Snow caused by the south bank low pressure on February 10th yesterday Why is the impact reduced in Tokyo? –Weathernews

2022/02/11 21:00 Weathernews

Yesterday, February 10th (Thursday), it snowed in the Kanto Koshin region and Shizuoka prefecture due to the influence of the so-called “south bank low pressure”, in which a low pressure system advances over the southern seas of the Kanto region.

In the 23 wards of Tokyo, the impact was lighter than the previous day’s forecast, and there was no heavy snowfall. On the other hand, in Chiba and Ibaraki, there were places where snow was equal to or greater than the forecast.

Weathernews is analyzing the mechanism to utilize it for future forecasts, but first we will review the phenomenon in a breaking news and consider the reason.

Maximum snowfall of 2 cm in central Tokyo

Comparison of snow depth (thickness of snow from the ground) with the time of heavy snow last month

It was forecast that it would snow in the morning in central Tokyo, but in reality it was almost raining until following noon. Although the snow area expanded following noon and it snowed in the city center, the snowfall was not as expected as 5 to 10 cm the day before, and only 2 cm of snow was observed at the maximum.

Comparing the difference from the heavy snowfall on January 6th last month in terms of snow depth (thickness of snow from the ground), we can see that the situation is less than the previous time in the 23 wards of Tokyo. On the other hand, it can be seen that there are places in northwestern Chiba prefecture and Ibaraki prefecture where the amount of snow is equal to or greater than last month.

Why it was raining in the morning in Tokyo

First of all, I will explain the reason why it was raining instead of snow in the morning using images. It is a map of the Kanto region with the temperature and wind direction and speed as high as the tip of the Tokyo Sky Tree, which is the lower layer in the atmosphere.

Looking at the figure at 6 am on the left side, we can see that the cyclone was in the south of Shizuoka prefecture, and the wind blowing toward the cyclone was blowing from the east-northeast. On the other hand, in the figure at 9 pm (21:00) on the right side, it can be seen that the cyclone went east of the Boso Peninsula, and the wind toward the cyclone was blowing from the north-northeast.

Looking at the colored part of the cold air, in the figure at 6 am, it is pushed by the wind from the east and stays in the northwestern part of the Kanto plain, but in the figure at 9 pm, it goes south according to the flow of the wind. You can read each of them.

In short, it is thought that it melted because the cold air was not enough while it was snowing from the sky, and it rained instead of snow in the morning. It is probable that in the followingnoon, due to this change in wind direction, cold air gradually flowed into the area around Tokyo, and it fell without melting the snow.

Why did it snow in Chiba and Ibaraki instead of heavy snow in Tokyo following the cold air became sufficient?

After the night when the cold air is strong enough, another change will begin to be seen. It’s the intensity of precipitation. In Tokyo and Kanagawa, precipitation began to weaken by the time the cold air became stronger. On the other hand, in Chiba and Ibaraki, band-shaped clouds stagnated in the morning, and heavy precipitation continued.

Of course, if there is a lot of precipitation, the amount of snow that falls will increase, and if the precipitation is strong, the effect of “lowering the ambient temperature when the snow melts” will be stronger, and it will also lower the temperature on the ground. It will connect. For this reason, it is thought that the temperature on the ground has dropped, making it even easier for snow to accumulate.

As a result, it is probable that heavy snow did not occur in Tokyo, where it rained for a long time instead of snow, and heavy snow occurred around the band-shaped clouds that stagnated from the vicinity of Chiba City to the vicinity of Mito City.

These are one of the factors that can be considered at this time, and it is thought that various factors such as the situation where water had accumulated on the ground were intertwined and the heavy snowfall in Tokyo was avoided. Weathernews has analyzed the phenomenon in detail and has begun efforts to improve the forecast in the future.

Real-time analysis of the boundary between rain and snow

The boundary between rain and snow can be easily estimated by “ground temperature” and “humidity”. This “rain and snow discrimination map” is published in textbooks as a rough guide from past research.

However, since this boundary line may shift depending on the region and weather conditions, Weathernews used user posts to analyze the boundary line in real time, and it is slightly higher than the past empirical rule in the textbook. I found out that it was snowing due to the temperature.

Also, from this figure, it can be read that rain or snow changes even with a difference of only regarding 1 ° C.

Rain or snow changes even with a difference of only 1 ° C

Compared to “heavy snow due to winter-type pressure distribution”, the south bank low pressure is a very difficult phenomenon to forecast. Moreover, in the south bank low pressure, it is often the case that rain or snow changes even with a temperature difference of only 1 ° C, and it can be said that there is a big difference in the effect.

Precipitation in central Tokyo was 29.5 mm until this morning, so if it had all fallen as snow, it would have been regarding 10 cm of snow. It’s safe to assume the same confusion as last month.

The trouble is that even from the 13th (Sun) to the 14th (Mon) the day following tomorrow, it is undeniable that the snowfall will be affected by the passage of the south bank low pressure. The forecast may change by the time just before, so be sure to anticipate cases where the forecast will be bad and heavy snow will occur.

It is a phenomenon that the effect is greatly different even if it is a slight difference, so it is helpful to consider various possibilities to reduce the damage.

Reference materials, etc.

Photo: Weather Report (Posted from Weathernews App) Takeyanko-san