The “Pi-Day” Coastal Storm of 14 March 2017

This presentation will provide an overview of the flow patterns that governed the evolution the “Pi-Day” Storm of 14 March 2017. This storm brought 50–100 cm of snow to parts of the interior northeastern U.S. and is of interest because synoptic-scale forecast uncertainty limited the predictability horizon to no more than 4–5 days. This relatively limited predictability horizon was associated with upper-level disturbances over the western Pacific, an omega block over the north-central Pacific, trough development downstream of this omega block, undulations in the position of the North Pacific jet stream, and the strength of a western CONUS upper-level ridge.

At shorter lead times (24-96 h), the predictability horizon was also limited by uncertainty associated with the establishment of an arctic air mass over the Northeast U.S. and the potential for upper-level trough phasing in split flow east of the Rockies. Cumulative trough passages in the northern part of this split flow introduced a low-level arctic air mass into the Northeast U.S. as a polar anticyclone spread eastward across southern central and eastern Canada. A strong lower tropospheric baroclinic zone along the southern boundary of this arctic air mass anchored strong lower tropospheric warm-air advection ahead of mobile upper-level disturbances in the aforementioned split flow. These disturbances phased near the Atlantic coast, resulting in a stronger and more northwestward-displaced surface cyclone for which the predictability horizon was only 48–72 h.

Mesoscale snowbands embedded within the larger-scale cyclone further limited the predictability horizon to < 12 h. Observations from the New York State Mesonet, and radar and satellite loops revealed that two mesoscale snowbands accounted for the heaviest snowfalls. One snowband formed in central Pennsylvania after 0600 UTC 14 March, pivoted cyclonically, and became quasi-stationary from northeastern Pennsylvania to the southwestern Adirondacks. A second snowband formed across Delmarva, eastern Pennsylvania and central New Jersey after 1200 UTC 14 March and moved northward while rotating cyclonically about a pivot point ~75–100 km west of Albany, NY. Snowfall rates of 12.5–15.0 cm per hour occurred during the passage of this second snowband. Areas impacted by both snowbands received storm-total snowfall accumulations near 1 m with a maximum of ~1.2 m reported in the vicinity of Cooperstown, NY.