Improving Flash Flood Warning Services in Northern New England

Timely flash flood warnings are critical to the mission of the National Weather Service to protect lives and property. The current goal for flash flood warning lead time is 65 minutes. The goal of this project is to identify synoptic and mesoscale patterns that exist up to 24 hours prior to flash flood events in order to be better aware of flash flood potential before events occur. In this study, a climatology of flash flooding for Northern New England (defined as Maine, New Hampshire, and Vermont) was established from 2008-2018 using flash flood events from the Storm Events Database. Using Maddox et. al. (1979) as a reference, this study focuses specifically on 500 millibar trough/ridge patterns as well as the presence of a surface front and the vertical wind profile. While there is not a single definitive pattern responsible for flash flood events in northern New England, several “most likely” situations have been identified. Flash flooding was found to be more common in large trough patterns with weak to moderate vertical wind shear through the profile.

Unwarned events, as well as false alarm days, were also analyzed from a climatological perspective and for their synoptic characteristics. Identifying similar patterns in missed events, events that did not occur, and events that did occur within a warning will allow forecasters to better identify flash flood situations with more advanced notice leading to warnings that meet the NWS lead time expectations.