How are flight paths determined?
The reason I'm asking is because I discovered that my flight between EWR and LAS last month took a rather odd one.
I did not realize this until I compared some pictures that I took with what I found in an atlas. It turned out that we flew over the Finger Lakes of Upstate New York. I took pix of them while in the air, but did not know what they were until I compared the shapes of the lakes in the pix to that in the map book when trying to match them up.
On a flight from EWR to LAS, why would we have been flying that far north?

Matt,

Do you recall what else you flew over, did the rest of the flight take a more northerly type path towards the west, or did you cut back south and then go across?

Of course, the pat reasons would be either weather, or traffic.

With regards to STAR routings, departures off EWR could go to Huguenot or Sparta VOR's northwest of EWR, then on course. I've got a set of high altitude enroute charts on me, so I can check the jet routes for you (unless, of course, you flew direct to someplace way down the road).

Tom in NO (at MSY)

Tom is pretty much right on the ball..

There are three main reasons why you'll take one route one day and another the next.. There could be a weather system enroute that the dispatcher or ATC is working to avoid, although this is a lot more common in the summer with thunderstorms than it is at any other time of year. If people are paying attention, they'll also work to route you around areas that have severe turbulence SIGMETs (SIGnificant METeorological conditions) issued for them. An interesting note on this topic - about a week ago there was some serious shear from the jet stream over the Ohio/Indiana area.. an A320 reportedly even suffered structural damage from the turbulence. ATC basically shut down all the flight levels between 240 and 410ish.. so on the situational display, there were a bunch of 757s, etc cruising around at 18,000ft. The amount of fuel they must have been chowing down on is incredible. Anyhoo.. if your dispatcher or ATC is watching for things like this, they'll take you around.

Enroute winds are probably the biggest reason why routes change. From my personal perspective, there's not so much effort put into taking advantage of favorable winds as there is to avoid strong headwinds. What you'll notice typically in the winter time is westbound flights stay way to the north (I've seen IAD-LAX flights over Wisconsin and South Dakota) and eastbound flights take more of a direct path. In the summer, all bets are off.. One day, the eastbounds were flying over PHX, ELP, IAH, MSY, and CLT to avoid a mess of thunderstorms in the midwest. So weather and winds play a big part there.

Traffic is another big reason. If you've got a lot of planes flying into NY Center from the west and southwest at one time, your departures are going to stay out of their hair. ATC takes this into account by issuing preferred routes that they like airlines to file their flights on. I don't have them right here, but I'll check to see what the preferred route between JFK/EWR and the LAX/LAS area is and see how that compares to what you experienced.