Observations of an Atmospheric River over New England in Late October 2017

Atmospheric rivers (ARs) are defined as long narrow corridors of enhanced integrated water vapor (IWV) and IWV transport (IVT) that are typically along a low-level jet stream ahead of the cold front of an extratropical cyclone. ARs are common to Northeast Pacific and Atlantic Oceans where they frequently influence occurrences of orographic precipitation along the U.S. West Coast and Europe, respectively. Extreme precipitation (>100 mm) is common in these areas where ARs transport tropical and subtropical air along a south-southwesterly trajectory that is orthogonal to north-south mountain barriers. This presentation will provide an overview of observations of an AR that produced strong winds, orographic enhanced precipitation, and flooding in the White Mountains of central New England on 30 October 2017.

The AR was well forecast by numerical weather prediction models and the National Weather Service several days prior to development. The NH Department of Homeland Security and Emergency Management pre-emptively evacuated flood zones in anticipation of the accompanying heavy rainfall and possible flooding on the Baker and Pemigewasset Rivers. The AR occurred in association with a south-southeasterly transport of subtropical and tropical air into central New England perpendicular to the White Mountains that led to CoCoRaHS-gauge measured 24-h precipitation amounts of ~100 mm across valley locations and >150 mm of precipitation in the mountains. Flooding on the Baker River eclipsed high water marks from both the Great New England Flood of 1936 and Tropical Storm Irene of 2011, whereas flooding on the Pemigewasset River brought the highest water levels since Irene.