According to the IRS Publication 547, under "Casualty", losses are deductible
during the taxable year that the loss occurred. You will find that page 14 and 15 is most helpful as it has information directly referring to Hurricane Harvey.
You can find more information here:
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p547.pdf">https://www.irs.gov/pub/irs-pdf/p547.pdf</a>
