What Is a Squall Line? A Cause of the Semarang Floods?

Semarang has been submerged in floods since Wednesday (13/3/2024) after heavy rain poured all day.

According to Climatology Expert from the National Research and Innovation Agency (BRIN), Erma Yulihastin, this flood is caused by the squall line phenomenon.

In her Twitter account, Erna stated that the heavy rain in Semarang is attributed to the contribution of the 18S cyclone seed, which is moving slowly. Due to its slow movement, this cyclone seed triggers numerous squall line storm formations.

"Since the beginning (March 11) from the prediction based on the meso-scale model we developed, it is predicted that the vortex (091S) which has turned into the 18S cyclone seed will tend to move slowly with an orientation from west to east. This is because of the low pressure in the east, which has now become two vortices (swirls)," Erma said on her Twitter account @EYulihastin, Friday (15/3/2024).

"The slow movement and not immediately moving away towards Australia are what triggered the strong rain propagation and the proliferation of squall line storms causing persistent rain for days, and even the intensity of rain can be extreme, accompanied by strong winds," she added.

So, what is a squall line?

According to the National Weather Service (NWS) website, which is the US equivalent of BMKG, a squall line is a group of storms arranged in a line. This phenomenon is often accompanied by severe windstorms and heavy rain.

Squall lines tend to pass quickly and can be hundreds of miles long, but typically only 10 or 20 miles wide.

"Sometimes thunderstorms will form in lines that can extend sideways for hundreds of miles. These 'storm lines' can last for hours and produce damaging winds and hail," according to the NWS statement.

Continuous upward airflow forms again in front of the storm system. Often rain and hail accompany it. The updrafts and downdrafts of storms along this storm line can become very strong.

Although the front part of the storm line sometimes forms tornadoes, it can also produce damage due to "straight-line winds," due to the force of downward airflow spreading horizontally when it reaches the Earth's surface.

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