A flood claim goes to your NFIP or private flood carrier, not to your homeowners insurer — they are separate policies and a homeowners claim will be denied for flood water. Report it the day you find it. NFIP requires a signed Proof of Loss, normally within 60 days of the loss, and the documentation you gather in the first hours is what it is built from: video of every affected room before anything is moved, photographs of the high-water line, a list of damaged contents with what you paid, and receipts for emergency mitigation.
Missouri homeowners in Joplin should review their policy for wind-driven rain exclusions, as severe thunderstorm damage that breaches the building envelope may be denied if the primary cause is determined to be wind rather than water intrusion. Mitigation is not optional — every policy requires you to stop further damage, so drying starts before the adjuster arrives, and you keep the invoices.