Northeast Extreme Precipitation Events

By: Zachary Labe , 5:31 PM GMT on August 15, 2014

According to the 2014 National Climate Assessment, increasing frequency and intensity of heavy precipitation events are likely over the next several decades. For those cleaning up from this past week's record breaking flash flooding in the Northeast, this comes as no surprise. The northeastern contiguous United States is expected to see the greatest increase in this threat as heavy precipitation daily events already having increased by approximately 71% from 1958-2012. This observational evidence is highly supported by the scientific community without question. However, the cause is a bit more uncertain.


Courtesy of the 2014 National Climate Assessment)

Flooding and in particular flash floods are on the rise and will continue across our area. The particulars of impact though are bit more difficult to quantify.


(Courtesy of the National Climatic Data Center)

Urban sprawl and a rapidly growing population across the Northeast corridor are moving communities into previously designated flood zones and/or exacerbating inefficient ground/storm water draining.

Observational evidence after past flash floods clearly designate that retention ponds and other storm water methods are not nearly as efficient in capturing and filtering flood water as trees and other natural vegetation forms.

Therefore, in locations that previously may have been able to handle high intensity rain events, development may actually have increased urban flooding without directly being a result of a climatic change. These questions are important to consider when analyzing this round of flash flooding this past week.

Let's note the estimated radar storm total precipitation from August 13 over Long Island, NY.



We can clearly see a line of training thunderstorms over the central portions of Long Island. Rising air along a warm front, a small mesoscale boundary, and a strongly southerly gulf flow set the stage for heavy rain over the area. 13.26" of rain fell in less than 24 hours breaking the previous New York State record for the most rain during a 24 hour period. A recent Capital Weather Gang article Link also points out this interesting fact, "Consider, too, Islip almost received about as much rain in three hours as downtown Los Angeles did in all of 2012 and 2013 combined (11.75″)."

Consider positioning this line of convection 30-45 miles to the west.

New York City. The impact of the short duration and high intensity of the rain would likely have made world news as it fell during their morning commute. It was the same record breaking rain as before, yet the damage and impacts would have increased two-fold.

Forecasting flash flood events are particularly difficult for meteorologists given their often localized impact zone which can widely range from rural western Maryland to the densely populated Boston metro area. Computational demanding models that solve the atmosphere non-hydrostatically and Eulerian can model mesoscale and convective features, but often lack the capabilities to model these high intensity rain events. Although progress is being made. The High Resolution Rapid Refresh Model (HRRR) is making significant gains in accurately forecasting convective events, and its verification was one of the highlights at the 2014 American Meteorological Society Conference in Atlanta, GA. The Rapid Refresh Model (RAP) and HRRR have already replaced the NCEP's previous high resolution model, the Rapid Update Cycle (RUC). For more information on the latest upgrades to the RAP and verification charts, the NCEP has a great new presentation out...Link

Remarkable gains are being made from a meteorological perspective with forecasters being often able to pinpoint geographic regions highest at threat for flash flooding. However, detailed specifics leave a lot to be desired. Most of this is a result from not enough computing power preventing full computer model potential, but it is also due to a lack of knowledge on threat zones and the impacts of rain on an urban infrastructure.

The climate system is far from understand; in fact my current research will be running through controlled 1000-year model simulations to possible find flaws in the present thinking behind the timing onset of spring (trends signify a dipole dichatomy in the USA with earlier in the east, but later in the northwest). However, the why is in question.

Nevertheless, it is likely flash flooding stories will continue to populate media forms across the Northeast due to a higher frequency of these extreme rain events and the addition of urban/population sprawl.

Forecast meteorologists will face a tightning scrutiny on their flash flood forecasts. But for the time being, computing power and inconsistent meteorological explanations for these convective rain events will limit forecast warning time.

Lower Susquehanna Valley Doppler...

(Courtesy of WGAL)

http://www.wunderground.com/blog/Bli...l?entrynum=283