[Plugin: WordPress SEO by Yoast] Change frequency of pages in sitemap

I have a WordPress page that includes an external php code displaying meteo data of my region (usign the plugin allow PHP). The content of the page is updated twice a day.
Inside sitemap XLM the last mod of the page is the date when I last modified the page but not the actual date. The update freq. is weekly, it should be daily.
Is there a way to tell SEO to define a page with update freq. daily?

Thank You

p.s.
this is the page: http://www.walter.bz/meteo-bolzano-alto-adige/

http://wordpress.org/extend/plugins/wordpress-seo/