I would like to ask a question but I am concerned about it and would prefer to ask it anonymously. Is this possible?
Yes, it is possible, but you must be careful.
There is no way to have a post that belongs to your account but is not publicly attached to it. All the content produced by an account is publicly linked to it. So if you want to make a publicly anonymous post, you cannot do it from your main account.
Stack Exchange doesn't need to know about your identity elsewhere. It needs to identify you only to keep track of the posts and other content that your account owns. You can create an account purely to post this question.
Be careful not to communicate anything about your “real” identity. For example, don't use an existing OpenID (use Stack Exchange's own), and provide a dedicated email address. Note that by default your avatar is a hash of your email address (and if you don't enter an email address, your IP address is used), so use a non-identifying email address. You don't need to use a valid email if you don't care about recovering a lost password.
If you don't care to retain any ownership of the question, you can post it with a “throwaway” account. Just post without logging in (presumably from Tor Browser), enter a bogus email, and the account will become unreachable as soon as you clear the associated cookie.
Don't log in with your two accounts at the same time in the same browser. I recommend using two different browser profiles. If you just have the one “anonymous account”, use a regular browser for your regular account and Tor Browser for the anonymous account. If you want to juggle multiple anonymous accounts, make sure to log out from the Stack Exchange questions and answers sites and from the OpenId provider.
It is permitted to have more than one account, but you must make sure that the two accounts never interact. Don't vote for one another, don't use both accounts to vote on the same post, and so on.
If tor.stackexchange will see soon an anonymous post, there won't be an anonymity set in which you can hide. And even if you could, since you asked here, you're the main suspect. Probably won't matter for a forum post here, just useful to thoroughly think through it.
Needless to say, when it's about anonymity questions, it's useful to think about the advice given in all answers, not only the highest voted one. (Not saying everyone is always right and you have to do everything.) For that reason, I won't repeat what others already mentioned.
My advice is to hide in the probably upcoming crowd of new users. Just register an anonymous e-mail account over Tor, never link to any real life stuff (such as your phone), create a separate account, ask your question. I wouldn't delete the account afterwards, there is no gain. Just don't use it anymore if that is what you wish (depending on your threat model).