First lets assume that all the gains are taxable.
Lets also go with the fact that gains are double your normal income.
Then yes you need to be concerned about owing a large payment in April next year.
You have two options regarding avoiding a penalty:
- Make a quarterly payment.
- Make the safe harbor.
The safe harbor requires less knowledge of what your taxes will be next April. The idea is that if your withholding this year, and any tax payments (like quarterly payments) exceed your tax responsibility on the tax forms you submitted in April of this year, then you can avoid underpayment penalties.
From IRS pub 505
In most cases, you must pay estimated tax for 2018 if both of the
You expect to owe at least $1,000 in tax for 2018, after subtracting your withholding and refundable credits.
You expect your withholding and refundable credits to be less than the smaller of:
a. 90% of the tax to be shown on your 2018 tax return, or
b. 100% of the tax shown on your 2017 tax return. Your 2017 tax return must cover all 12 months.
Note. The percentages in (2a) or (2b) just listed may be different if
you are a farmer, fisherman, or higher income taxpayer. See Special
Rules , later.
Higher Income Taxpayers
If your AGI for 2017 was more than $150,000 ($75,000 if your filing
status for 2018 is married filing a separate return), substitute 110%
for 100% in (2b) under General Rule , later.
For 2017, AGI is the amount shown on Form 1040, line 37; Form 1040A,
line 21; and Form 1040EZ, line 4.
So if you adjust your withholding for the rest of the year to make sure that you have 110% of the Total Tax on your previous years form. Calculate how much as been withheld so far, estimate how much will be withheld the rest of the year, and if you need to have additional money withheld submit a new W-4. Make very sure you make the safe harbor. If you do then you will avaoid a tax penalty.
Of course you still have to make sure you can pay what you owe in April of next year.