A Forecast Strategy for Anticipating Cold Season Mesoscale Band Formation within Eastern U.S. Cyclones

Abstract

An ingredients-based, time- and scale-dependent forecast strategy for anticipating cold season mesoscale band formation within eastern U.S. cyclones is presented. This strategy draws on emerging conceptual models of mesoscale band development, advances in numerical weather prediction, and modern observational tools. As previous research has shown, mesoscale band development is associated with frontogenesis in the presence of weak moist symmetric stability and sufficient moisture. These three parameters—frontogenesis, weak moist symmetric stability, and moisture—are used as the ingredients for identifying mesoscale band development in this strategy. At forecast projections beyond 2 days, the strategy assesses whether cyclogenesis is expected. Within 2 days of the event, the strategy places the band ingredients in the context of the broader synoptic flow, with attention to where deformation zones are present, to assess whether banding is possible. Within 1 day of the event, the strategy focuses on assessment of the ingredients to outline when and where band formation is favored. Plan-view and cross-sectional analyses of gridded model fields in conjunction with high-resolution model guidance are used to assess the likelihood of banding and to outline the threat area. Within 12 h, short-range forecasts of the band ingredients are evaluated in concert with observations to make specific band predictions. Particular emphasis is placed on the evolution of the frontogenetic forcing and moist symmetric stability. During the event, trends in observations and short-range model forecasts are used to anticipate the movement, intensity, and dissipation of the band. The benefits and practical challenges associated with the proposed strategy are illustrated through its operational application to the 25 December 2002 northeast U.S. snowstorm, during which intense mesoscale snowband formation occurred. Forecast products from this event demonstrate how the forecast strategy can lead to heightened situational awareness, in this case resulting in accurate band forecasts. This application shows that accurate operational forecasts of mesoscale bands can be made based on our current conceptual understanding, observational tools, and modeling capabilities.

Banded Convection Caused by Frontogenesis in a Conditionally, Symmetrically, and Inertially Unstable Environment

“…In this paper, we calculate the instability parameters using the both the geostrophic wind and the total wind. In general, calculations performed using the geostrophic wind indicated more instability, as found in previous studies (e.g., Clark et al 2002;Jurewicz and Evans 2004;Novak et al 2006). Whether the geostrophic or total wind is more appropriate in calculating symmetric and inertial instability parameters remains an open question (e.g., Gray and Thorpe 2001;Novak et al 2006, 19-21;NielsenGammon and Gold 2007).…”

“…It would appear that the circulations that spawned the cloud bands formed first in an unsaturated environment, then saturated later. This process is different than banding in extratropical cyclones (e.g., Novak et al 2004Novak et al , 2006 where large-scale saturation poleward of the warm front is typically established before bands form. Thus, we eliminate moist symmetric instability from consideration as a possible mechanism.…”

“…2713).] In contrast, others prefer using the total wind (e.g., Gray and Thorpe 2001;Clark et al 2002;Jurewicz and Evans 2004;Novak et al 2004Novak et al , 2006. In this paper, we calculate the instability parameters using the both the geostrophic wind and the total wind.…”

Convective Snowbands Downstream of the Rocky Mountains in an Environment with Conditional, Dry Symmetric, and Inertial Instabilities

“…The bands in this case, however, developed in a generally unsaturated synoptic and mesoscale environment (Fig. 9)-like the banding case discussed by Schultz and Knox (2007) but unlike the banding that occurs in the saturated comma-cloud heads of extratropical cyclones (e.g., Novak et al 2004Novak et al , 2006Novak et al , 2009. Consequently, moist symmetric instability is an unlikely candidate for the bands in this present case.…”

“…13d-f and 15c,d). This formation of inertial instability results from the lifting of low-momentum air near the surface to a level where winds are stronger, creating a deficit of momentum and strong horizontal gradients in the wind speed (e.g., Holt and Thorpe 1991;Blanchard et al 1998;Novak et al 2008). Inertial instability develops on the anticyclonic-shear edge of this wind speed gradient ( Fig.…”