The use of ensembles and anomalies to anticipate extreme flood events in the northeastern United States

Widespread extreme flood events in the northeastern United States during the past 20 years have caused millions of dollars in damages and numerous casualties. Recently, the flood events of October 2005, May 2006 and June 2006 were characterized by as much as 14 inches of rain in portions of New York and New England. Heavy rainfall produced record river levels at many locations. Meteorologists anticipated the potential for heavy rain and flooding across the region during these events. However, the magnitude and impact was greatly underestimated. Flood watches were issued with at least 12 hours lead time, but forecast rainfall amounts were roughly half of what was observed. Analysis of past flooding events showed two dominant patterns supporting widespread extreme flood events in the northeastern United States. One pattern has been defined as the Atlantic Flow pattern, characterized by strong high pressure over eastern Canada and low pressure tracking east and off the East Coast of the United States. This event type produces strong deep southeasterly flow off the Atlantic Ocean, which is further enhanced by the direct impingement of the flow onto the northeast-southwest oriented mountain ranges of the northeastern U.S. The second pattern has been defined as the Gulf/Tropical Origin pattern, characterized by a slow moving trough of low pressure over the eastern United States, with successive upper-level impulses originating from the Gulf of Mexico and western Atlantic subtropics. This enhances deep southerly flow and moisture advection which can persist for several days. Both patterns are characterized by strong deep moisture advection evident in precipitable water (PWAT) and as a tropical connection seen in water vapor satellite imagery. Ensemble guidance in a variety of formats showed the likelihood of more significant rainfall and flooding than was forecasted 24 to 48 hours prior to observed flooding. Means, spreads, probabilities and anomalies from short-range and medium-range ensemble forecast guidance products (SREF and MREF) were evaluated for the 2005 and 2006 events. The anomalies were based on departures from the 1971-2000 climatalogical normals. The ensemble mean and the departure of key fields and probabilities of precipitation at discrete times and thresholds are presented. It will be shown that the effective use of ensemble guidance can aid forecasters in increasing lead-times for watches and warnings, improve forecast accuracy and confidence of the magnitude of high impact and significant precipitation events. Forecasters can then add value to forecast and warning products, in an effort to give users more specific information and more lead time to make critical decisions.