how to do display: block after user login

WordPress has a class when the user has been log in to the site.

.logged-in

You can use that to check with javascript and show the pop up.

This can be hidden if the user has not been logged in.

And if this you want to show only one time of x number of times you can do it with cookies (among other options)

$(document).ready(function() {
    if ($.cookie('cookiepopupcheck')){ 
        $('#welcome').hide();
    }else {
        $("#close-welcome").click(function() {
            $(".welcome").fadeOut(3000);
            $.cookie('cookiepopupcheck', true);    
        });
    }
});

File can be included with Enqueue
And the this is the file github project