CME
should give some warning of a couple of days.
Enough for us preppers to take a day off from work and head to the BOL , unless we already live there.
of if neither then at least buy extra food/fuel and/or get the Silver/Gold out of the Bank lock boxes.
EMP attack like JoJo said will be hard on everyone even preppers due to its surprise characteristic.
I am changing my job to one where I will no longer do all that international travel I did in the past ( remember my Chernobyl/priyat picture thread?)
International travel can be fun+educational but from a preppers perspective is very risky.
Pandemic is a threat we know is coming eventually.
there is no way around it.
Humanity has never been as connected as this, never as mobile , never as densely populated and never has so much live-stock living in close proximity to each other in high density "range".
Unless a different catastrophe ends the factors above a pandemic will come.
Question is.. is it in 4 weeks? 4 years? or 40 years? Its impossible to tell
But this is IMHO on of the most likely causes the other one is societal unrest.
I direct everyone's attention to Matt Bracken's brilliant essay "when the music stops" which see.
Thats the other high likelihood mechanism IMO.