Extreme Rains Swamp Baltimore and Long Island

An extreme deluge nearly unprecedented in Baltimore history swamped the city in flood waters that closed multiple expressways on Tuesday afternoon. Officially, 6.30" of rain fell at the Baltimore Airport on Tuesday. This was their second wettest calendar day in history, behind only the 7.62" that fell on August 23, 1933 during the Chesapeake-Potomac Hurricane. Remarkably, 3.95" of yesterday's Baltimore deluge fell in just 73 minutes. According to the NOAA Precipitation Frequency server, we would expect such a heavy 2-hour rainfall event to happen only once every 100 years. A 6.30" rainfall in 24 hours has a recurrence interval of once every 25 years.


Figure 1. Flooding in Baltimore, Maryland on Shell Avenue between Curtis Bay and Route 2 on August 12, 2014. Image posted to Instagram by blueagavebalto.

Extreme rains swamp Long Island and Connecticut
Dangerous flash flooding is occurring this Wednesday morning across Central Long Island, New York and Southern Connecticut as a low pressure area centered near New York City brings bands of heavy rain to its east. Rainfall rates as high as 5.34" per hour were observed in Islip, New York, triggering flooding that has forced the closure of multiple freeways, including the Long Island Expressway. Thunderstorms that repeatedly trained over the same point brought Islip 5.34" of rain between 5 - 6 am EDT, then another 4.37" between 6 - 7 am. The NWS reported that 13.20" of rain had fallen in Islip so far this morning, as of 10 am EDT. The record rainfall total for the entire month of August in Islip is 13.78".


Figure 2. Radar-estimated rain in Long Island, New York on August 13, 2014 exceeded 8" along a narrow swath.

Why such heavy rains?
Baltimore and Long Island's deluges comes on the heels of Monday's torrential rains in Detroit, whose roads were virtually shut down when the city's 2nd heaviest 24-hour rain since 1874, 4.57", fell. Portions of four major expressways remain closed in Detroit today due to flood damage, and states of emergency remain in effect for much of Detroit and some of its northern suburbs. All of these floods had two things in common: an unusually high level of water vapor in the atmosphere, and an unusually amplified jet stream. Precipitable water (a measure of water vapor) in Detroit on Monday and near Long Island last night was in the 99th percentile historically. The jet stream was in an unusually contorted configuration, with a strong trough of low pressure over the Eastern U.S., and sharp ridge of high pressure over the West. This allowed colder air than usual to move in aloft, increasing the instability of the atmosphere, causing stronger thunderstorm updrafts and heavier rains.


Figure 3. I-94 East in Detroit at Livernois on August 11, 2014. Image posted to Twitter by Ali B. (@AABaydoun.)

Monday's rains meant that four of Detroit's top ten rainiest days since 1874 have occurred in the past seventeen years. Yesterday's rains in Baltimore means that three of Baltimore's top ten rainiest days since 1871 have occurred during the past five years, and four of the top ten rainiest days have occurred in the past 15 years:

1) 7.62" August 23, 1933 (the great Chesapeake-Potomac Hurricane
2) 6.30" August 12, 2014
3) 6.02" September 30, 2010
4) 5.97" September 24, 1912
5) 5.85" July 8, 1952
6) 5.51" October 29, 2012 Hurricane Sandy)
7) 5.02" September 16, 1999 (Hurricane Floyd)
8) 5.00" September 27, 1985 (Hurricane Gloria)
9) 4.91" August 12, 1955 (Hurricane Connie)
10) 4.76" September 5, 1895


Figure 4. Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) from 1958 to 2012 for each region. There is a clear national trend toward a greater amount of precipitation being concentrated in very heavy events, particularly in the Northeast and Midwest. Image credit: NCA Overview, updated from Karl et al. 2009.

If these numbers make you suspect that record heavy rains may be occurring more frequently in these cities due to a changing climate, then you're in good company. The U.S. National Climate Assessment, issued every four years by NOAA, is an effort by more than 300 U.S. scientists to assess how the climate is changing in the U.S. The just-released 2014 report said: “Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.” Fundamentally, a warmer atmosphere will evaporate more moisture from the oceans, resulting in more days with 99th percentile water vapor in the atmosphere, and increased chances of very heavy rainfall events like this week's deluges in Detroit, Baltimore, and Islip.

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days.

Jeff Masters