United Airlines do not fly LAX-AMS directly, so I'm presuming that you were due to fly via somewhere like Chicago, Cleveland or Washington DC which were experencing some of the worst and coldest weather conditions those areas have ever experienced due to a "Polar Vortex". Presuming that's the case, the claim of "weather" as the cause of the cancelation/delay appears to be justified.
EU Regulation (EC) No 261/2004 covers compensation in the case of flights EITHER by carriers based in the EU, and/or DIRECT flights by non-EU carriers to countries within the EU.
In your specific circumstances there's at least 2 reasons why compensation would NOT be due.
1) It appears that your initial flight cancellation was NOT a flight on either an EU-based airlines or a flight DIRECTLY to the EU. There have been claims by some people that such connceting flights are covered, however in general it is accepted that they are not.
2) EC 261/2004 does not apply "when the cancellation occurs in extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken." It goes on to state that :
(14) As under the Montreal Convention, obligations on operating air
carriers should be limited or excluded in cases where an event has
been caused by extraordinary circumstances which could not have been
avoided even if all reasonable measures had been taken. Such
circumstances may, in particular, occur in cases of political
instability, meteorological conditions incompatible with the operation
of the flight concerned, security risks, unexpected flight safety
shortcomings and strikes that affect the operation of an operating air
(15) Extraordinary circumstances should be deemed to exist where the
impact of an air traffic management decision in relation to a
particular aircraft on a particular day gives rise to a long delay, an
overnight delay, or the cancellation of one or more flights by that
aircraft, even though all reasonable measures had been taken by the
air carrier concerned to avoid the delays or cancellations.
In your case it is likely that both of these were true - not only were the "meteorological conditions incompatible with the operation of the flight concerned" , but also the airports in question had significant traffic limiting programs in place, severely restricting the number of aircraft able to arrive/depart.
If you had been on a direct flight to the EU it's possible that you would have been due compensation for meals and possibly accommodation, but as your first flight was likely a US domestic leg your chances of getting anything there are at best extremely low.