Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
(not a complete answer, as the related literature is quite abundant).
When it comes to "extreme" design conditions, you could first go over the (updated) ASHRAE Climatic Design Information (Chapter 14, Fundamentals). More specifically the Extreme Annual Design Conditions (e.g. n-Year Return Period Values of Extreme Temperature). You can sift through the References section (at the end of the Chapter) for a deeper dive.
Depending on the location of interest, you may find precompiled "future weather" files (EPW format), like here (Canadian locations). In this case however, the design day conditions remain those of Fundamentals 2017. Yet one could pre-process the actual "future weather" file and extract monthly min/max values and reconstitute design day conditions.
