The reason the weather widgets jump around is because the icon pictures of the current weather stick out above the widget itself. For example, the clouds or rain images. When the weather changes, the images change, and since they stick out different amounts, they move around to accommodate the change.
This is fixable but results in a tiny bit of chopping-off of the top of the widget image. Worth it in my opinion, you can hardly notice the chop-off.
Edit the file:
/Library/Widgets/Weather.wdgt/Weather.js
On line 359, change:
maxOffset = entry.voffset > maxOffset ? entry.voffset : maxOffset;
To:
maxOffset = 0;
It may be a different line number - 359 is correct for Snow Leopard. You should search the file for the correct one.
You need to redo this when you update OSX.