Your phone buzzes. A weather alert. You glance at it, maybe read the first few words, and then go about your day — because honestly, how bad can it be?

Every day, meteorologists at National Weather Service forecast offices across the country monitor the atmosphere, track developing storms and issue alerts designed to give the public the time and information needed to stay safe. These alerts, advisories, watches and warnings (all of which you can sign up for here) form a tiered system to communicate not just what is coming, but how serious it is and how quickly you need to act.

This guide breaks down every major weather alert issued in the U.S. — what each one means, why it was issued and exactly what you should do when you receive one. Because the next time your phone buzzes, you'll want to know whether to grab an umbrella or head for the basement.

The big three: Advisory, watch, warning 

What's the difference between a weather advisory, a watch, and a warning?

The National Weather Service uses three alert levels - think of them as levels of urgency. 

• Advisory: Be aware. Conditions will be inconvenient and potentially hazardous. Use caution. 
• Watch: Be prepared. Dangerous weather is possible in your area within 24-48 hours. Have a plan ready in case the situation worsens. 
• Warning: Take action. A hazardous weather event is occurring in your area. 

Severe weather

• Severe thunderstorm
• • Watch: Severe thunderstorms capable of producing damaging winds and large hail are possible in your area. Conditions are coming together for these storms to fire up. So stay alert, have a plan and keep an eye out for updates.
  • Warning: A severe thunderstorm is actively happening or headed your way, packing winds of 58 mph or stronger and/or hail the size of a quarter or larger. Get indoors and away from windows.
• Tornado:
• • Watch: Conditions are favorable for tornado development. It hasn’t happened yet, but now is the time to prepare and know where to seek shelter. 
  • Warning: A tornado has been spotted or confirmed on radar and is an immediate threat to life and property. Seek shelter now in an interior room on the lowest floor of a sturdy building, away from windows.
  • Emergency: The highest tornado alert the NWS issues — reserved for a confirmed, violent tornado posing an imminent threat to life in a heavily populated area. This is extremely rare, but when it is issued your phone will automatically alarm. You should already be sheltered. Do not wait for this alert to take action.

Winter & cold weather

• Winter storm:
• • Watch: A significant winter storm is possible in your area within the next 24-48 hours. Now is the time to stock up, plan ahead, and avoid unnecessary travel. 
  • Warning: A dangerous winter storm is occurring, bringing some combination of heavy snow, sleet, and/or freezing rain that will make travel dangerous or impossible. Stay off the roads and stay indoors until conditions improve.

• Blizzard warning: 
• • A blizzard is occurring or expected within the next 12 to 18 hours. Blinding snow and sustained winds of 35 mph or greater will reduce visibility to a quarter mile or less, making travel life-threatening. Stay home. This is not a storm to drive through. 
• Ice storm warning:
• • An ice storm is occurring or expected, bringing ¼ inch or more of ice accumulation. That may not sound like much,  but a thin glaze of ice can make roads virtually impassable, snap tree limbs and knock out power for days. Stay home if at all possible.
• Winter weather advisory:
• • Expect freezing rain or 2 to 4 inches of snow, sleet or a combination of both — enough to make travel slippery and slow but not quite reaching warning levels. Give yourself extra time, use extra caution on the roads and consider delaying travel if possible.
• Cold weather advisory: 
• • Dangerously cold temperatures or wind chills are expected that could be hazardous with prolonged exposure. Temperature thresholds vary by region. Bundle up, limit time outdoors and check on elderly neighbors and family members.
• Extreme cold warning: 
• • Life-threatening cold temperatures or wind chills are occurring or expected. Exposed skin can develop frostbite in a matter of minutes. Stay indoors, dress in layers if you must go outside and never take this alert lightly — extreme cold kills.
• Frost advisory: 
• • Temperatures are expected to drop between 33 and 36 degrees on clear, calm nights during the growing season. Not a hard freeze — but cold enough to damage sensitive plants and crops. Cover vegetation and protect outdoor plants overnight.
• Freeze:
• • Watch: Significant and widespread freezing temperatures are possible within the next 24 to 36 hours during the growing season. Now is the time to prepare — cover plants, protect exposed pipes and plan to bring sensitive vegetation indoors before the cold arrives.
  • Warning: Significant and widespread freezing temperatures are expected during the growing season — most commonly issued in fall before the first hard freeze or in spring when new plants and crops are most vulnerable. Protect vegetation, cover exposed pipes, and bring sensitive plants indoors now.

Flooding & water hazards

• Flash flood:
• • Warning: Flash flooding is occurring. This is one of the deadliest weather hazards in the U.S., as flood waters can rise in minutes with little to no warning. Never drive through flooded roads. Turn around, don't drown.
  • Emergency: The highest flash flood alert the NWS issues — reserved for an exceedingly rare and catastrophic flash flooding situation that poses an unprecedented threat to human life. If this alert hits your phone, move to higher ground immediately. Do not wait, do not drive, do not hesitate.
• Flood:
• • Watch: Flooding is possible in your area. Unlike a flash flood, this type of flooding develops more gradually, but that doesn't make it any less dangerous. Stay informed and be ready to act.
  • Warning: Flooding is occurring in your area. Stay off flooded roads, avoid flood-prone areas and follow any evacuation orders issued by local authorities immediately.
• River flood:
• • Watch: River flooding is possible at one or more points along a river in your area. If you live or work near a river, now is the time to monitor updates closely and know your evacuation options.
  • Warning: A river is flooding or expected to flood at one or more points along its path. Stay away from riverbanks, avoid low-lying areas nearby, and follow any evacuation orders immediately.
• Coastal flooding:
• • Advisory: Minor coastal flooding is occurring, enough to flood low-lying roads and properties near the coast. Use caution in affected areas.
  • Watch: Moderate to major coastal flooding is possible and could pose a serious risk to life and property. If you live or work in a coastal flood-prone area, now is the time to prepare and know your evacuation options.
  • Warning: Moderate to major coastal flooding is occurring and poses a serious threat to life and property. Follow evacuation orders immediately and stay out of flooded areas.

Tropical

• Tropical storm 
• • Watch: A tropical storm may threaten your area within the next 48 hours, bringing sustained winds of 39 to 73 mph along with the potential for storm surge, coastal flooding, and river flooding. It's not certain yet, but now is the time to prepare, not wait.
  • Warning: Sustained winds of 39 to 73 mph from a tropical storm are expected within the next 36 hours, along with potential storm surge and flooding. Complete your preparations now — once those winds arrive, it will be too late to act safely.
• Hurricane
• • Watch: A hurricane may threaten your area within the next 48 hours, bringing sustained winds of 74 mph or higher along with the potential for storm surge, coastal flooding and river flooding. This is not a guarantee but do not wait for a warning to start preparing. Finish your preparations, know your evacuation zone and be ready to move.
  • Warning: Sustained winds of 74 mph or higher are expected within the next 36 hours. Storm surge, coastal flooding and river flooding will likely accompany these winds. Do not wait to evacuate if ordered and know that this warning can remain in effect even after winds drop below hurricane force if dangerous flooding and high waves continue to pose a threat. When local authorities say evacuate, you should go. 
• Storm surge 
• • Watch: Life-threatening flooding from rising ocean water moving inland from the shoreline is possible within the next 48 hours. Storm surge is the deadliest part of a hurricane — a wall of ocean water that can submerge entire neighborhoods in a matter of hours. If you are in a watch area, do not wait for a warning to prepare or evacuate.
  • Warning: Life-threatening flooding from rising ocean water moving inland from the shoreline is expected within the next 36 hours. This is not flooding you can wait out — storm surge can rise faster than you can escape it. If you are ordered to evacuate, do so immediately. 

Heat & air quality

• Heat advisory:
• • Heat index values are expected to be hot and uncomfortable — below warning levels but still dangerous, especially for those who work outdoors, the elderly, and young children. Limit strenuous outdoor activity, stay hydrated and take breaks in the shade or air conditioning.
• Extreme heat:
• • Watch: Dangerously high heat index values are possible within the next 24 to 48 hours. The heat index can make conditions far more dangerous than the thermometer alone suggests. Stay hydrated, plan ahead and know where your nearest cooling center is.
  • Warning: Dangerously high heat index values are expected within the next 12 to 24 hours. Stay indoors in air conditioning, drink plenty of water and check on elderly neighbors, young children and pets.
• Air quality alert
• • Issued on behalf of state and local environmental agencies when air pollution reaches unhealthy levels — an Air Quality Index of 151 or higher. At these levels, everyone can experience health effects from prolonged exposure outdoors. Limit time outside, keep windows closed and avoid strenuous outdoor activity until the alert is lifted.

Fire weather & visibility

• Fire weather watch:
• • Conditions are becoming favorable for dangerous wildfire spread within the next 24 to 36 hours. A combination of low humidity, dry vegetation and strong winds can turn even a small spark into a fast-moving fire. If you live in a fire-prone area, now is the time to prepare.
• Red flag warning: 
• • Critical fire weather conditions are occurring or expected within the next 12 to 24 hours. Low humidity, strong winds and dry fuels create the perfect environment for wildfires to ignite and spread rapidly. Avoid any activity that could spark a fire and follow guidance from local fire officials.
• Dense smoke: 
• • Smoke from nearby wildfires is reducing visibility to ¼ mile or less. Similar to dense fog, except what you can't see through is smoke, not moisture. Limit time outdoors, keep windows closed, and wear a mask if you must go outside.
• Dense fog advisory: 
• • Widespread fog is reducing or expected to reduce visibility to ¼ mile or less for two hours or more. At that level of visibility, hazards can appear with almost no time to react. Slow down, use your low beam headlights, increase your following distance and consider delaying travel until conditions improve.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her masters in Earth and Atmospheric Science from the Georgia Institute of Technology.