Incredible East Coast Rainfall Event of August 12-14

Incredible East Coast Rainfall Event of August 12-14

What must have been one of the most anomalous non-tropical-storm-related precipitation events on record affected a wide area from North Carolina to Maine on August 12-14. The heaviest precipitation was confined to a relatively narrow band from the Baltimore, Maryland area, across southern New Jersey, and into coastal areas of New England as far north as Maine. Here are some details.



Submerged automobiles litter the Belmont Parkway on Long Island following the intense rainfall that deluged Suffolk County on the morning of August 13th. Photo from New York State Department of Transportation.

Peak storm totals by state (all occurring within 24 hours) but beginning on August 12th in the south and ending August 14th in the north included:

NORTH CAROLINA: 5.63” at Rendezvous Mountain RAWS site

VIRGINIA: 4.30” at Tysons Corner 2 NW

MARYLAND: 10.32” at Green Haven 1 WNW

DELAWARE: 4.10” at Odessa

NEW JERSEY: 8.94” at Millville Municipal Airport

PENNSYLVANIA: 4.30” at Beaverton

NEW YORK: 13.57” at Islip Airport

CONNECTICUT: 4.50” at Madison

RHODE ISLAND: 4.53” at Coventry

MASSACHUSETTS: 5.46” at Hatfield

VERMONT: 2.78” at Westminster West 0.9 E

NEW HAMPSHIRE: 5.26” at Newbury 4 SE

MAINE: 6.59” at Scarborough

Unfortunately (and surprisingly!), it appears no comprehensive map of the rainfall totals for the entire region has yet been produced.

The most extraordinary report was that from Islip, New York where 13.57” fell in the 24-hour period of 11 p.m. August 12 to 11 p.m. on August 13th. This established a new all-time New York State record for a 24-hour precipitation event (although NOAA reports 13.70” falling in Brewster during Hurricane Floyd on September 16-17, 1999—this value has been difficult to verify). Of the 13.57” total that accumulated at Islip an amazing 13.27” came down in just 12 hours with 9.71” in the two-hour period between 5 a.m.-7:00 a.m. on the morning of the 13th.



METARS at Islip, New York Airport during the heaviest period of rain on the morning of August 13th. NWS-New York.



At its greatest intensity, 1.76” of rain fell in one 15-minute period at Islip between 5:39 a.m. and 5:54 a.m. as this graphic above displays.

Given the intensity of the event it is remarkable that only one storm-related fatality (a traffic accident) was reported and flood damage seems to have been largely confined to submerged vehicles. This is in sharp contrast to the devastation that took place earlier in the week around Detroit, Michigan where at least three fatalities were reported and preliminary damage estimates are said to be close to $1.2 billion with over 18,000 homes flooded. Detroit Metro Airport picked up 4.57” on August 11th, its 2nd greatest calendar day precipitation event on record (following 4.74” on July 31, 1925). Dearborn, Michigan had the top rainfall report in the area with 6.31”.

Aside from the rain on Long Island, the Baltimore, Maryland region also saw record-breaking rainfall with the 6.30” on August 13th being their 2nd greatest calendar day total (following 7.62” on August 23, 1933 during a tropical storm). However, Hurricane Connie dumped 7.95” in 24 hours on the city on August 12-13, 1955. Again, it was the rainfall rates that were most extraordinary during the recent event. 3.91” fell at the Baltimore-Washington Airport between 12:29 p.m. and 1:32 p.m. on August 13th, basically a one-hour period. The NWS said this rate of rainfall had an occurrence period of once in 500 to 1000 years.

Further north, Portland, Maine picked up 6.43” on August 13th, its 5th greatest calendar day rainfall and greatest such during a non-tropical-storm-related event (the city’s greatest daily rainfall occurred during Tropical Storm Lili on October 22, 1996 when 11.74” was measured, part of a 24-hour record of 13.32” on October 21-22). Of interest, similar to Islip and Baltimore, was the intensity of the rainfall with 2.57” falling during the one-hour period between 9 p.m.-10 p.m. on August 13th, a new record (tropical storm-related or not). The previous hourly record was 2.08” during Tropical Storm Bob on August 19, 1991. In just two hours (9 p.m. to 11 p.m. Wednesday night) 4.21” fell, another all-time record, this time for a two-hour period. The figure was said to be a once in a 200-year extreme precipitation event.

The cause of the excessive precipitation was unusually moist air from the south over running an occluded front that was draped over the eastern U.S. A small low-pressure system developed on the front and dragged the attendant precipitation northeastward to Maine. The core of the heaviest precipitation followed a narrow slot up the eastern seaboard with the heaviest rain cells training over one another. This is why Central Park in Manhattan, just 40 miles west of Islip, picked up only .78” and sites 40 miles east of Islip only .50”.





The 7 a.m. ET Daily Weather Maps for August 12th (top) and August 13th (bottom). NOAA Daily Weather Maps.

As always happens when something exceptional weather-wise, such as this week’s extreme precipitation events, occur the question of how this might relate to global climate change pops up. I am not qualified to answer that question but here is a recent blog posted on WU by Dr. Marshall Shepherd of the University of Georgia’s Atmospheric Sciences Program that addresses this question.

Christopher C. Burt
Weather Historian