Centering a Modal dialog box

I'm displaying a dialog box centered. It works fine as long as I

don't scroll down on the web page. If I scroll down and clink my link to display the dialog, the browser scrolls back to the top and the dialog is display centered horizontally but not vertically. I would like the dialog box to display centered in the browser regardless of where on the page the user has scrolled.

$('#new_event_form').addClass('flora').dialog({

autoOpen: false,

height: 260,

width: 400,

modal: true,

center: true,

resizable: false});