The first thunderstorm of the flood season is expected to cause flooding and disrupt traffic, the Shanghai Meteorological Bureau said yesterday.

Rain was coming down in most of the city by 8pm last night, the bureau said.

At 9:50pm, it issued a yellow lightning alert for the whole city, warning of thundershowers overnight and this morning

The city is expected to get between 40 and 70 millimeters of rainfall and in some areas it could be as much as 70 to 100 millimeters. Between 20 and 40 millimeters of rainfall are expected per hour.

The storm will push the mercury down by 5 degrees Celsius to a high of 22 degrees.

After a one-day dry break on Thursday, another round of showers will arrive on Friday night and last through Saturday daytime.

Cloudy skies will return on Sunday as the mercury rises.

On Thursday the high will rise to 25 degrees but dip to 22 degrees on Friday and Saturday. On Sunday the mercury will soar to 27 degrees when the skies clear. The fair weather is expected to continue for five days.

More than half the month of May was rainy and by Monday, it reached 142.2 millimeters, nearly doubled the amount of precipitation the city saw at the same time last year.

Part of Shanghai suffered several floods at this time last year, which led to traffic congestion and flight and train delays or cancellations.

In a rain storm on June 17 last year, more than 20 roads in districts including Hongkou, Baoshan, Yangpu and Jiading were flooded to a depth of between 10 and 20 centimeters. Ten residential communities experienced flooding of 5 to 10 centimeters. The heavy rains flooded parts of local Metro lines and caused train delays.

This spring has been the longest recorded — 113 days.

The bureau will likely announce the arrival of summer on Sunday if the average temperature between June 5 and 9 is at least 22 degrees.

The solar term mangzhong, or grain in ear, falls on Sunday. It indicates the ripening of some crops, such as barley and wheat, and it's a busy time for farmers to harvest. Frequent rainfall is seen during this period.

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