The 8 December, 2006 banded snow event

Another in a series of moderate impact, poorly forecast, banded snow events occurred in Maine on 8 December, 2006. During this event, sub-synoptic scale forcing yielded several inches of snowfall over small geographical areas. This is problematic as other recent banded cases have also shown similar snowfall distributions which have not been explicitly captured by numerical models. This includes the 3 December, 2004, 8 January, 2005 and 11-12, March 2005 events.

The 8 December, 2006 event showed characteristics consistent with many of the features associated with the less common Norlun Type II instability trough. Norlun events are noted for quasi-stationary, inverted, surface troughs around the northwest periphery of a departing surface low undergoing cyclogenesis. The formation of snowbands represent an organized mesoscale response to large scale ascent associated with the approach of a cyclonic vorticity maximum containing cold air aloft. Ascent is then concentrated and enhanced near and parallel to a coastal front in a region of confluent, relatively warm and moist boundary layer conditions. The marine-modified, buoyant air interacts with advancing cold and dry continental air, leading to enhanced coastal baroclinicity.

During the 8 December, 2006 event, a steady state situation developed as an offshore surface low pressure system intensified and exited over the North Atlantic. Simultaneously, an inverted surface trough allowed a convergent low-level flow to develop along the Maine coastline, while an upper-level low progressed east from Upstate New York. Cold air advection on the west side of the surface trough coincided with warm boundary layer conditions to produce steepening lapse rates during the early morning hours. This increased instability necessary for additional ascent, allowing snow bands to form in shallow upright convection. Radar images showed these signatures converging along and parallel to the Maine coast despite a downslope northwesterly flow.

Diagnostic procedures developed for office winter training were reviewed to determine the location of best forcing, which was focused at the 850 mb level. Snow bands oriented themselves along this low-level frontogenetic region and intensified as lapse rates increased. NWP model vertical profiles further depicted accentuated lift within the maximum dendritic formation zone for enhanced snowflake production, creating snow-water equivalent ratios as high as 25:1 during the event. This more frequently observed hybrid case will serve as a companion study during the ongoing production of a pure Norlun trough climatology.