MetService meteorologist Alwyn Bakker said a frontal system over the Tasman Sea was forecast to move on to the far south about noon today, then move slowly northwards over southern and central New Zealand by midnight tomorrow.
"This front is preceded by a period of northwesterly rain in the west of the South Island, while a period of southerly rain follows the front in eastern regions.
"There is the possibility of some heavy falls over southern New Zealand, and severe weather watches are in force for some areas."
In Otago, some heavy falls were possible between 4pm today and 7am tomorrow, he said.
"Rainfall amounts may approach warning criteria, especially in the east, where the threshold for heavy rain is lower."
In Southland and Fiordland (north of Bligh Sound), heavy falls were also possible and may approach warning criteria between 2pm today and 3am tomorrow.
"The parched south and east of the South Island will be in for some relief, with some locations forecast to see 40mm of rain in 24 hours.
"That’s as much rainfall as parts of inland Otago have had all year.
"People are advised to keep up to date with forecasts as changes may be made, including the possibility that some of these watches may be upgraded to orange warnings."
Mr Bakker said there would also be a "dramatic" drop in temperatures this week, as the cold front moves over the country from the Southern Ocean.
"Plenty of places in Otago and Canterbury will see a fall of 10 degC from Tuesday to Wednesday.
"Alexandra is in for a particularly remarkable drop, with a forecast high of 30 degC on Tuesday and a mere 15 degC on Wednesday.
"This isn’t the largest change in temperature they’ve ever recorded in 24 hours, but it is close.
"And if Alexandra doesn’t rise above 15.7 degC on Wednesday, they will see their coldest maximum temperature this summer."
Coupled with the cold temperatures, the forecast rain may fall as snow, dusting the peaks of the ranges down to 1000m, he said.
