Assuming that the Dublin in 2019 Worldcon bid wins its (unopposed) election, there will be a need for a 2019 NASFiC. By WSFS rules, the Worldcon (or NASFiC, if there is one) one year before the convention year will hold the NASFiC election. In this case, it means that the 2018 Worldcon, Worldcon 76 San Jose, will run the 2019 NASFiC election.
Any site in North America more than 500 miles from San Jose, California will be eligible to host the 2019 NASFiC. That effectively means that no place in California or Nevada is eligible. The 500 mile circle around San Jose does allow places like Portland OR, Salt Lake City UT, and Phoenix AZ to bid. Of course, any place farther away than that is also eligible.
NASFiC does not have a date restriction, other than it has to be in the year for which they are bidding. Unlike Worldcon, NASFiC doesn't administer awards, and therefore it has even fewer practical limitations on how early in the year it can be held. This year's NASFiC is the weekend after Westercon.
Combined with the rules for Westercon bidding (see my previous post), it is technically possible for a site to bid for and host both Westercon and NASFiC simultaneously. The elections take place at different times and places and are run by different groups. (Westercon is selected two years in advance and voted at Westercon. NASFiC is selected one year in advance and voted at — in 2019's case — at Worldcon.) However, any site in North American west of 104° west longitude and also more than 500 miles from San Jose is eligible to host both.
There are of course many places in North America east of 104° west longitude, and they are all eligible to host NASFiC but not Westercon. There's never been an official NASFiC outside of the USA (Puerto Rico is part of the USA, folks, and don't forget it!), but sites in Canada, Mexico, and the elsewhere are all eligible as well.
To my knowledge, no group has ever actually tried to bid for both Westercon and NASFiC, but that's only due to the practical issues such a bid might encounter, not due to any restriction in the rules of either Westercon or WSFS.