Blue Alert rain is a type of extreme rainfall event that causes widespread flooding. It gets its name from the emergency alerts that are often issued when severe thunderstorms are forecast to produce heavy downpours over a large area.
What causes Blue Alert rain?
Blue Alert rain is caused by slow-moving thunderstorms that stall over an area and dump copious amounts of rain. This can happen when the atmosphere is very moist and unstable, allowing storms to repeatedly develop and redevelop over the same region. Certain weather patterns can also help lock a storm system in place. Here are some of the ingredients that come together to cause a Blue Alert rain event:
- Warm, humid air – Provides moisture that storms need to produce heavy rainfall.
- Atmospheric instability – Allows thunderstorms to readily build and strengthen.
- Weak winds aloft – Prevents storms from pushing through, causing them to remain nearly stationary.
- Fronts or other boundaries – Can focus storm development in one area.
- Orographic lift – Terrain like mountains can enhance rainfall when moist air is forced upslope.
How much rain falls during a Blue Alert event?
To qualify as a Blue Alert rain event, an area must receive at least 6 inches of rain over a 12-hour period, or 8 inches over a 24-hour period. However, totals often exceed these criteria, with over a foot of rain commonly reported. The highest ever recorded was 43 inches of rain in just 24 hours during Tropical Storm Claudette in 1979.
|Blue Alert Rain Event
|Tropical Storm Claudette
|43 inches in 24 hours
|July 25, 1979
|60.58 inches over 5 days
|August 25-29, 2017
|May 2010 Tennessee floods
|19.41 inches in 24 hours
|May 1-2, 2010
|14.17 inches in 12 hours
|Morgantown, West Virginia
|August 27-28, 2011
Where do Blue Alert rain events occur?
Blue Alert heavy rain can occur anywhere, but tends to be more common in certain regions:
- Gulf Coast – Warm waters provide moisture for storms and fronts can stall over the region.
- Central U.S. – Colliding air masses and abundant moisture feed heavy thunderstorms.
- East Coast – Tropical systems like hurricanes and their remnants often produce extreme rainfall.
- Mountain West – Orographic lift squeezes out heavy precipitation.
The Gulf Coast is prime territory for Blue Alert rain events. Slow moving tropical storms and hurricanes often dump tremendous rainfall totals as they drift along the coast or inland from the Gulf of Mexico. Warm ocean water provides storms with a nearly unlimited supply of moisture. Stalling fronts are also common in this region.
Central United States
Clashing air masses, plentiful moisture, and severe thunderstorms contribute to prolific rainfall in the Central Plains and Midwest. Tornado alley overlaps heavily with the regions most prone to extreme rainfall. Storm systems can repeatedly develop and backbuild over the same areas.
From the Mid-Atlantic up through New England, tropical systems are a major cause of extreme rain events. Hurricanes and their remnants often slink up the coast, wringing out copious precipitation. Nor’easters can also produce phenomenal rainfall, especially when they tap tropical moisture.
Orographic lift helps enhance rainfall where storms run into mountain ranges like the Rockies, Sierras, and Cascades. Upward vertical motion as air ascends slopes can rapidly squeeze moisture out of storm systems as they move inland from the Pacific Ocean.
Impacts of Blue Alert rain
The excessive rainfall produced during Blue Alert events leads to major impacts:
- Widespread flooding – Streets, homes, and infrastructure are inundated with water.
- River flooding – Streams and rivers spill over their banks, flooding adjacent areas.
- Landslides & debris flows – Saturated soil gives way on steep slopes.
- Delayed planting & harvesting – Fields remain too soaked for farming equipment or crops rot.
- Infrastructure damage – Roads, bridges, and utilities are damaged or disrupted.
- Loss of life – People drown in floodwaters or are killed in landslides.
The financial costs and loss of life can be substantial. Flooding is the #1 weather-related killer in the United States. Both Hurricane Harvey and the May 2010 Tennessee floods caused over $10 billion in damage each.
How to prepare for Blue Alert rain
Here are some tips to prepare for extreme rainfall that could accompany a Blue Alert:
- Monitor weather alerts and stay attuned to heavy rain forecasts.
- Have an emergency kit ready with supplies like water, non-perishable food, and medications.
- Identify and move valuables to upper floors or elevated surfaces.
- Clear storm drains and gutters so they don’t get clogged.
- Review flood evacuation routes and identify higher ground.
- Keep important documents secured in waterproof containers.
- Ensure sump pumps are functioning properly.
- Reinforce basement walls and seals.
Following evacuation orders is also essential during major flood events. Turn around, don’t drown when encountering flooded roads.
Blue Alert rain events are extreme heavy rainfall situations that can bring widespread flooding. A variety of ingredients like ample moisture, atmospheric instability, and weak steering winds come together to cause storms to repeatedly develop and unleash torrential downpours over the same areas. Gulf and Atlantic tropical systems, Midwest thunderstorms, and upslope-enhanced mountain rainfall all contribute to the most prolific Blue Alert rain events around the country. Advanced preparation and proper safety precautions are key to staying safe when extreme flooding rainfall is forecast.