I started weight watchers on Saturday, weighing 343, and today, on Thursday I am down to 333. Is it possible/healthy to lose 10lbs in your first week? I remember reading in the past that the heavier you are starting the more weight you can lose each healthily. I also thought, maybe my body was shocked by the change it lost more weight. Also, my home scale matched the WW scale when I weighed myself on saturday.

People often (but not always) see a big drop on the scale at the start of a diet. One simple reason is that if you are eating less, there is less food in the "pipeline." I've read that folks can have up to about 20 lbs of undigested food hanging around. Another reason is water weight. If you are eating less than you are burning, you will be using up your supply of glycogen as an energy source, which is stored with water. You might also be eating less sodium, which also helps drop water. Hormones also play a role, and bloating around the time of one's period can add several pounds (and release them later). Another thing that can pile on water weight temporarily is exercise; if your muscles are sore, they will hold extra water while repairing.

So, at the beginning of a diet, I wouldn't worry about the large drop. Things are likely to stabilize at a sustainable rate. But since the numbers on the scale have a lot to do with mass that isn't actual body weight, you should also prepare yourself for the possibility of weeks that you make sacrifices, eat perfectly according to plan, exercise, and end up seeing a random increase on the scale.

Congratulations on getting off to a good start! Getting started can be really hard and you seem to be concerned about making sure you are in this for the long run, which is really important.

Great going for first week
Food awareness/water weight/tracking are all factors

You obviously followed that plan really well - keep it up
Its quite normal also to have a big weight loss week followed by a small one Too

Keep tracking keep on top of it and you will get through your goals

Well done