Incredible East Coast Rainfall Event of August 12-14

By: Christopher C. Burt , 8:43 PM GMT on August 15, 2014

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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

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14. WunderAlertBot (Admin)
8:58 PM GMT on August 19, 2014
weatherhistorian has created a new entry.
13. Astrometeor
2:59 AM GMT on August 18, 2014
CoCoRaHS data for Long Island:



Top measurement was 13.02", so that would seem to bolster Islip Airport's data. CoCoRaHS gauges are not automated, you have to dump them out yourself, so there could be a little bit missing if the owner of the gauge didn't notice the gauge filling up rapidly.

Link for Picture
Member Since: July 2, 2012 Posts: 101 Comments: 10364
12. maitelanusse
2:29 AM GMT on August 18, 2014
Christopher, I will like to know if the data from Islip is from an official observatory ?, or it is from a weather station ?, I' m asking these because it is well known that during big rains the digital pluviometers give erroneus data, for instance last year during a big rain in La Plata, Argentina (where I'm from) the official data from the official La Plata analog pluviometer gave a real data of seven inches in a 24 hour period, but the media gave a erroneus data of fourteen inches (in the same period of 24 hours) taken from a digital pluviometer from a not oficial weather station.
Member Since: August 18, 2014 Posts: 0 Comments: 0
11. ColoradoBob1
7:52 PM GMT on August 17, 2014
Quoting 3. Neapolitan:



I'm with Chris: on which key point do you feel Dr. Shepherd "wimped out"? Seems to me he stated in plain terms the situation as science knows it. That is, while it's not at all wise to claim that any particular extreme weather event was "caused" by climate change, that change is to some degree affecting every single weather event metropolitan areas were built to standards that didn't include the increasingly frequent and severe precipitation events we're seeing nowadays,...


Neapolitan:
I am reminded of the City Engineer for Amarillo, Texas just a few years back . When they got their new record for an extreme rain event, something on the order of over 6 inches in 12 hours. Months later, everyone was howling in a city hall meeting, and he said no ever dreamed it would rain this hard, and this fast in Amarillo, Texas.

As far as I'm concerned, these extreme rain events , coupled with the extreme drought events, tell me our climate is moving to a new state , and we will not wait for decades to see it . These extreme rain events are very common now. Each one is a record setter. And each one carries more rain in a shorter period of time.
Member Since: August 13, 2010 Posts: 0 Comments: 2607
10. ColoradoBob1
4:28 PM GMT on August 17, 2014
Quoting 4. rlk:

This was the same system that flooded Detroit earlier in the week, correct?


Was the Detroit Flood a Billion-Dollar Disaster?

The Detroit metro flood on August 11 may end up being a billion-dollar disaster, according to initial damage assessments.

Monday afternoon anywhere from 4-7 inches of rainfall hammered parts of the metro. Between one-third and one-half of homes in the hardest hit areas were damaged in the flood, amounting to over 34,000 structures, according to the Detroit Free Press.

City officials in the hard-hit suburb of Warren estimated 18,047 structures were damaged, including one-third of its homes, with a total value of $1.2 billion.

An average claim for water damage, however, ranges from $20,000 to $25,000. Unfortunately, a typical homeowners insurance policy does not cover flood damage, so there will be a large number of uninsured homes with at least basement flood damage.


Link
Member Since: August 13, 2010 Posts: 0 Comments: 2607
9. georgevandenberghe
4:00 PM GMT on August 17, 2014
Quoting 6. barbamz:


For August, 13 2014 (Click to enlarge). Source.


You beat me to the punch :-)
Member Since: February 1, 2012 Posts: 18 Comments: 1876
8. georgevandenberghe
3:57 PM GMT on August 17, 2014
Quoting 4. rlk:

This was the same system that flooded Detroit earlier in the week, correct?


Not sure. I think not although they were related. The system that flooded detroit went to the north of the mid atlantic Tuesday. The system that flooded MD, DE, NJ, LI and ME was the same.
Member Since: February 1, 2012 Posts: 18 Comments: 1876
7. georgevandenberghe
3:56 PM GMT on August 17, 2014
On weather.gov you can find seven day radar estimates which give an idea of the scale of the rain.

Click on "observed precipitation" then go to the bottom menu and click on "last 7 days"




Although I've argued in other posts that flash floods are often local events without much of a recognizable synoptic signal, this one I think did have one, the combination of a seasonally reasonably intense (i.e. not incredible or unprecedented) shortwave and surface low reflection, seasonably pretty high PWATS and (the key) a focusing mechanisim in the form of a coastal front. The DTW event was different from the other three but the Maine, Long Island and BWI events all were on that front. Radar shows a fourth area of intense rain in Northern DE and the sticks of southern NJ which are not heavily populated.


The combination of these coastal fronts, high PWATS and fair (only needs to be fair} dynamics always needs to be watched for flooding. Coastal fronts and tropical cyclones running northward after partial recurvature are commonly found together and create conditions for well described predecessor rain events. Remmanants of tropical cyclones sometimes hit these fronts also and fire off large amounts of rainfall. Eloise and Lee in the DC area are two salient examples, both produced large stream flood events over an area 300km long by 75km wide.. very large mesoscale or small synoptic scale.

I was wrong in some of my previous posts that generalize intense flash floods to be local events without much large scale contribution or signal. A large fraction of them are but this one was not in hindsight and merits much further analysis.


The key factor for a flash flood remains FOCUSING or CONCENTRATION mechanisms that cause a basin's dose of water to be dumped over a small fraction of the basin. The obvious one here was that coastal front and a flow aloft that was just about balanced by backward propagation rates so cells could backbuild over the same area for several hours.

I remain optimistic that with about 20 years of further research progress and computer and analysis progress we're going to be able to capture a large fraction of even the local events several hours prior rather than being totally surprised as we are now. I'll be 76 and by then my health will have failed enough I'll probably be pushed into retirement so you younger guys will have to make it happen.
Member Since: February 1, 2012 Posts: 18 Comments: 1876
6. barbamz
6:55 PM GMT on August 16, 2014

For August, 13 2014 (Click to enlarge). Source.
Member Since: October 25, 2008 Posts: 56 Comments: 6081
5. wxgeek723
4:21 PM GMT on August 16, 2014
Excellent post.

I suppose the silver lining here (if there is one) is that if the severity of these events continues to ramp up faster than anyone expected, then hopefully the public will start to catch on.

I'm not holding my breath though.
Member Since: August 28, 2008 Posts: 79 Comments: 3602
4. rlk
3:26 PM GMT on August 16, 2014
This was the same system that flooded Detroit earlier in the week, correct?
Member Since: January 30, 2005 Posts: 0 Comments: 97
3. Neapolitan
12:36 PM GMT on August 16, 2014
Quoting 1. spbloom:

Marshall's post wimps out on a key point, unfortunately.


I'm with Chris: on which key point do you feel Dr. Shepherd "wimped out"? Seems to me he stated in plain terms the situation as science knows it. That is, while it's not at all wise to claim that any particular extreme weather event was "caused" by climate change, that change is to some degree affecting every single weather event that takes place. And he also stated the truth when he noted that many of the highway and drainage systems in major metropolitan areas were built to standards that didn't include the increasingly frequent and severe precipitation events we're seeing nowadays, and that's going to continue to be an issue.

On another note: 0.14" in one minute?! 1.76" in 15 minutes? Though I've long lived in Florida--home of the daily tropical deluge and the occasional hurricane--I believe I've probably only seen that kind of rate very, very few times. Incredible, indeed...
Member Since: November 8, 2009 Posts: 4 Comments: 13579
2. Christopher C. Burt , Weather Historian
11:31 PM GMT on August 15, 2014
I'm curios. What key point did he miss?

Quoting 1. spbloom:

Marshall's post wimps out on a key point, unfortunately.
Member Since: February 15, 2006 Posts: 308 Comments: 290
1. spbloom
10:27 PM GMT on August 15, 2014
Marshall's post wimps out on a key point, unfortunately.
Member Since: May 12, 2010 Posts: 0 Comments: 428

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About weatherhistorian

Christopher C. Burt is the author of 'Extreme Weather; A Guide and Record Book'. He studied meteorology at the Univ. of Wisconsin-Madison.