently chaotic, and our understanding is limited.

Therefore ren I am constantly amazed by our ability to provide the forecasts we do receive, and how we are able to identify where dangerous weather will strike as oftens as we do.

Meteorologists now make the nearly impossible look easy, then we thank them by complain about when they do get it wrong.

[Sequoyah101]

7.3141592653589793238462643383279502884197 years left to go or something like that.

Top men know these things.

Top men.

[AndyJackson]

But climate change is settled science.

Actually I think it is, and the answer is that the underlying physics is so chaotice we cannot predict climate since we cannot even predict weather 2 weeks out.

[Swirl]

Emanuael and the late Doug Lilly of NCAR and his ilk espoused this stuff for at least four decades now. Then it was preached that deterministic forecast had a valid range of a few days or less, and there was little hope for improvement.

The main problem, other than the governing equations being nonlinear, is an incomplete and less-than accurate description of the initial conditions. This is due to measurement errors and a sparsity of measurements in time and space with wide gaps through the earth’s atmosphere. It is my opinion that the choice of emphasizing satellite technology rather than ground-based technologies have slowed our advancement in achieving a better initial condition.

[Maine Mariner]

We are in Ellsworth, Maine and expect high winds and rain.
The amount of rain and velocity of the winds in the forecasts have changed every couple of hours: the last forecast much less rain and wind speed less than predicted.

We were worried about the 60- 70 mph gusts.

[Iron Munro]

Hmmph!!! Must be intentional misinformation.

The entire world knows that Al Gore can predict the weather at least 20 years out

And “Grating Greta” Thunberg can beat tha!.

/s

[fruser1]

I don’t see much accuracy two weeks out.

[maddog55]

As a friend of mine said, they can’t get yesterday’s forecast right.

[larryjohnson]

I’ve seen your posts before. I am visiting daughter’s family in Whiting. I lived in Roque Bluffs for 18 years before vacating to Florida. I am a retired AF weather officer and engineer so I check many weather sites. For this storm I found Carabu weather forecasting much lower winds than AccuWeather. I chech BarHarbor and Ellsworth observations. As a mariner, I am not, you know what to look for.
Larry

[daniel1212]

Say there is a hypothetical model that perfectly simulates our reality down to the last atom in a droplet of water in the atmosphere. You run the model to forecast the weather, but moments later, in the real world, a tree falls down that is not accounted for in the model. The more time that passes, the further away the model and the reality with the fallen tree will be from each other since the fall will undoubtedly have a ripple effect that will deviate more from the model with time. You might know this as “the butterfly effect.” So the further you look into a forecast on your weather app, the higher the chance it is to be wrong because of the impossibility of accounting for future events. How accurate are longterm weather forecasts? Ullrich explains that essentially unobservable effects that are impossible to predict occur all the time at small scales and can grow over time to become important for the weather forecast. “It’s a fundamental limitation, both of our observational network and just the inherently chaotic nature of the atmosphere,” Ullrich says.

The NOAA seems to take the safe approach, which is to almost always predict warmer to much warmer than normal (which only goes back about 150 years) for the NE. Predictions last Spring:

https://web.archive.org/web/20230511102349im_/https://www.cpc.ncep.noaa.gov/products/predictions/long_range/lead01/off01_temp.gif
https://web.archive.org/web/20230511102349im_/https://www.cpc.ncep.noaa.gov/products/predictions/long_range/lead01/off01_prcp.gif

In reality: Last year at this time, we were smack dab in the middle of a drought. This summer, we are challenging the all-time seasonal rainfall records in Boston and Worcester. By this date last summer, Boston had 19 days of 90 degrees or higher including a 6-day and 7-day heatwave. This summer, the city has reached 90 just 4 times. - https://www.cbsnews.com/boston/news/boston-summer-temperatures-90s-climate-change-weather/

Northeast's hottest weather of summer may come in early September ....for the summer season as a whole, temperatures in many areas during the three-month period from June 1 to Aug. 31, cumulative temperatures are finishing within 1 degree of the historical average and rainfall has ended up near the historical average.
A notable exception is in New England where summer rainfall was nearly two times that of the historical average, thanks in large part to excessive rain in July. - https://www.accuweather.com/en/weather-forecasts/northeasts-hottest-weather-of-summer-may-come-in-early-september/1575776 Composite from above: past and present NOAA predictions:

[SMARTY]

Right, they can’t forecast 2 weeks out but demand BILLIONS to ‘fix’ the climate?????

[sauropod]

Science.

[CFW]

Weather forecasts aren’t even accurate 24 hours out. Last Sunday, my husband wanted to do an outside project and asked me what the weather was going to be like for the upcoming week. I told him practically no chance of rain according to the forecast. It rained on each and every day.

[DoodleBob]

I’m with you.

I remember vividly, in the 70s, getting rained in when tomorrow’s forecast was for sunshine. Or packing an umbrella and then getting sunshine. Very often.

The accuracy of the 24-48 hour forecast over the past few decades is an unsung benefit of the proper application of science and statistical/numeric forecasting methods. Parenthetically, 1 out of every 4 meteorologists came out of Penn State since the 1980s, and as a Nittany Lion I approve of this message.

The problems have come from these new forecasting godz, who believe their own hype, and go out 5 days or two weeks. It’s like someone getting the gold in the 100-yard dash now thinking he can win the NY Marathon. I ignore those forecasts except to monitor for general trends - and even then it’s a coin toss at best.

Then there’s global warming or cooling or whatever they’re pushing nowadays. These wizards sniff at us little people, “weather isn’t climate.” Maybe. But a vital element in ANY data-driven exercise is consistency and homogeneity in data collection and quality. The improvements in 24-48-hour forecasts are as much a function of good data as it is better techniques. But the temperature and flood etc data from 1979 on backward was collected with little focus in consistency, or accuracy….in fact I bet the margin of error in the old thermometers is within the 1-degree C “rise” in global temperatures they thrust upon is.

Therefore, I applaud meteorologists as they execute their day-to-day job of forecasting tomorrow. There is great value in that activity. But please, you’re a plumber: stop trying to lord over the sewer system.

[Georgia Girl 2]

Here in GA its hardly accurate for one week.

[minnesota_bound]

But the socialist know to 1/2 degree the temperature 100 years in the future.
I look at the weather forecast at night and the next day and the rest of the week I see the forecast constantly changing. They cannot get 1 day accurate.