I forgot to add a 1099-Int that had a significant interest amount.

[Penguin0319]

I just found a 1099-INT form that was over $1,000, that I forgot to add to my 2020 taxes.  

I already filed my taxes and had to pay on federal (complete). 

I am still waiting for the state to receive my forms and issue a refund .  What do I do?

[SweetieJean]

The IRS might adjust for this themselves, but it could take time.  Come back at the end of the month, when the 1040-X amended return form is ready here.

[Penguin0319]

Since my Fed return was already processed (I owed and paid), is it best to wait for the State return to process completely and amend it within this year (TurboTax will allow me to start the amendment process now)?

[JohnB5677]

The 1040X (Amended Return) Form will be available on 02/25/2021. 

You can do an amended return starting from that date, but right now the IRS is deeply under water.

It may be worth contacting the IRS for some guidance.  You can call toll-free at 1-800-829-1040 or visit a Taxpayer Assistance Center Local IRS Office

The staffing of the IRS is likely going to cause a lot of delays for early tax filers. The reason is simple: with many IRS workers stuck at home or sick due to the pandemic, there will be delays.

Combine that with the additional stimulus checks (which require staffing and programming to send out), and the last minute tax changes that passed in December (which also require programming and testing), the IRS is stretched to the limits.

In 2021, the IRS will began e-File on February 12, 2021. That's the latest start to tax season we've ever seen.

However, if you're mailing documents to the IRS, expect massive delays. It took months and months to clear through the backlog in 2020, and the IRS is still delayed on taking care of mailed returns.