Severe Weather

Severe weather occurs frequently in McKinney. Whether it is thunderstorms, lightning, extreme heat or winter weather, residents need to be prepared for all these hazards.

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Extreme Heat

Located near the urban Dallas / Fort Worth Metroplex, McKinney is at a greater risk from the effects of prolonged heat.

  • Very common occurrence for the summer months of June through September, but can be recorded as early as April and late into November. 
  • Extreme heat is generally defined as temperatures that hover 10 degrees or more above the average high temperature for the region and last for several weeks. 
  • Air quality conditions may also deteriorate due to trapped pollutants, such as ozone from the atmosphere or carbon monoxide from vehicles.
  • What to Do Before, During and After Extreme Heat | Spanish

The Heat Index takes the relative humidity into account, providing the "feels like" temperature.

Alert Levels

  • Excessive Heat Outlook: Issued when the potential exists for an excessive heat event in the next 3 to 7 days.
  • Excessive Heat Watch: Issued when conditions are favorable for an excessive heat event in the next 12 to 48 hours. A watch is used when the risk of an event has increased, but the timing and occurrence is still uncertain.
  • Excessive Heat Warning / Advisory: Issued when an excessive heat event is expected within the next 36 hours. These are used when an event is occurring, is imminent or has a very high probability of occurring. The warning is used for conditions posing a threat to life or property, while the advisory is used for less serious conditions that cause significant discomfort or inconvenience and if caution is not taken, could lead to a threat to life or property.
  • Ozone Health Advisory: Issued when ozone levels in outdoor air are expected to be greater than 0.08 parts per million when averaged over an eight-hour period.


Flooding Hazards

A condition where two or more acres of normally dry land or two or more properties are inundated by water or mudflow. Flooding in McKinney can result from hurricane remnants, dam / levee failure, clogged drainage systems or rapid accumulation of rainfall.

Flood Insurance

The National Flood Insurance Program was created in 1968 by Congress to help provide a means for individuals to protect themselves financially since most homeowner's insurance does not cover flooding.

Flood Insurance Rate Maps (FIRMs) display the following flood risk categories:

  • High-risk areas | Special Flood Hazard Area)
  • Moderate-to-low risk areas | Non-Special Flood Hazard Area
  • Undetermined-risk areas

Visit Explore McKinney to see what areas in McKinney are at risk from flooding.

Flash Flood vs. Flood

  • Flash floods occur during heavy rain events and happens very quickly. They also end quickly,
  • Flooding occurs as the result of a more prolonged rain event or lake / creek overflow. It is gradual, more predictable and lasts longer.
  • What to Do Before, During and After a Flooding Event

Alert Levels

  • Flood Watch:  Issued when flooding is possible in a given area.
  • Flash Flood Watch: Issued when heavy rains and flash flooding are possible. You should be ready to move to higher ground immediately.
  • Flood Warning: Issued when flooding is occurring or is imminent. If you are advised to evacuate, do so immediately.
  • Flash Flood Warning: Issued when a flash flood is occurring and moving to higher ground is necessary.



Hail is produced when updrafts within a storm carry water droplets to a height in the sky where freezing occurs. These ice particles continue to grow as they circulate within a storm, allowing ice layers to build. When the ice, or hail, becomes too heavy for the storm to circulate, it falls to the ground. Hail is considered to be severe when it reaches a diameter of 0.75 inch or more. 

Hail occurs during severe thunderstorms and tornadoes so look and listen for warning information about these storms.

Estimating Hail Size

  • Pea = 1/4 inch diameter
  • Marble/Mothball = 1/2 inch diameter
  • Dime/Penny = 3/4 inch
  • Nickel = 7/8 inch
  • Quarter = 1 inch
  • Ping-pong ball = 1 ½ inches
  • Golf ball = 1 ¾ inches
  • Tennis Ball = 2 ½ inches
  • Baseball = 2 ¾ inches
  • Tea cup = 3 inches
  • Grapefruit = 4 inches
  • Softball = 4 ½ inches


Thunderstorms & Lightning

To generate a thunderstorm, the surrounding area needs moisture, unstable air and lift (such as a cold front). Thunderstorms have the capability to produce straight-line winds, large hail, flash floods and even tornadoes. 

A dry thunderstorm can occur when the rain produced by the storm does not reach the ground, but evaporates in the air. Lightning will still strike and reach the ground, potentially starting structural and wildfires.

The National Weather Service classifies a thunderstorm as severe if its winds reach or exceed 58 mph, it produces a tornado or if it drops surface hail at least one inch in diameter.

Lightning is present in all thunderstorms and results from the buildup and discharge of electrical energy between positively and negatively charged areas. 

  • A cloud-to-ground lightning strike begins as an invisible channel of electrically charged air moving from the cloud toward the ground. When one channel nears an object on the ground, a powerful surge of electricity from the ground moves upward to the cloud and produces a visible lightning strike.

Alert Levels

  • Severe Thunderstorm Watch: Issued when the conditions are favorable for a thunderstorm to develop. A watch will tell you when and where severe thunderstorms are likely to occur.
  • Severe Thunderstorm Warning: Issued when severe weather has been reported by spotters and indicated by radar. Warnings indicate imminent danger to life and property to those in the path of the storm.


Tornado Safety

Peak tornado season for McKinney is March - May, but they are still possible in the summer and fall. Tornadoes may strike quickly with little or no warning and may appear translucent until dust and debris are picked up or a cloud forms in the funnel.

Tornadoes spawn from strong thunderstorms, a tornado appears as a rotating funnel-shaped cloud that extends to the ground with whirling winds. Tornadoes are rated on the Enhanced Fujita (EF) Scale between 0 and 5.

Operational Enhanced Fujita Scale

EF Number3 Second Gust (MPH)
5Over 200


Go to the link below for sheltering-in-place actions to take during a tornado. 

Alert Level

  • Tornado Watch: Issued when the conditions are favorable for a tornado to be produced. Watch the sky for signs of the storm and stay tuned to weather and news sources
  • Tornado Warning: Issued when a tornado has been sighted by storm spotters or indicated on radar. Take shelter immediately when there is a tornado warning for the City of McKinney or Collin County.


Winter Weather

Although not common, winter weather does occasionally come to McKinney in the form of freezing rain, ice, snow and extreme cold (temperatures below freezing -32 °F).

Alert Levels

  • Winter Storm Watch: Winter storm conditions are possible within the next 36 to 48 hours. People in a watch area should review their winter storm plans and kits and stay informed about weather conditions
  • Winter Weather Advisory: Winter weather conditions are expected to cause significant inconveniences and may be hazardous. When caution is used, these situations should not be life threatening
  • Winter Storm Warning: Life-threatening, severe weather conditions have begun or will begin within 24 hours. People in a warning area should take precautions immediately
  • Blizzard Warning: Sustained winds or gusts of 35 miles per hour or greater and considerable amounts of snow (falling or blowing) are expected to occur for a period of three hours or longer
  • Frost / Freeze Warning: Below-freezing temperatures are expected
  • Wind Chill: The wind chill index is the temperature your body feels when the air temperature is combined with the wind speed