A tropical low formed earlier this week some 400km (250 miles) southwest of southern Guatemala, in what meteorologists call a gyre, or a large area of disturbed weather.
On Sunday, it was named Tropical Storm Amanda.
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The storm will produce torrential rains all the way from southern Mexico to the Costa Rico-Panama border.
On Wednesday, parts of El Salvador recorded a total rainfall of 80-120mm (3-5 inches) which prompted the National Emergency Commission to issue weather warnings across the country.
Areas in both El Salvador and Guatemala could see as much as 600mm (24 inches) of rain in the next few days.
Many areas across the region could receive as much as 50mm (two inches) of rain in just a few hours.
Such heavy downpours are likely to produce life-threatening flash floods and mudslides in mountainous areas.
While tropical systems often form near Central America, or even make landfall along the coast with the Caribbean Sea, a Pacific system making landfall is much less common.
The last tropical system to do this was Tropical Storm Selma, which formed late in October 2017. Selma was the first ever tropical storm to make landfall in El Salvador.