That's because weather is a chaotic system, and it's very sensitive to boundary conditions. In such a system, very small disturbances can make very big changes, far enough out.
Now, for Wisconsin (as that's what the OP's profile gives as their location), let's think about the external air temperature at 05.00 on 1 February 2017, and at 14.00 on 1 July 2017. Which do you think will be higher? Will it be a difference of a few Kelvin, or more than ten Kelvin (18°F) difference? How confident are you in your answer? You're probably very very confident that the July temperature will be higher, by more than ten Kelvin. The climate is still reasonably predictable months or years ahead, even though it gets very hard to predict the weather reliably more than ten days ahead. Climate is a long-term system, and is not sensitive to boundary conditions in the same way that weather is.
If you roll a fair six-sided dice once, you can't be very confident about whether a one will come up, however much you know about the air movements in the room when you throw the dice (you can be reasonably sure that the probability is 1/6, however much you know about the air). However, if you roll that dice 6000 times, you can be reasonably confident that one will come up a thousand times, give or take a hundred times, and you don't need to know anything about the air movements in the room.
That's the difference between weather and climate; even though weather is just a manifestation of climate, it's just a sample from the climate distribution.
We've built the global economy, patterns of habitation and movement, food supplies, energy systems, around the climate that we've had for the last few hundred years. And that should be ok, because climate, sea levels and sea pH levels normally take between thousands and millions of years to change radically. But our releases of greenhouse gases are now causing big changes to happen within years to decades: and it will be enormously expensive, disruptive, and cause huge suffering to billions of people, to reorient the global economy, patterns of habitation and movement, food supplies, and energy systems, to newer, unknown, unfamiliar climates, coupled with rising sea levels and ocean acidification.
He ends it with
We must therefore leave our original question unanswered for a few more years, even while affirming our faith in the instability of the atmosphere. Meanwhile, today’s errors in weather forecasting cannot be blamed entirely nor even primarily upon the finer structure of weather patterns. They arise mainly from our failure to observe even the coarser structure with near completeness, our somewhat incomplete knowledge of the governing physical principles, and the inevitable approximations which must be introduced in formulating these principles as procedures which the human mind or the computer can carry out.
That extreme sensitivity to initial conditions means that in theory, a scenario in which a butterfly in Brazil does or does not flag its wings just so are enough to create slightly different conditions that eventually result in tornado hitting Texas. Even if this is the case (and Lorenz did leave that as an open question in hist talk), it's not quite fair to say that the butterfly caused the tornado. The Lyapunov time for the disturbances created by a flap of a butterfly wing is very short. Saying that some event caused some later event else when the two events are separated by hundreds of Lyapunov times just doesn't make sense.
What this means is that forty plus years after Lorenz's talk, weather forecasters still can't make an accurate two week forecast, and they may well not ever be able to do so. They can make now a fairly accurate five or seven day forecast, and that was something that was beyond the skills of meteorologists forty years ago.
Caveat: Don't believe the forecast by your local TV station. They are notoriously inaccurate. If the US National Weather Service says there's a 100% chance of rain tomorrow, it's best to cancel your barbecue. If your local TV station weatherman says the same, there's a good chance tomorrow will be sunny.