It is a sunny start to the school holidays for many areas today, but the good weather is not expected to last.
MetService says heavy rain and strong winds can be expected as the middle of the week approaches,
Meteorologist Lewis Ferris said: "Wednesday and Thursday are the main risk period for severe weather and heavy rain watches are already in force for western and northern parts of the South Island.
"It's likely some of these will be upgraded to orange warnings and areas in the North Island may be added."
Heavy rain watches are in place for the Richmond and Bryant Ranges, including the Rai Valley, Tasman, northwest of Motueka, the Buller and Grey Districts, and the Westland District.
MetService said the "potentially severe weather" was the result of a low pressure system coming from the Tasman Sea, bringing warm and humid air.
"While this does mean some warmer than average temperatures, it also increases the chance of heavy rain as the system passes over.
"Strong northeasterly winds will also be seen before the rain arrives."
It said the North Island would experience mostly warm and humid air, but snow could be seen in the South Island.
"Thursday brings the risk of snow falling down to around 500 metres for inland Otago and southern parts of Canterbury. Prolonged rain is also possible around these areas and severe weather forecasts might be issued."
MetService said people should keep up to date with forecasts this week as the weather was likely to be much different that it currently was, and "severe weather may impact numerous regions".
