Are you trying to get it as a shiny or just non shiny? The thing is with the roamers is that the frame advancement is SUPPOSED to be irregular because of the weather. There's no guaranteed way to hit your desired frame unless you RAM Hack (I don't know how, considered illegal in trades, etc.), you get lucky or you abuse an ID/SID where you know for sure that you can hit your desired frame (more useful when trying to catch a shiny roamer)
However, that's mostly the case for carts. It's easier with emulators, and here's how.
Use the lua script for Pokemon White provided by Kaphotics. It helps confirm you hitting your seed and what frame you're on
HOWEVER in my experience so far, it always displays it as -2 of what the frame is supposed to be. Example being if my target frame is supposed to be 72 (according to RNG reporter), in the game it would be frame 70. So for me so far it's a -2 differential, you might have to test and do trial and error yourself to see if that applies to you as well (it might be just me though, for whatever reason)
To confirm the above problem, hit your seed (which is always accurate) advance to your desired frame and catch a pokemon (preferably a stationary). Save the file, open up pokegen and load the save file. Then look at the IVs of the pokemon you caught. If they're the right IVs cool, if not, look at the PID. Then go to RNG reporter, look on the top right under options and change the PID from hex to decimal and look back to RNG reporter results. Look at frames close to your desired frame and see what frame matches the PID of the pokemon you caught. From there you can see what the difference is.
Whoops, major brain lapse. The above paragraph only applies to breeding, not wild pokemon.
http://projectpokemon.org/forums/showthread.php?16634-Pokemon-B-W-Lua-scripts
Copy the code he provided and create a new notepad file. Paste it in, save it, then change the extension of the notepad file from .txt to .lua and put it in the same folder as your Desmume folder.
Don't tamper with season changes, just keep the date as it is and advance as usual. However, with this script you'll be able to see what frame you're on. Also remember to ignore the "Current seed" and look at "Initial seed" when using the script so you know that you hit your seed. The picture in the link provided shows what the script is like and how you can use it to your advantage when using an emulator.
I quit RNGing 2 years ago so I don't know how to do anything past Black/White, but I remember this much. I didn't RAM hack the roamer, didn't have the lua script either, so I had to do it by trial and error. You have the luxury of the lua script to confirm what frame you're on and to know you hit your seed.
What you could do now, with the lua script, is let the frames advance when it rains (because now you can see the frames advance but they advance quickly so prepare save states accordingly), then stop advancing the frames near 1000 and then chatot all the way to 1105, thereby hitting your frame. Keep in mind the NPCs and frame differential I stated earlier when using the lua script (but it could just be for me)
Yeah...I think that should work. I don't come here often/post at all, so if you message me I can get back to you on questions. I RNG'd on both emulators in Black/White/4th gen/3rd gen (RAM Hack though) so I have some experience.
I wrote this on your wall but for reference, I think it's File -> Lua Script for Desmume 9.8, but for 9.10 it's under Tools at the bottom.