The likelyhood of the attack of the U.S. weather system originating from China is incredibly high, as these kind of cyber attacks happen on a daily basis. Even if the culprite isn't China, it'll be another country such as Iran testing out the capabilities and vulnerabilities of the U.S. computer systems. If important networks and systems such as weather and even the power grid are taken offline, then the entire country would be plunged into the dark ages over night.
Computer security experts agree that the vast majority of hacking attacks of all kinds on the Internet are sourced from China or Russia/Eastern Europe. Considering the strategic value of the U.S. weather system, and repeated evidence that the Chinese government is deliberately targeting U.S. government servers with hacking attacks, it is overwhelmingly likely that this hacking is a deliberate foreign threat.
There is no real evidence claiming that the threat was from a foreign country, so we must always give the benefit of the doubt to others. Making assumptions like this can lead to inter-country wars that may have no real cause (just look at the U.S.S Maine and the Spanish-American War, people think that it was just a freak accident, not the Spanish, now) Innocent until proven guilty, or something like that, right?
No I do not think that it is a threat from a foreign country that the US weather system what hacked. It could have been any country including somebody from our own country that hacked it. It should not be seen as a threat either it was just the weather system.
There are plenty of people in the U.S. that would hack our systems, so it's more likely that it was one of us than from a foreign country. There are too many people who would be capable of doing something like that acting alone. Also, it's harder for foreign countries to access our systems to hack us, so it's more likely that it was an American.