Hurricanes and tropical storms are notoriously prolific rain producers no matter where they strike. In the Mid-Atlantic, in particular, warm Gulf Stream ocean water ensures a vast supply of moisture. Mountains, just a couple hundred miles west, lift saturated air and wring out the moisture even more. Frontal systems and jet stream energy often combine with tropical remnants, further stoking the rainmaking machine. This is a geographical setup that has produced more than its share of flooding catastrophes over the years.
With Hurricane Florence walloping the Carolinas, and its potential to generate several inches of rain in the Mid-Atlantic early next week, it is useful to have some perspective on historically rainy storms in the past half-century. To start with, the following figure shows the all-time highest rain totals delivered to each state from a tropical cyclone (hurricane or tropical storm). For the purposes of this report, I include North Carolina in the Mid-Atlantic region.
Maryland’s “flood of record” was delivered by Hurricane Eloise in 1975 (more than 14 inches). In Virginia, the record flood from a hurricane came in 1969 from the remnants of Camille (27 inches). Pennsylvania’s storm of record was Agnes in 1972, which dumped up to 19 inches over several days. And in North Carolina, the record rainy storm was Floyd (24 inches) in 1999.
Below, we take a quick tour of the Top 10 most prolific tropical rainmakers dating from the late 1960s. I discuss them chronologically, starting in 2016, working back in time. Each image includes a detailed rainfall map, with the storm track and location of maximum rain accumulation. In the title introducing each image, I include the intensity of the storm either at landfall, or at the point closest to landfall. One of the key things to note: The amount of rain delivered by a storm is not well correlated with its intensity.
2016 Hurricane Matthew (Category 2)
This is a classic recurving storm track in the western Atlantic, with the eyewall just brushing the coastline along South Carolina. Yet note the impressive amount of rain, for a storm not officially making landfall, along the entire Southeast. The swath of heaviest rain (10 inches or more) is quite narrow, confined to the coastal plain of the Carolinas.
2011 Tropical Storm Lee
It’s amazing how much water a humble tropical storm can dump. What’s bizarre is that this storm barely grazed the Mid-Atlantic, approaching our region from the Ohio Valley. But how do we explain the huge swath of 10-plus inches running from Virginia through Pennsylvania … and up to 21 inches more than 100 miles east of the track? Lee’s remnant moisture combined with a front draped across the Mid-Atlantic, aided by orographic uplift of moisture and cold air damming along the Appalachians.
2011 Hurricane Irene (Category 1)
Another classic “coastal runner” with a long, narrow stripe of heavy rain extending from the Carolina coastal plain up the Delmarva and into the boot of New York.
2003 Hurricane Isabel (Category 2)
This storm took a rare track west of the Chesapeake Bay, then exited to the northwest across Lake Erie. Rain amounts were generally modest, thanks to the rapid forward speed of the storm, with one exception: Sherando, Va., along the steep slopes of the central Virginia Blue Ridge, where a small pocket saw 20 inches fall. Around the D.C. region, Isabel was known more for its fierce winds and saltwater storm surge that ran up the bay into the headwaters regions of Annapolis and Baltimore.
1999 Dennis (TS) and Floyd (Category 2)
We are treating these two storms together because they inundated the same region of coastal North Carolina less than two weeks apart. That’s some mighty rotten luck. Dennis, after executing a Figure 8 loop off the Outer Banks, dropped 15 to 20 inches along the coastline. Then came Floyd, a massive storm that morphed into an intense nor’easter as it clipped eastern North Carolina — delivering another 15 to 20 inches. The combination of these two storms created a flood catastrophe over the coastal plain that took years from which to recover.
1996 Hurricane Fran (Category 3)
This was a major hurricane that delivered pockets of heavy rain in two spots — along the coast during landfall and deep inland, over the Shenandoah Valley of Virginia. Flooding along the Shenandoah River was extreme and orography (Big Meadows is on a high plateau atop the Blue Ridge Mountains) played a significant role in focusing the heavy rain.
1979 Hurricane David (Category 2)
Here, another “great arc” storm pinwheeled around the Bermuda High, depositing a strip of heavy rain from Florida to New England. The heaviest amounts fell in the Carolinas, on the coastal plain. David also produced some inland hazards including a significant tornado in Fairfax County, Va., part of a tornado outbreak that swarmed across Virginia and Maryland.
1975 Hurricane Eloise (Category 3)
Many have never heard of this storm but it arrived just three years after of Agnes, and it led to more extreme flooding of the Mid-Atlantic. It’s remarkable that, like Agnes, Eloise struck Florida but did not unload its heaviest rain until arriving in the District-Maryland-Virginia area. Mountainous regions including the Catoctins and Appalachians fared worst … including the greater Harrisburg, Pa., region.
1972 Hurricane Agnes (Category 1)
It’s still remarkable that a Category 1 hurricane striking Florida is capable of unleashing so much rain, hundreds of miles to the north and days later. The storm underwent one of the most complicated post-tropical evolutions ever documented, which included splitting into two vortices that culminated in a large, upper level storm that stalled over western Pennsylvania. As the rain map below shows, the Appalachians once again played a major role in raining out this storm.
1969 Hurricane Camille (Category 5)
Our final storm is regarded as the most amazing, and most deadly, of all flood-generating tropical cyclone remnants in the Mid-Atlantic. Note where the storm made landfall: Mississippi, as one of only three Category 5 storms to hit the U.S. in the 20th century. Also note when and where the most catastrophic rain unfolded: Three days later, over central Virginia, to the tune of 27 inches falling in eight hours. This, after post-tropical remnants approached from the west, from the Ohio Valley.
The Weather Bureau forecast for the region that night was: “Showers likely, clearing in the morning.” What happened: Close to 150 fatalities, many of them buried alive beneath trillions of tons of landslide-loosed rock, mud and trees. At the time, this event defined the record flood discharge for all rivers east of the Mississippi.