templates/messages.html
{% with messages = get_flashed_messages(with_categories=true) %}
{% if messages %}
<div class="modal fade" tabindex="-1" role="dialog" id="modal_alert">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span></button>
<h4 class="modal-title">Alerta</h4>
</div>
<div class="modal-body">
{% for category, message in messages %}
<div class="alert alert-{{ category }}"><strong>{{ message }}</strong></div>
{% endfor %}
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Fechar</button>
</div>
</div>
</div>
</div>
<script>
jQuery(function(){
$('#modal_alert').modal('show');
});
</script>
{% endif %}
{% endwith %}