This presentation will examine the 15 May 2018 severe event from the synoptic scale to the storm scale. Fast 500-hPa flow of 25–33 m s-1 (50–65 kt) existed across the area impacted by severe thunderstorms between an anomalously deep low over Quebec and a high in the western Atlantic. Steep midlevel lapse rates of 6.5 to 8.0 K km-1 along with surface temperatures near 30°C (86°F) and dewpoints near 20°C (68°F) contributed to a tongue of mixed-layer CAPE of 1000–2000 J kg-1 collocated with the fast 500-hPa flow. The steep midlevel lapse rates had origins from an elevated mixed layer, which has been associated with past significant severe weather events in the northeastern United States (Banacos and Ekster 2010). Convection initiated in eastern Ohio ahead of a surface cold front and mesoscale convective vortex in the late morning hours of 15 May and quickly evolved into a broken line of storms that raced eastward across Pennsylvania during the early afternoon. This line of storms produced severe weather all the way to Connecticut and the New York City and Philadelphia metro areas before weakening in the early evening. Particularly intense damage was observed with the northernmost storm in the line, with several confirmed microbursts, macrobursts, and QLCS tornadoes across northern Pennsylvania, southeastern New York, and western Connecticut. The radar presentation of this storm featured a persistent well-defined rear inflow notch associated with strong straight-line winds, as well as occasional front inflow notches associated with low-level rotation. Ahead of this line of storms, discrete supercells formed over the Catskill Mountains of New York in the early afternoon. These storms were responsible for a swath of hail measured at 5–7 cm (2–2.75 in) diameter along with three confirmed tornadoes in southeastern New York and northwestern Connecticut. Remarkably, these were the first tornadoes to occur in a tornado watch box in the NWS Albany forecast area since 2003.
The prospect of a significant severe weather event was relatively well anticipated by National Weather Service (NWS) forecast offices and the Storm Prediction Center (SPC). With the 1630Z day 1 convective outlook, the SPC upgraded a portion of the northeastern U.S. to a moderate risk of severe thunderstorms, a relatively rare occurrence in this geographical area. Given the convective environment, knowledge of storms with a history of producing severe weather, and key radar signatures, NWS offices issued impact-based warnings with enhanced wording to convey the high-end nature of the threat. This presentation will provide a verification of impact-based warnings for those storms which resulted in significant severe weather [i.e., wind gusts ≥ 33.4 m s-1 (65 kt), hail ≥ 5 cm (2 in), tornado intensity ≥ EF2, or any storms resulting in injuries or fatalities].