It is totally true that there’s no way that these climate variations could be this dramatic over 600 miles of latitude (which would only be slightly more than 8.6 degrees), especially with all that maritime moderation. But, this is still a D&D game, and I want a small scale, but different climate zones. So, I’ll do what I want. :-p
The deserts around Silith are high scrub desert (not rolling dunes), but can still be very hot in the day and cold at night. Think the Sonora desert in Arizona. The southlands of the Swelter are also quite hot, but the thick humidity traps the heat and gives the area it’s name. Think the Everglades in Florida. Summer daytime highs can reach nearly 100, and winter lows rarely dip below 50 in the Swelter, or 30 in the Damara desert.
The northlands, Far Borea, Skywall Mountains, and Galdurang are more boreal climates, with cold, biting winters, and warm summers that, at least in Far Borea, are riddled with buzzing, biting insects. Think northern Minnesota (for the woods of Far Borea) or northern Montana (for the mountain areas). Summer daytime highs are usually around 65-75, and winter lows frequently approach 0. Not too much farther off the map to the north is ice sheets, and the mountains have numerous year-round glaciers.
Most of the region is temperate forests, grasslands, and scrubby hills. Think rural Massachusetts.. Summer time highs can be mid-80s and the Winter time lows average around 20.