Aftermath of the “Big Rain”

The summer started out hot and dry. July rainfall was just over an inch below the seasonal average. 24 days hit 90 degrees or hotter and three days recorded temperatures in the low 100s.

August flipped on rainfall, recording 10.38 inches at BWI-Marshall (7.09 inches above average) and so far September has followed suit in the first half of the month with 9.38 inches of rain at BWI-Marshall (7.77 inches above average) with some areas recording more than 11 inches of rain.

The most recent rains have pushed the 2011 precipitation total to 42.32 inches, 12.86 inches above average.

Last week’s rain totals from the remnants of Tropical Storm Lee were the most spectacular. The National Weather Service in Sterling, VA reported that the Bowie area recorded 4.57 inches of rain in 3 hours on Wednesday, September 7. That is a once in 200 year event. The rainfall in Ellicott City and Upper Marlboro on that day was a 50-100 year event. Westview rainfall was a 10-25 year event.

Two Weeks of Rain Swamps Eastern U.S.

From Hurricane Irene to Tropical Storm Lee, the Eastern U.S. has seen a seemingly endless stream of rainfall over the past two weeks. This image plots all rainfall from August 26 – September 9, 2011, as analyzed by the National Weather Service’s River Forecast Centers. The measurements use a combination of radar, rain gauges and satellite rainfall estimates. The largest rainfall totals, almost 3 feet so far, are seen in Northern Virginia through New York. More rainfall is expected throughout the weekend, possibly up to 7 inches. Flooding throughout the Susquehanna River and watershed has prompted the president to declare states of emergency in Pennsylvania. The Susquehanna crested today at around 39 feet. At least 5 deaths have been attributed to the flooding. (Source: NOAA)

The NOAA image above shows the rainfall concentrations over a two week period. The image below shows the impact all of that rain had on the Chesapeake Bay.

Rain and Runoff in the Chesapeake

The Chesapeake Bay is part of the largest watershed in the Northeast U.S. It receives river and stream input from much of Central New York through Virginia, including the Susquehanna and Potomac Rivers. Over the past two weeks, this same region has received in excess of 32” of rain in some areas. The Susquehanna River experienced record flood levels. All of this rain and runoff eventually has made its way into the Chesapeake Bay. The result can be seen here in this total suspended matter data acquired by the NASA Aqua satellite and processed by NOAA CoastWatch. Before (August 23rd) and after (September 11th) images show stark differences in the amount of suspended matter (silt, mud, debris) found in the waters of the Chesapeake and its tributaries. Water conditions caused the modification of the Nation’s Triathlon held in DC on September 11th due to poor water quality, currents, and debris.

Suspended matter poses a hazard to the Chesapeake Bay by reducing the Bay’s water quality. Oysters that normally filter the water can be smothered by the sediment. In addition, sediment reduces the light needed by sea-grass beds, which ordinarily provide nursery grounds for many species of fish and crabs. The restoration of oysters and sea grasses has been a priority in efforts to revive the health of the Bay, but catastrophic events such as those brought by Tropical Storm Lee hamper these efforts. Management practices that will bring about long-term reduction in sediment and nutrient pollution from upstream agricultural and urban/suburban sources, will have a positive impact on the Bay by making the Bay more resilient when catastrophic events occur. (Source: NOAA)

Most of this excessive rain came from tropical moisture, some of it related to tropical cyclones. These storms have an interesting effect on the ocean waters that they pass over. tropical storms and hurricanes thrive on warm water but as a storm passes over the water, the sea surface temperature cools down as demonstrated in the NOAA image below.

The Wake of Katia and Nate

Hurricanes cool ocean temperatures by absorbing energy from the water and by churning up colder water from below. There is no better way to show this process than by looking at the sea surface temperature after a storm. In this case, the image shown here compares the satellite sea surface temperature data from September 12, 2011 to the average for that day over the 1981-2000 time period. The signatures of Hurricane Katia and Tropical Storm Nate are clearly visible as areas of colder than normal water in areas that are otherwise warmer than average. Similar features can also be seen in the actual sea surface temperature data from the same day (below). (Source: NOAA)

The hurricane season is still in full swing, although there is only one active storm in the Atlantic Basin at this time.

A slight shift in the weather pattern could allow for an increase in tropical activity and additional opportunities for soaking rains. Stay tuned.

John Collins



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