That's a good point, the time Rick was in a coma is not explicitly mentioned - though it certainly looked hot during the premiere and Lori's flashback to when Shane came to tell her Rick was hurt, and everyone was wearing summer clothing. However, we do have this:
"They don't make it clear in the TV show or the comics, but in episode 5 (Wildfire), Jenner says it is "Day 194 since wildfire was declared and 63 days since the disease abruptly went global". This would be the fourth day since Rick woke up. This means the disease has been spreading for over 6 months and has been an international pandemic for 2 months.
(Aside: I generally assume that "Wildfire" is a reference to "The Andromeda Strain" which features a "Wildfire" team being activated to study and defeat an alien pathogen - thus implying whatever caused the outbreak had been around for at least four months, permeating the population before somehow reaching critical mass or effectiveness and spontaneously activating, causing the global outbreak to begin.)
Anyway, assuming this is accurate, the CRAZY part of the zombie outbreak had only being going for a couple months. It would still be pretty hot for everyone to be wearing what we typically see them wearing. Not to say zombies hadn't been migrating around in the intervening time, but they generally aren't portrayed as long-distance types.