Little is as important in the Japanese spring than knowing when cherry trees will bloom.

Last year's prediction was off by several days, leaving forecasters at the Meteorological Agency red faced.

Japanese couples in row boats pass beneath cherry blossoms in a moat near the Imperial Palace in Tokyo, in this March 20, 2002 file photo. Little is as important in the Japanese spring than knowing when cherry trees will bloom. [Reuters]

Stung by a storm of criticism, the agency has revised its forecasting model to predict when the delicate pink flowers, a national obsession, will emerge.

"It is true that last year our predictions for the cherries were off by about four days, and we got a lot of complaints," an agency spokeswoman said.

Global warming, it seems, is to blame.

Until this year, the agency based its prediction on data from the past 50 years but global warming is making those figures less trustworthy and the period has been narrowed to 30 years.

The arrival of warmer weather prompts the blooming and Japanese follow the event intensely as it moves south to north over about a month. Media publish frequent updates.

The traditional appeal of cherry blossom viewing is said to lie in its poignant reminder of the shortness of life, but for many people it is an excuse to celebrate in often raucous "hanami" (cherry-viewing) parties.

Planning these parties is a matter of prime importance, which is why accurate predictions are crucial.

The cherries began to bloom in the western prefecture of Kochi on Thursday. Tokyo's trees are predicted to start blossoming on March 22.

The weather agency has erred in more than cherry blossom predictions in recent months.

Last autumn, they said the winter just ending would be warmer than usual. Instead, it was the most severe since World War Two.

"Newspapers say our reputation is at stake over the cherries, but that isn't true," the spokeswoman said. "We give weather predictions and cherry predictions the same scientific treatment."