Although, weather and local news might be one of the favorite segments of the viewers and low airtime of these resulting in falling viewership and ultimately low advertising revenue, the stated argument contains sizable amount of flaws in its assumption and reasoning.
Firstly, the author of the memorandum assumes that lower airtime of weather and local news is resulting in lower viewership of the late night news program based of 'most complaints' without giving any sound quatification. What portion of the viewers are complaining is not mentioned. Also, there might be multiple complaints from a few viewers askewing the statistics. In addition, the memorandum doesn't tell what is the number of interesting and newsworthy weather and local phenomenon happend past year. If the number of actual news is low then the coverage time is bound to be low. To stregthen the argument by author, these information should be clarified in a consistent manner.
Though an assumption with multiple hole, even if we consider that low airtime of local and weather news caused lower viewership, another flawed assumption comes into play. That is the lower viewership resulted in low advertising revenue. There can be multiple reasoning behind the low revenue. The local business might be undergoing economic recession and had to cut their spending for surviving. Another competitor TV channel might have introduced and business decided to advertise their product in that channel. This assumption without proper reasoning weakened the argument of the author and needs to be addressed with concrete evidence to strengthen the argument.
Apart from the above two points, another assumption is former level of advertising revenue amassed by the TV channel is enough for todays business situation. This might be not true and the TV channel's cashflow might be still not adequate if they able to achieve former level. The author should consider a proper analysis for projecting its required revenue and include in the memorandum.
The argument while interesting and plausible, contains some weak assumptions which can turn it invalid. To strengthen this argument and its assumption mentioned questions should be answered with proper evidence.