CHAPTER THIRTEEN - PRE-TROUGH WINTER PRECIPITATION FORECASTING

Abstract

This chapter focuses on pre-trough winter precipitation forecasting. It presents a method for qualitative and quantitative forecasting of winter precipitation for the central and eastern portions of the United States and eastern Canada, which is not too time-consuming and selects its parameters directly from the surface synoptic chart and standard upper-level surfaces, at 850 mb, 700 mb, and to a lesser degree, from the 500-mb charts. The broad field of winter precipitation for the extensive areas considered may be divided arbitrarily into categories, each relating to the primary cause of the precipitation. The causes are listed as: (a) over-running of warm and moist air, (b) cyclonic activity, (c) orographic forced lifting, and (d) thunderstorm activity associated with cold fronts and line squalls. For qualitative forecasting of precipitation, the major parameters used are (1) moisture distribution and movement and (2) existing vertical wind shear. The chapter also presents objective yet practical methods by which the forecaster can make a decision as to whether the precipitation in winter will be rain, snow, freezing rain, sleet, or some combination of these.