Drupal 7: Show ctools modal via JS.

backend.module

<?php
drupal_add_js('misc/ajax.js');

ctools_add_js('modal');
ctools_add_css('modal');

// Add your modal style to the settings. You find the defaults in modal.js near the top:
drupal_add_js(array(
  'my-modal-style' => array(
    'modalSize' => array(
      'type' => 'fixed',
      'width' => 250,
      'height' => 250,
    ),
    'closeText' => 'Dismiss',
    'closeImage' => ...
  ),
), 'setting');

frontend.js

Drupal.CTools.Modal.show('my-modal-style');
$('#modal-title').html('my-title');
$('#modal-content').html('some markup here').scrollTop(0);
Drupal.attachBehaviors();