Why is it difficult to accurately forecast snowfall?

First of all, it needs to be clear that rain and snow are water with different phases, so snowfall forecast and rainfall forecast can be collectively called precipitation forecast in meteorology.

We know that whether it rains or snows, the basic conditions of precipitation need to be met, and snowfall mostly occurs in the precipitation weather under the winter climate background.

Similarity: Meteorological factors such as large-scale circulation background, water vapor conditions and dynamic conditions that need to be paid attention to in rainfall forecast also need to be paid attention to in snowfall forecast. At this point, the forecasting methods and skills of the two are also consistent.

Difference: Winter precipitation is often accompanied by the phase change process of rain and snow. Accurate phase prediction is the basis of snowfall and snow depth prediction, which is not available in rainfall prediction.

When we pull the viewing angle to the vertical height of several kilometers, we can find that the temperature distribution in different heights of the atmosphere is not uniform. Under different heights and different temperature and humidity conditions, the morphology of precipitation particles has various changes, and the precipitation phase will also undergo very complicated changes.

Schematic diagram of atmospheric temperature changing with height. Snowfall only occurs in the troposphere.

When ice crystals or snowflakes fall in the clouds, it is like an unknown adventure.

Only when the whole floor is cold (such as below 0℃) will the snow fall to the ground. If the temperature of the whole layer is higher than 0℃, it will become rain. In fact, the temperature changes between layers of the atmosphere are often more complicated than imagined:

Case 1: If the temperature of the upper and lower layers is lower than 0℃, but there is a shallow warm layer in the middle, and the snow in the upper layer freezes in the middle and lower cold layers after melting in the middle warm layer, then it is ice particles.

Case 2: If the middle warm layer is thick, the snow on the upper layer will rain if it is too late to freeze after the middle warm layer melts.

Situation 3: If the ground temperature is high or the water content of snowflakes is high, it will snow sometimes, and it is difficult to form obvious snow on the ground, but it is wet.

In reality, due to the influence of atmospheric circulation system and complex dynamic environmental conditions, the real vertical temperature of the atmosphere is not so simple stratification. In addition, the temperature near the ground is different, which makes the "urchin" of precipitation enter the public eye when and in what form? Need complex comprehensive judgment. This is also the reason why it is difficult to accurately predict snowfall.