What does Mark 13:18 mean?

Although the warnings given in Mark 13:14–18 have relevance to the fall of Jerusalem in AD 70, they also reveal details that will not happen until the midpoint of the end-times tribulation. In taking the form of humanity, Jesus gave up His omniscience (Mark 13:32), but there's no reason that God would have hidden the date of the destruction of the temple by the Romans (Mark 13:2). The siege against Jerusalem ended in late August. The temple was destroyed by September 8th. Winter was not an immediate concern. Details here seem to suggest that Jesus' warnings are not meant to end in 70 AD, but apply to something larger.

Jesus specifically says, however, that He does not know when He will return (Mark 13:32). Since His return will be three and a half years after the mid-point of the tribulation, that means He doesn't know when these events will take place, either. Thus, His concern that it will happen in winter.

The Antichrist's violence against the Jews will come so quickly people will not have the chance to return to their homes to collect supplies (Mark 13:15–16). In January, the low temperature in Jerusalem is typically around 44° F, or 7° C. This doesn't seem that cold to some people. However, for those who are not even able to grab a jacket, it will make for miserable sleeping. December and January are the rainy season in Israel. The creeks and rivers will run fast, and at higher elevations it may snow. In addition, even if the hills are tended for agriculture, there will be nothing to harvest in the winter. If the midpoint of the tribulation occurs in winter, the refugees will be cold and hungry.