/* center modal */
function centerModals(){
  $('#myModal').each(function(i){
    var $clone = $(this).clone().css('display', 'block').appendTo('body');
    var top = Math.round(($clone.height() - $clone.find('form').height()) / 2);
    top = top > 0 ? top : 0;
    $clone.remove();
    $(this).find('form').css("margin-top", top);
  });
}
$('.modal').on('show.bs.modal', centerModals);
$(window).on('resize', centerModals);



<div id="myModal" class="modal fade in" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="false" style="display: block; padding-right: 17px;">
    <form action="" method="post" name="Login_Form" class="form-signin" style="top: 138.5px; margin-top: 173px;">
		    <h3 class="form-signin-heading">Welcome Back! Please Sign In</h3>
			  <hr class="colorgraph"><br>

			  <input type="text" class="form-control" name="Username" placeholder="Username" required="" autofocus="">
			  <input type="password" class="form-control" name="Password" placeholder="Password" required="">

			  <button class="btn btn-lg btn-primary btn-block" name="Submit" value="Login" type="Submit">Login</button>
		</form><!-- /.modal-dialog -->
  </div>


<button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
      Launch demo modal
    </button>