This would be a shock to the system for many who had been enjoying temperatures in the mid 20s with recent northwesterly winds causing temperatures to climb, he said.
"It's been quite nice over the last few days."
A high pressure over the Tasman Sea would bring cooler air, showers and stronger winds especially on the coast, Mr Law said.
"It's amazing how much a wind change can affect things."
Temperatures wouldn't be anywhere near as low as parts of the South Island though, he said.
"Even in Hawke's Bay it's cooler than what you've been used to, the weekend at least will be good."
Overnight lows would drop into single figures with 7C forecast for Tuesday and Wednesday in Napier and Hastings.
Wednesday would be the coldest with a high of 16C for Napier and 15C for Hastings.
"It's going to feel cold, especially since it's been so warm for so long."
However, the temperatures were not uncommon for this time of year and would be warming up for the rest of autumn according to Niwa.
Niwa Climate scientist Petra Pearce said Hawke's Bay could expect above-average temperatures until early winter due to elevated sea temperatures since November.
Rainfall looked to be slightly above average for the next three months, also because of the warm sea surface temperatures.
