The storm is on the radar. You have 30 minutes. Here's exactly what to do, in order of impact.
Most flash-flood damage inside a home is preventable in the last half-hour before the rain arrives. But only if you spend those 30 minutes on the right things. Most homeowners waste the first 20 minutes on low-impact work — taping windows, filling bathtubs, moving cars — and don't get to the two or three actions that actually matter until water is already at the door.
This is a minute-by-minute drill written in impact order. If you get interrupted after ten minutes, the first ten minutes have already reduced your risk more than the last twenty. Every step assumes you're a homeowner in a residential neighborhood watching a flash-flood watch or warning move toward you on radar.
Before you start: know what alert you're under
A flash flood watch means conditions are favorable — you have time to prepare. A flash flood warning means flooding is imminent or already occurring — skip anything below that requires leaving the property. A flash flood emergency is life-threatening and rare; shelter high, do not attempt any of this. Full breakdown: flash flood watch vs. warning vs. emergency.
Minutes 0–5: The two actions that reduce risk the most
If you only have five minutes, do these two things.
1. Clear the storm drain closest to your property (2 minutes)
Grab work gloves. Walk to the storm drain on the curb line nearest your house. Physically pull off any leaf mat, litter, plastic bags, sediment, or debris covering the grate. If a drain is 30% blocked at the start of a storm, it will be 100% blocked within ten minutes of heavy rain — and water that can't drain will pool at the low point of your street, which is usually near a driveway, walkway, or garage.
This is the single highest-leverage act you can take before a flash flood, because it protects not just your house but the four to six houses upstream and downstream of you. It is also the one most homeowners skip because it feels like the town's problem. It is not. On a rain-rate day of 1.5 inches per hour, whether that drain flows or clogs is the difference between water on the street and water in your garage.
2. Move your car uphill or to a garage (3 minutes)
Not to protect the car, but to protect you. Cars stranded in flood water are the largest single source of flash-flood fatalities in the United States — more than half of flash-flood deaths involve vehicles, according to National Weather Service data. Move your vehicle before you have to think about whether it's safe to drive somewhere else. Two feet of moving water will float a mid-size SUV. You cannot judge depth from a windshield.
If your driveway slopes down to your garage door, this is the most important step in this entire article and it is worth arguing with. Yes, your car "belongs" in the garage. If the garage is at the low point of your property, park at the high point of the block and walk back. Then read the next section.
Minutes 5–15: The failure points you can defend
Flood water gets into houses through predictable places. In order of most-to-least common:
- Attached-garage doors that face downhill. The bottom seal of a garage door is designed to keep out weather, not water. A one-inch gap under a 16-foot garage door will admit hundreds of gallons per hour if water is pooling against it.
- Exterior doors on the ground floor. Especially side doors, sliding patio doors, and walk-out basement doors where the exterior grade is at or above the interior floor level.
- Window wells around basement egress windows. If a window well fills with water faster than it can drain, the window becomes the failure point — not because the glass breaks, but because the seal doesn't hold against standing water.
- Sump-pump discharge lines that dump water right back against the foundation. A pump that moves 2,000 gallons per hour is useless if the water it discharges runs 15 feet and then drains right back into the well it just left.
- Floor drains and toilets on the lowest level of the house. During surge events, water can come up through the drain system if street storm sewers back up.
3. Position physical flood protection at the doors and low windows most exposed (10 minutes)
Every home has different failure points, but the drill is the same. Look at your property from the street and identify the two or three lowest openings that face uphill or the direction water will come from. Stage protection there, in this priority order:
- Garage door(s) facing downhill or on the low side of the driveway. This is almost always the first place water enters.
- Any exterior door with a threshold that's flush with the exterior grade or lower.
- Sliding patio doors and French doors on ground level.
- Basement egress window wells that aren't already covered.
Traditional sandbags take 45 to 60 minutes to fill, weigh 30 to 40 pounds each, and produce sand contamination that has to be disposed of after the event. If you have them pre-filled and staged, great — place them now. If you don't, you are almost certainly out of time.
Expandable StormBag sandbags are designed for this window. Each bag weighs about a pound dry, expands to 33 pounds in under three minutes with fresh water from any hose, and stacks like a traditional sandbag to build a six-inch hydrated barrier. A 10-pack ($99.99) covers a single garage door or sliding patio door. If you're not sure how many you need for your specific setup — door width, expected water depth — our sandbag calculator handles the math in about 15 seconds. If you're comparing options against traditional sandbags or considering alternatives, our sandbag alternatives page covers eight ranked options with their tradeoffs.
If you have no physical barrier and no time to source one, roll up towels or blankets against the base of the most vulnerable door as a stopgap. It won't hold back a real flood, but it will delay water intrusion by five to ten minutes, which is often the difference between damp carpet and a flooded finished basement.
Minutes 15–25: Protect what's inside
Once the perimeter is as defended as it's going to get, move to what can be moved off the floor.
4. Elevate anything valuable on the lowest floor (7 minutes)
Priorities:
- Electronics on the floor or on low shelves. Computers, gaming consoles, media systems, routers, network switches, surge protectors. Get them onto a table or higher shelf.
- Photographs, documents, and anything irreplaceable. If you have paper records or heirlooms stored in a basement, put them on a top shelf or bring them upstairs. This includes tax returns, birth certificates, passports, and family photos not yet digitized.
- Boxes stored directly on concrete floors. Corrugated cardboard wicks water aggressively; once wet, everything inside is contaminated even after the water is gone.
- Rugs on the lowest floor. If they're small enough to roll and carry, do it. If not, at least lift a corner onto a chair — a rug soaked to its backing is almost always a total loss.
5. Verify sump pump and battery backup, then move to unplug (3 minutes)
If you have a sump pump, listen for it. It should be cycling normally — not running continuously (means it's undersized for what's coming), not silent (means it's failed or the water hasn't reached it yet). Confirm the discharge line is unblocked and directs water at least 10 feet from the foundation, ideally toward a downhill grade.
If you have a battery backup for the pump, verify the indicator light is green. If it's red or off, you're one power outage away from the pump being decorative. Even a moderate thunderstorm event will knock power out to a percentage of houses.
Finally, unplug any non-essential appliances or electronics that would sit in water if the basement floods. Chest freezers, dehumidifiers, workshop tools, sump pumps for finished-basement sinks. You want them off the circuit before they're wet, both to prevent damage and to prevent electrical hazards when you're wading through to clean up after.
Minutes 25–30: The last five minutes
6. Charge everything, gather essentials, stage a "go" bag (5 minutes)
By this point the perimeter is defended, valuables are elevated, and the sump-pump system is verified. Use the last five minutes for personal readiness:
- Plug in phones, laptops, tablets, and any medical devices to top them up. Scattered power outages are the norm in flash-flood events.
- Fill any pitchers, water bottles, or clean containers with drinking water from the tap. If a boil-water advisory follows the event (common when treatment plants get overwhelmed), you'll want a day's worth of unadulterated water ready.
- Move flashlights, headlamps, and battery lanterns to accessible spots — not the basement.
- Stage a "go" bag near the front door: prescription meds, wallet, keys, phone charger, a change of clothes, and any pet essentials. You almost certainly won't need it. But if you do, you'll want it ready in 30 seconds, not 30 minutes.
- Bring pets to the highest habitable floor of your house before the storm hits. Basements are the worst place for a scared dog during a flash flood.
What NOT to do in the last 30 minutes
These are common actions that use time without meaningfully reducing risk. Skip them.
- Taping windows. Tape does nothing to prevent breakage. It just makes cleanup harder if the window fails.
- Filling the bathtub with water. This is preparation for a multi-day loss of utilities, not for a flash-flood event. Storm-related bathtub water is contaminated water anyway.
- Driving somewhere for supplies. If the storm is 30 minutes out, the roads are already getting bad. Any run to a hardware store, gas station, or grocery is now high-risk for low return. Whatever you don't have, do without.
- Photographing your house for insurance. Insurance photos are useful before the season, not during a warning. Do them in July, not the day of.
- Reading Twitter, Facebook, or Nextdoor for local flood reports. The information density is too low and the time cost is too high. Follow the NWS office for your area and nothing else.
Once the rain starts
Stop the outdoor work and stay inside. Nothing on the checklist above is worth the risk of being outside during a severe thunderstorm with lightning, hail, and potentially a spin-up tornado. Move to an interior room on the lowest safe floor (highest floor if flooding is your main concern, lowest interior room if tornadoes are on the threat list — the two guidance sets conflict, and you have to pick based on your specific risk profile).
Watch official alerts on your phone. The Flood Watch tool aggregates active NWS flood watches, warnings, and advisories by state and county in one view, updated in real time.
After the event: the first 24 hours
If water entered the house:
- Photograph everything before you clean anything. Insurance claims move faster with time-stamped photos of the damage in place. Include serial numbers of ruined appliances and electronics.
- Do not use grid electricity in wet areas. Wait for a licensed electrician to verify the panel and any affected circuits before restoring power to a flooded floor.
- Discard anything that sat in flood water and can't be sanitized. This includes upholstered furniture, mattresses, particleboard cabinetry, and food (including canned goods that were submerged — the can seals fail invisibly).
- Start drying within 24 hours. Mold begins colonizing within 48 hours in wet materials. Open windows, run fans, get commercial dehumidifiers if you can source them, and pull baseboards and lower drywall as soon as possible to expose the wall cavity.
- File your insurance claim within 60 days. Standard homeowners insurance does not cover flood damage — only the National Flood Insurance Program does. If you don't have flood insurance, this event is the reminder to get it now; there is a 30-day waiting period on new NFIP policies, per the FEMA National Flood Insurance Program.
The bigger point: 30 minutes is late
Everything above is what to do when the storm is 30 minutes out. But most of these actions are more effective done hours or days in advance. The homeowner who cleared storm drains yesterday, staged sandbags last month, and bought a battery backup for the sump pump in April is not doing this drill on radar day. They are watching the storm from a house that's already prepared.
If you found this checklist because a storm is approaching, run it. If you found it before a storm, use it to identify the gaps and close them in July, not September:
- Our FEMA flood preparedness checklist is the long-horizon version — six months to a year of prep, organized by season.
- The sandbag calculator tells you the exact quantity of bags you need for your specific setup, so you're not guessing at 5 PM.
- The sandbag alternatives page compares eight ranked options with their tradeoffs, USACE test data, and use cases.
- The Flood Watch tool lets you monitor active NWS alerts in your state — useful for setting up your own advance warning.
StormBag is FEMA and DHS approved and made in Chico, California. We built the company around the premise that flood prep should be done before a watch is issued, not during one. But if the watch is already up, the next 30 minutes matter more than the last 30 days you didn't spend preparing.
Frequently asked questions
How much rain does it take to cause a flash flood?
It varies by geography, but in most U.S. urban and suburban areas, rainfall rates of 1 inch per hour or greater sustained for more than 30 minutes will produce flash flooding. Areas with saturated soil, extensive impervious surfaces, or steep terrain flood at lower thresholds.
Can StormBags be deployed in the last 30 minutes before a storm?
Yes. Each bag weighs about a pound dry and expands to 33 pounds in under three minutes when hydrated with fresh water from a hose. A stack of 10 bags across a garage door can be staged and hydrated in about 10 to 15 minutes. StormBag cannot be hydrated in salt water, but will work to repel salt water once they are hydrated with fresh water.
What if I don't have any physical flood protection at home?
Use towels and blankets rolled against the base of the most vulnerable exterior doors as a stopgap. This is not a real barrier — it will delay water intrusion by five to ten minutes at most — but it's better than nothing. Then, after the event, use it as the reminder to stage physical barriers before the next storm.
Should I evacuate for a flash flood watch?
A watch alone does not require evacuation. A warning — especially in low-lying areas, near creeks and rivers, or in known repeat-flood neighborhoods — may require moving to higher ground. Follow local emergency management guidance, not internet advice.
Does homeowners insurance cover flash-flood damage?
Almost never. Standard homeowners policies exclude flood damage from surface water regardless of source. Coverage requires a separate National Flood Insurance Program policy or a private flood insurance policy. NFIP has a 30-day waiting period on new policies.
Is 30 minutes really enough time to make a difference?
Yes, if you spend the time on the right things. Studies of post-flood damage repeatedly find that homes where owners cleared drains, staged physical barriers, and elevated valuables in the last hour before a storm sustain significantly less damage than homes where owners spent that hour on lower-impact tasks. The type of preparation matters as much as the quantity.
