If any day chosen by a rail union for strike action has been formerly announced, then weather systems such as Storm Doris arrives, can they claim another strike day in lieu?...
Browsing websites such as Real-Time Trains, Open Train Times and Recent Train Times (all of which use open data feeds from Network Rail) it can be seen that on many routes Doris caused significantly more cancellations than Wednesday's strike.
It does not seem to be possible to gets stats from these websites for Southern as a whole, but here are some stats for selected routes on the number of trains that actually ran on them on Wednesday (strike) and Thursday (Doris). For comparison, I have also shown how many trains ran on these routes last Wednesday (15th), which as far as I am aware was a normal day. For each pair of stations, the stats are listed in chronological order, with the number that ran on 15th first, then 22nd followed by 23rd. They show the number of through passenger trains that ran from the first station to the second station (in the stated direction) anytime that day.
Clapham Junction - Watford Junction 20 0 12
Victoria - East Croydon 265 268 220
Victoria - Redhill 56 57 49
Victoria - Gatwick Airport 199 212 163
Victoria - Horsham 81 80 44
Victoria - Brighton 62 59 59
Victoria - Lewes 29 30 26
London Bridge - Uckfield 18 21 15
London Bridge - Littlehampton 3 1 2
London Bridge - Eastbourne 2 1 0
East Croydon - East Grinstead 43 43 38
East Croydon - Uckfield 19 22 15
Horsham - Barnham 54 54 32
Barnham - Bognor Regis 67 64 54
Barnham - Portsmouth & Southsea 45 39 37
Barnham - Southampton Ctl 29 23 22
Barnham - Brighton 38 36 39
Brighton - Lewes 86 84 77
Lewes - Seaford 36 36 30
Lewes - Eastbourne 64 64 55
Eastbourne - Hastings 48 50 37
Hastings - Ashford 18 15 9
Epsom - Guildford 4 0 4
This is not a comprehensive set of routes, but I think it includes most of the routes that retain conductors. Some services may appear in multiple groups (e.g Victoria - East Croydon and then East Croydon - East Grinstead).
Unless I've made some mistakes, across this set of routes the number of trains that ran on 15th was 1286, with just 27 less during the strike and a total of 1039 yesterday. It would be interesting to know what the PPM stats were on those days for Southern/Gatwick Express as a whole.