Are you using water or a dry pan?
What model 22" WSM do you have? I'm guessing you have the newer version with the arched doorway in the center section?
Lump or briqs and how much?
What type/brand of briqs?
What method of firing your pit did you use?
Did you bring it up to temp on its own or did you fire it off and let the ATC do the work?
With ATC's you just barely crack the dome vents. (at least with the Guru on my BGE's & 18" WSM's)
Do you have your controller set at 375* because you are trying to raise the temp or is that your target pit temp? { I see you are shooting for 300* } I have to ask!
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1) First off, I would do all I could to use something for a windbreak.
2) Then I would close the lid vent almost closed but not all the way.
3) Open the door and let the temps rise without the ATC. Let the pit get up to @ 325 on its own then shut the door and resume cooking with your controller, (set for 300*) I'm fairly confident the temps will rise then drop to what you are looking for.
Don't FREAK OUT!
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And next time take into consideration the weather conditions and be prepared to deal with it.
Use a dry pan.
Use a weed burner to fire off the charcoal, the whole ring until the edges ash over. Assemble the cooker and bring it past your target temp, because the temps will drop when you add the meats.
The big bullet holds a lot of charcoal and can take a long time to get up to temp using a Minion with a few briqs using an ATC. Consider a mix of lump and briqs.
I'll have to dig my bullets out of the trailer to take a pic of my magnetic door prop mod for the Gen2 22" WSM's.
Hope this help in some little way.