Wine growers are on high alert after a forecast drop in the mercury threatened vineyards with frost throughout Napier and Hastings this morning.

Frost was expected to ice the region as overnight temperatures as low as 2C were forecast. Metservice meteorologist Claire Flynn said: "There is a ridge over us at the moment which means there isn't much wind at night."

The lack of wind meant heat would radiate away from the ground rather than mixing with warmer surface air.

Shaded areas were at most risk of the frost, Ms Flynn said.

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Winemaker Rod McDonald said many grape growers would have known about the frost for a few days already.

Viticulturists had specialist weather forecasters alerting them in these sorts of situations, Mr McDonald said.

Grapes had already been through a stage of growing called bud burst so shoots would be at least 30cm long.

Despite this crucial stage already passed, flowering had not yet happened which meant bunches of grapes and leaves were still susceptible to getting frosted.

Mr McDonald said frost had the potential to impact on yield. He also said there were three common ways to deal with forecasts like these.

"Some people use a water irrigation system, a windmill, or have a helicopter ready in case frost does appear."

All three of these methods were very effective because they each pushed the warm air down towards the ground to stop it from becoming too cold.

Cloud and some rain were expected for tomorrow and north-west winds would soon change to a southerly in the evening.