I've had this frustration for many years with regards to these flash amber warnings. The only one in recent times I can remember them being right on the money on was the snowfall of December 17th 2010 (Friday night), an amber warning was issued at tea time roughly about 5pm & the snow arrived about 7:30pm and dropped over a foot before petering out at roughly 2:30am. Apart from that its been epic fail after epic fail from the Met Office for our area in my book.