12mb Pressure drop - who counted that?!??
1020 - 998 = 22mb....not just a nasty storm - a *really* nasty one...i think in pilot terms it comes under the 'lively' and 'interesting' categories.....i.e. Shouldnt be flying to start with - how silly can you be?!
If your altimeter reads correctly on QNH and then -60 on QFE - even though there is the correct difference between QNH & QFE then you should check your altimeter operation... +50/-75 is typical accpeted tolerences - so you are still inside - however - check the altimeter by winding it through 300' if the difference isnt precisely 10mb it probably needs recalibrating...