Extended Abstract (632K)P1.7
Short-range prediction of banded precipitation associated with deformation and frontogenetic forcing
Peter C. Banacos, NOAA/NWS/NCEP/SPC, Norman, OK
NOAA's Storm Prediction Center (SPC) is responsible for the short-term (0-9 hour) prediction of significant winter weather hazards (i.e. heavy snow, freezing rain, and blizzard conditions) and convective heavy rainfall for the contiguous U.S. as part of NCEP's “seamless suite” of forecast products. The SPC places particular emphasis on precipitating structures exhibiting considerable mesoscale organization and, as such, holds a strong interest in the predictability of banded precipitation found in extratropical cyclones and mid-level baroclinic zones. These features are often attributable to frontogenetic forcing in the 850-500mb layer, become established perpendicular to the local quasi-horizontal thermal gradient, and exhibit steady-state character over localized regions for a period of several hours or longer. The potential for locally enhanced precipitation rates represent an important short-term forecast problem, with bands in the cold season commonly associated with snowfall rates in excess of 1”/hr and up to 5”/hr in extreme cases. Rarer warm season banded events are often associated with training convective storms producing rainfall rates of 2”/hr or greater.
Recognition of frontogenetic forcing as a mechanism for ascent, and stability as a modulating factor in band intensity and structural character, is central to proper diagnosis of banded systems. Case examples are used to illustrate distinguishing characteristics between narrowly banded systems, multiple banded systems, and precipitation areas absent of banding through consideration of spatial and temporal continuity of quasi-horizontal frontogenesis, gravitational and slantwise instability (evaluated using equivalent potential vorticity), and traditional quasi-geostrophic forcing. Favorable large-scale environments for the development of deformation zones, which are often highly frontogenetic, and characteristic straight rather than strongly curved mid-level hodographs are described as clues the forecaster can use to better anticipate the potential for long-lived banded precipitation in conjunction with explicit measures of frontogenesis. Lastly, operationally pertinent questions are posed in regard to banded precipitation systems to encourage avenues of research that may result in forecast improvements. Specifically, the author argues for the need of a comprehensive climatology of mesoscale bands – particularly those not associated with cyclones - which quantifies vertical shear, baroclinity, and stability for both warm and cold season events.
Poster Session 1, Poster Session P1 with Coffee Break
Tuesday, 24 June 2003, 2:30 PM-4:00 PM
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