The New York City Metro Area Transportation Apocalypse Event of 15 November 2018

A rather ordinary early season Nor’easter produced an epic transportation disaster in the New York City (NYC) metro area on the afternoon and evening of Thursday 15 November 2018 when the NYC metro area was nearly paralyzed by a 6” (15 cm) snowfall over a period of a few hours. The NYC media and the social media outlets had a field day hyping snow-induced transportation gridlock across the metro area while serving as “outrage outlets” for hundreds of thousands of aggrieved travelers who were frustrated at their inability to go from point A to point B. A critical question is how could a run-of-the-mill ordinary Nor’easter that was reasonably well-predicted and well-advertised by the available numerical guidance produce such a disproportionately high-impact disastrous transportation debacle? This presentation is motivated by the need to understand what happened, how it happened, and why it happened from both science and forecasting perspectives.

Observations from from Central Park (KNYC) show that light snow (S-) began at 1836 UTC, transitioned to moderate snow (S) at 1859 UTC (23 min after snow began), and further transitioned to heavy snow (S+) by 1943 UTC (67 min after snow began). After the transition to S+, KNYC reported liquid water equivalents of 0.15”, 0.25”, 0.18” and 0.15” in the hours ending 2051, 2151, 2251, and 2351 UTC, respectively. These precipitation rates equated to snowfall rates of 1–3” h^-1, which were more than enough to disrupt transportation and produce widespread gridlock. This “transportation apocalypse” was compounded by a quasi-simultaneous mass exodus for home by millions of people when everyone panicked as snow began, rapidly intensified to S+, and accumulated on untreated roads. People were stuck in their cars, stuck cars made it impossible for road crews to treat and plow the roads, and massive gridlock ensued.

Confluent jet-entrance region dynamics contributed to an equatorward-directed surge of low-level cold Canadian air into the Northeast prior to the storm. This surge of cold Canadian air increased the baroclinicity across the Northeast and set the table for a subsequent warm-air advection-driven precipitation event. Heavy snow developed along a slow-moving east-west oriented axis of strong low-level frontogenesis beneath vigorous ascent aloft. Sensible and evaporative cooling from heavy snow established an isothermal (0°C) lapse rate in the planetary boundary layer (PBL) and delayed the changeover to mixed precipitation and rain. A critical forecast problem was determining the timing of this changeover, given the competing effects of sensible and evaporative cooling, adiabatic ascent, and horizontal temperature advection on the temperature tendency in the PBL. This talk will focus on the applicable meteorology, the governing dynamics, and relevant forecast issues.