When we drove a car on Rt. 18 in Annaka city, Gumma Pref. in Japan towards Nagano, we came occasionally under big black clouds at 16:00, Sep. 23 (Fri.), 2011.
In this year, heat of late summer continued till Sep. 18, 2011. Subsequently, the typhoon No.15 landed in Japan, and passed through slowly in the length direction of north Japan in three days since Sep. 19, 2011. After the typhoon passed away from Japan in the north direction at 16:00 in Sep. 22, 2011, cold air current began to flow from Northern Asia continent (in another word, Siberia).
So, big black clouds were yielded by the temporary winter weather as seen in the following pictures.