The snow versus rain problem usually depends upon relatively small-scale synoptic considerations, such as the exact track of the surface disturbance, whether the wind at a coastal station has an onshore component, the position of the warm front, and the orientation of a ridge east of the low.

In the larger sense, the snow versus rain zone is directly related to the position of the polar front. The location of the polar front is, in turn, closely related to the position of the belt of strong winds in the middle and upper troposphere. When the westerlies extend farther to the south, storm tracks are similarly affected, and the snow-rain zone may be farther to the south. As the westerlies shift northward of their normal position, the storm tracks develop across Canada. Concurrent with this northward shift, the United States has above normal temperatures, and the snow-rain problem exists farther to the north.

With a high zonal index situation aloft, the snow-rain zone will extend in a narrower belt, often well ahead of the surface perturbation and will undergo little north-south displacement, Those areas with precipitation occurring will not undergo a change from one form to another since there is relatively little advection of warm or cold air with a high zonal condition.

When the upper-level wave is of large or increasing amplitude (low zonal index), it is difficult to generalize about the characteristics of the snow versus rain problem without considering the surface perturbation. Up to this point, we have discussed the snow-rain pattern in association with an active low of the classical type, The rate of precipitation accumulation here is rapid, and the transition period of freezing rain or sleet is short, usually on the order of a few hours or less. Another situation in which there is frequently a snow versus rain problem is that of a quasi-stationary front in the southern states, with a, broad west-southwest to southwest flow aloft, and a weak surface low. The precipitation area in this case tends to become elongated in the direction of the upper-level current. The precipitation rate may be slow, but it occurs over a longer period. Often a broad area of sleet and freezing rain exists between belts of snow and rain, leading to a serious icing condition over an extensive region for a period of several hours or more. This pattern of precipitation changes either as an upper trough approaches from the west and initiates cyclogenesis on the front or as the flow aloft veers and precipitation ceases.

Approaches to the snow versus rain forecasting problem have generally fallen into three broad categories. The first category depends on the use of observed flow patterns and parameters to predict the prevalent form of precipitation for periods as much as 36 hours in advance. The second category consists of studies relating local parameters to the occurrence of rain or snow at a particular station, or area. In this approach, it is assumed thermal parameters will be obtainable from prognoses. This approach tends to have its greatest accuracy for periods of 12 hours, or less, since longer periods of temperature predictions for the boundary zone between rain and snow are very difficult to make with sufficient precision. A third category used involves the use of one of the many objective techniques available. A number of stations have developed objective local techniques. The method presented here is applicable to the eastern half of the United States. Thus, the general procedure in making a snow versus rain forecast at present is to use a synoptic method for periods up to 24 or 36 hours, and then consider the expected behavior of thermal parameters over the area to obtain more precision for periods of about 12 hours or less.

A number of methods based on synoptic flow patterns applicable to the United States are described in the U.S. Department of Commerces publication. The Prediction of Snow vs Rain, Forecasting Guide No. 2. These methods are mostly local in application and are beyond the scope of this manual.

Prognostic charts from the National Meteorological Center and other sources should be used whenever and wherever available, not only to determine the occurrence and extent of precipitation, but for the prediction of the applicable thermal parameters as well.

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