South Bank Low Pressure on 13th and 14th What are the differences in features and points of interest from the previous time? –Weathernews

2022/02/12 11:30 Weathernews

From February 13th (Sun) to 14th (Mon) following the three consecutive holidays, it is expected that it will rain and snow in western and eastern Japan due to the influence of the so-called “southern coastal cyclone” in which a low pressure system advances south of Honshu.

We have summarized the differences in characteristics between the south bank low pressure that caused heavy snowfall in the Kanto region last month and the south bank low pressure that left the local influence the day before yesterday.

Although the pressure distribution is different, the situation is similar even when compared with the past two times

In the table, the snowfall of 10 cm was brought to the center of Tokyo (1) heavy snow last month (January 6), the snowfall remained at 2 cm in the center of Tokyo (2) the snow of the day before yesterday (February 10), and (3) this forecast (February). We are comparing the three patterns (13th and 14th).

First of all, it is a characteristic of low pressure. (1) Last month and (2) the day before yesterday, there was a pattern in which a large-scale cyclone was moving eastward over the sea as far south as the latitude of Okinawa, and a small cyclone was generated on the north side due to the influence of the topography. (3) This time, it is expected that the large-scale cyclone itself will travel just south of Honshu.

At first glance, it looks different, but considering the speed at which the cyclone progresses, there is no big difference in the intensity and duration of precipitation.

Next is the strength of the cold air in the sky. This is (1) last month, (2) the day before yesterday, and (3) it is expected to be regarding the same this time.

Finally, there is a caveat due to the surrounding pressure distribution. (2) The day before yesterday, the atmospheric pressure is expected to be high following the vicinity of northern Japan, and (3) the relative high pressure part is expected to be near the Tohoku region this time. In that case, although the temperature on the ground is regarding 1 ° C, it may rise slightly, so attention should be paid.

The focus this time is the low pressure course

Examples of differences in the course and effects of cyclones

③ In this case, how far the low pressure system passes from the land seems to be a measure of the magnitude of the impact.

（A）

When a cyclone passes through a course that is relatively close to the land (toward the north), it often rains in the metropolitan area due to the influence of the warm air that the cyclone draws from the south.

（B）

If the cyclone passes moderately away from the land, and if the cyclone advances near the north of Hachijojima in the Kanto region, it will snow more often in the metropolitan area due to the influence of the cold air that the cyclone draws from the north. increase. Heavy snowfall may occur depending on the degree of low pressure development.

（C）

If the cyclone travels far away from the land (toward the south), the cyclone draws cold air from the north, but the developed clouds often do not reach the land and it does not snow or rain.

Also, not only the course but also the development of low pressure may change whether it is rain or snow. Even if it is a rain pattern as in (A), the surrounding air may be cooled and turned into snow as the rain gets stronger.

③ This time, the possibility of rain is high at the beginning of the rain. It is also necessary to pay attention to the timing when it changes to snow, and if it changes to snow early, there is a possibility that the amount of snow will increase as a result.

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. 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.

(2) Even in the case of the day before yesterday, if the temperature remained 1 ° C lower, it is possible that there was regarding 10 cm of snow in the city center. ③ Again, in Tokyo, the range of prediction error is quite large, from 0 cm to 10 cm or more.

In fact, there are large variations in the simulation results calculated by institutions around the world such as Japan, Europe and the United States, and it is difficult to identify not only whether it is rain or snow, but also when it will be affected.

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.