Detroit, Long Island, Baltimore Floods: Signs of a Wetter Future?

August 15, 2014


Detroit, Baltimore, Long Island; all inundated by torrential rain the likes of which were only seen once since the Great Depression, Roaring Twenties, or never before, all in the span of about 48 hours.

Let's first review the specifics of each of these events.

Detroit: August 11

- Second heaviest rainfall of any calendar day on record: 4.57 inches

- Only July 31, 1925 was wetter with 4.74 inches

- Parts of five metro freeways flooded with up to 12 feet of water. Some sections were closed 48 hours.

- The Clinton River at Clinton Township set a record crest, with a flow of five million gallons a minute.



Baltimore: August 12

- Second heaviest rainfall of any calendar day on record (BWI Airport): 6.30 inches

- This was the wettest day since the Chesapeake/Potomac hurricane of 1933: 7.62 inches on August 23, 1933.

- 3.95 inches of rain fell at BWI in just 73 minutes. This rainfall rate has a 0.1 to 0.2 percent chance of occurring in any year, according to a NOAA precipitation historical database.



Long Island/New England: August 13

- Islip, New York, set a new 24-hour rainfall record for the state of New York (13.57 inches).

- The previous state record was from Hurricane/Tropical Storm Irene, which dropped 11.60 inches at Tannersville on August 27-28, 2011.

- This was almost two inches more than the average rain from June through August and just a half inch shy of their wettest single month (14.07 inches in Oct. 2005).

- Islip picked up 5.34 inches of rain in one hour and 9.71 inches in two hours (roughly from 5-7 a.m.). This two-hour total is comparable to average June (9.67 inches) and September (9.86 inches) rainfall in Miami, their two wettest months.

- Portland, Maine. tallied their fifth wettest calendar day (6.43 inches), their wettest day not related to a tropical cyclone. New one-hour (2.57 inches) and two-hour (4.21 inches) record rain rates were set, besting previous records from Tropical Storm Bob (2.08 inches in August 19, 1991) and Tropical Storm Daisy (4.21 inches on October 6, 1962), respectively.


These events are incredible enough, but here is a sampling of other notable flood events since August 9:

- Hampstead, N.C. and Greenville, S.C. on August 9
- A Nebraska hospital is swamped
- August 10 in Lexington, Kentucky, became the fourth wettest day on record with 5.38 inches of rain.
- Sixteen rescued in Phoenix flooding August 12.
- Even during western Washington's dry season, flooding swamped the Seattle suburb of Bellevue August 13.

Of course, mid-late summer is prime time for flash flooding in the U.S.

The jet stream migrates north toward southern Canada, leaving behind light steering winds for thunderstorms sprouting in moist summer atmosphere. The wet phase of the North American monsoon brings slow-moving thunderstorms to the West. Slow-moving thunderstorm clusters called mesoscale convective systems, drench the Plains. Thunderstorms along the Gulf Coast along the sea breeze boundaries are notorious soakers. Tropical cyclones are notorious heavy rain and flash-flood factories.

A Heavier Trend

Is the extremity of rainfall we've seen this week solely a reflection of a favorable sluggish weather pattern, or is the Earth's changing climate also playing a role?

It is impossible to pin any single weather event — whether flash flooding, severe outbreaks, hurricanes, even winter storms — on climate change, alone. Weather and climate act on vastly different time scales.

The National Climate Assessment, a report compiled by 300 scientists and experts released in May 2014, offered compelling evidence that, in fact, heavy rain events are becoming, well, heavier in parts of the U.S.

Specifically, the NCA report first concluded that the intensity of heavy rain events is increasing nationwide, with the greatest rises occurring in the Northeast and Midwest (see map at right). In other words, the heaviest rain events are producing more rain now than in the middle of the 20th Century, particularly in the eastern half of the nation.

The rainfall rates we saw on August 13 in southern Maine and Long Island exceeded those from the area's tropical cyclone history. Shattering a New York state 24-hour rainfall record from the notorious flood-producing Irene is quite a feat.

Also, these heavy rain events are becoming more frequent, as you can see in the graph at right, taking off particularly since the 1980s.

Future climate forecasts suggest the number of heavy rain events will continue to rise nationwide, even in regions where overall less precipitation is expected, such as the Desert Southwest, according to the NCA report.

There is one likely and one possible reason for this.

1) Water vapor, the gaseous form of water which condenses to form clouds and rain, is increasing in a warming world. More available water in the atmosphere means higher precipitation, all other things being equal. 

2) The upper-level wind flow steering weather systems from west to east in the mid-latitudes may be buckling into high-amplitude blocking patterns more often, resulting in slower-moving weather systems and, thus, more and heavier precipitation events.

"While there was some randomness on the local scale, the anomalous nature of the (mid-August) rainfall was not just purely a coincidence or only due to a general increase in evaporation and atmospheric water vapor, there was forcing by the pattern," said The Weather Channel senior meteorologist, Stu Ostro in an internal email. 

A recently released study from the Potsdam Institute for Climate Impact Research found that the summer atmosphere is moving into these large-amplitude blocking patterns almost twice as frequently since the year 2000 as before.

The study states the Arctic is warming about twice as fast as the rest of the Earth since 2000, due in part to diminished sea ice and snow cover. This diminishes the north-south temperature gradient and, therefore, the west-to-east component of the jet stream slows down, slowing down the forward motion of weather systems.

In practical terms the theory suggests cold fronts may be slower to move through, showing a greater propensity to stall, areas of precipitation may move slower, and contain heavier precipitation rates in the summer.

Given this, it's a good time to remind you of some flash flood basics.

- Find out if you live in a flood plain. Home insurance does not cover flooding.
- Pay attention to all flash flood warnings, particularly if you're driving or live in a flood-prone area such as a small river or creek.
- Never drive into floodwaters of unknown depth, particularly at night. Roughly two-thirds of all flash flood deaths are in vehicles.

MORE: Satellite Images of Climate Change

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