Moderate to heavy rain will drench the Southland from Tuesday night through Thursday producing the wettest storm in Southwest California this rainy season.

Between late tonight and Tuesday morning, the atmosphere will moisten rapidly as subtropical moisture associated with an atmospheric river reaches the region, according to forecasters.

The atmospheric moisture is expected to build up in the tropical Pacific before reaching the West Coast, packing a huge amount of rain, said National Weather Service meteorologist Rich Thompson.

NWS Los Angeles tweeted today there is a moderate to high risk of debris flows near recent burn areas and flash flooding may occur anywhere with heavy rainfall.

The NWS said overnight that it would wait for additional computer models before issuing flash-flood warnings, even though it’s already clear that periods of moderate to heavy rain are possible Wednesday through Thursday night.

City News Service contributed to this report.