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Portland, Maine, experienced record setting rainfall of 163.3 mm (6.43 in) on 13–14 August 2014, which resulted in urban flash flooding. This paper examines the key ingredients for heavy convective rainfall: high specific humidity, intense vertical motion, and long rainfall duration at both the synoptic scale and mesoscale. A favorable environment for flooding is identified from positive anomalies in the specific humidity and a synoptic scale, upper-level trough just upstream of Portland. The existence of a low-level jet altered the propagation of the responsible convective system as it approached Portland. This jet resulted in a change in the mesoscale beta convective element propagation vector (VMBE). The concept of residence time as a function of the angle between the synoptic motion and VMBE is introduced and found to increase the duration of heavy rainfall in this case. In addition to the effects attributable to system motion, a coastal front (more commonly a winter phenomenon) is also identified in this case. This front enhanced the vertical motion, further increasing the local rainfall maximum over the Portland area. ABSTRACT (Manuscript received 30 June 2016; review completed 15 December 2016)

A Synoptic and Mesoscale Analysis of Heavy Rainfall at Portland, Maine, 13-14 August 2014