I think there are outfits like Accumweather that dares to go out 15 days, but.......
In the defense of the profession of meteorology, I can give several reasons why this craft never, has never, and will never be an exact science or have any real accuarcy, outside 1, and the very most 2 days.
1.And most around the issue of randomness. Watch a 30 day or longer loop of weather and cloud movement. It is a extreme hodgepoodge of randomness, and these rivers and pools of moisture that have a life of their own. The Mets do pretty good at a macro level with trending and overall patterns, but....
2. Sticking with the topic of randomness, weather patterns are greatly effected by (1) wobbling on the earth's rotation (frequency and severity), solar flares, and surface environmental factors like smoke, or dust.
3. Throwing the dice. Most of us in this country live in areas that are impacted summer thermal showers. Here near the gulf coast, I have seen an example of one summer shower dropping 3 inches of rain on one spot, and not a drop within eyeshoot a 1/4 of a mile away. The same can be said around lake effect snow squalls.
4. One of my favorite examples is tropical storm modeling and forecasting. Next hurricane season, take a gander at the 120 hour modeling data. I have seen certain Cape Verde storms have points of expected locale 3000 miles apart. And believe me, more science has been put in these prgnostication tools by far than any other tool in Meteorology. I'll give you an extreme example here in SE Texas. Back in 2007, a wave formed off the gulf, maintained strength, and NHC and everyone else thoought a minimal Depression might form and give the area a gully wash, like we see time to time. That was at 4PM. By 7 am, Cat 1 Humberto was hitting, downing trees and power lines in a swath at the coast to about 35 miles inland.
That is why as even an un-accurate science that this is, the professional mets are very meticulate in explaining that their wares are based on models, percentages, and ranges. It's only like hacks at the Weather Channel that might give you an idea that they think they know what they are talking about