Jquery window resize bug

I have a popup I'm trying to both re-size dynamically and keep horizontally centered. Since the popup needs to work in both mobile and desktop environments, I wrote a quick bit of jQuery in order to handle the re-sizing and remove the margins if the window width is less than the max-width of the popup. This all works as it should, except that whenever I slowly resize the window the popup defaults to hugging the left edge instead of staying centered. Here's my code, any help would be greatly appreciated as CSS is still going a bit over my head.

jQuery:

$(window).resize(function(){
    var w = $('.body').width();
    if (w > 1000) {
        $('.popup_modal').css({"left": (w - 1000) / 2 + "px"});
    } else if (w < 1000) {
        $('.popup_modal').css({"left": "0px"});
    }
});

CSS:

.popup_modal {
    display: none;
    width: 100%; 
    max-width: 1000px;
    float: left; 
    background: #fff; 
    border: 1px solid #d4d4d4;
    border-radius: 3px;
    box-shadow: 0px 0px 5px #ccc;
    position: relative; 
    margin-top: 20px; 
    padding: 20px;
}

Thanks!

Change to this code:

$(window).resize(function(){
    var w = $(window).width;
    if (w > 1000){
        $('.popup_modal').css({"left" : (w-1000)/2 + "px"});
    } else if (w < 1000){
        $('.popup_modal').css({"left" : "0px"});
    }
});