Precipitation Variability

Heavy Rain Storms

Among the expected consequences of global warming is an increase in the heaviest rain and snow storms, fueled by increased evaporation and the ability of a warmer atmosphere to hold more moisture. Extreme downpours are now happening 30 percent more often nationwide than in 1948. Moreover, the largest annual storms now produce 10 percent more precipitation, on average.

Flooding_Mt Washington Precipitation Flood Baltimore Sun

According to the U.S. Global Change Research Program, “heavy downpours that are now 1-in-20 year occurrences are projected to occur about every 4 to 15 years by the end of this century,” while producing 10 to 25 percent more precipitation per storm. Increase in heavy downpours is “one of the clearest precipitation trends in the United States” and is linked to global warming.


A Nor’easter is a type of storm that forms from a strong low pressure system in the Eastern United States. Cold air from the North, primarily from Canada, will often clash with the warm air masses that form over the ocean and from the Gulf Stream in the coastal areas. While most people associate Nor’easter storms with winter, these storms can actually form any time of the year.

These storms often move very slowly causing heavy amounts of precipitation. In the winter, the heavy snowfall and blizzard-like conditions cause icy road conditions, power outages, and other winter weather hazards. Nor’easter storms are also dangerous in Baltimore due to the potential for large wind-driven storm-surge waves and heavy precipitation that can cause flooding.

Hail and Ice Storms

Ice Storm 1994_maryland weatherAn ice storm is a type of winter storm caused by freezing rain. Ice storms form when a layer of warm air is between two layers of cold air. Frozen precipitation melts while falling into the warm air layer, and then proceeds to refreeze in the cold layer above the ground. This creates freezing rain or a glaze of ice. Warmer air in the winter months increases the possibility of a dangerous ice storms in Baltimore City.
OLYMPUS DIGITAL CAMERAHail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted higher and higher into the sky until they freeze into ice.  According to NOAA, hail causes $1 billion dollars in damage to crops and property each year.

Winter Storms

Winter storms in Baltimore usually form when an air mass of cold, dry, Canadian air moves south and interacts with a warm, moist air mass moving north from the Gulf of Mexico.


As the Earth gets warmer and more moisture gets absorbed into the atmosphere, we are steadily loading the dice in favor of more extreme storms in all seasons, capable of causing greater impacts on society. Large snowstorms that happened once every 12 months, on average, in the middle of the 20th century now happen every nine months. In fact, in each of the past two winters the northeastern United States has been hammered by three snowstorms that qualified as Category 3 storms or worse on the Northeast Snowfall Impact Scale.


The U.S. Army Corps of Engineers’ defines drought as “periods of time when natural or managed water systems do not provide enough water to meet established human and environmental uses because of natural shortfalls in precipitation or stream flow”.

From 2001-2002 the State of Maryland experienced one of the most severe droughts in the region’s history. The water levels in Baltimore’s three reservoirs slipped by more than 4 billion gallons — from 75 percent of capacity to 69 percent. The city even tested the pumps to evaluate moving Susquehanna water to Baltimore.

Reservoirs are designed to provide adequate storage when demand exceeds reservoir inflow. As the streamflows are lowest during the summer period and demand is also greatest, the most critical time begins at the onset of summer. Baltimore has three reservoirs that are monitored for the purposes of evaluating drought conditions: Loch Raven Reservoir, Prettyboy Reservoir, and Liberty Reservoir.

Although drought is not a major hazard impacting Baltimore City, it is important to understand how drought could impact the City and surrounding areas in the future. Drought will significantly affected the Chesapeake Bay ecosystem and the plants and animals that live in and rely on the bay. Drought conditions often provide too little water to support food crops which increases food prices and reduces food availability. Drought also sets the stage for wildfires that may cause injuries or deaths aswell as extensive damage to property.

drought outlook_2013


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