* Super typhoon bearing down on northern Philippines
* Could halve rice harvest - Cagayan governor
* Evacuations begin; landfall expected by 8 a.m. (0000 GMT) (Updates with expected arrival/exit time, quote, details)
By Manny Mogato
MANILA, Oct 17 (Reuters) - A super typhoon bore down on the northeastern Philippines on Sunday packing winds of more than 250 kph (155), and evacuations began before it makes landfall on Monday morning.
Typhoon Megi would be felt on Sunday night in the north of the main island Luzon, a rice and corn growing area, and the government advised up to 7 million people in its direct path to stock up on food and medicine.
Government forecasters said waves off the east coast could be greater than 14 metres (46 ft), and advised against travel to the region as Megi could bring flash flooding, landslides and storm surges. Manila was not expected to be affected by the typhoon.
Tropical Storm Risk (http://www.tropicalstormrisk.com) said Megi had intensified into a category 5 super typhoon, the highest rating, with winds of more than 250 kph, and would retain that strength as it crossed north Luzon on Monday.
The governor of northern Cagayan province, Alvaro Antonio, told Reuters the typhoon could cause great damage to the rice crop.
"Based on our estimates, about 50-60 percent of our rice production could either be destroyed or damaged. We are the top producer of rice," he said.
The Cagayan valley produces about of a third of the country's rice. The head of the state grain agency said last month that rice output would be about 3.2 million tonnes short of a projected 17.4 million tonnes this year. [ID:nSGE68G083]
Last year, the country lost 1.3 million tonnes of paddy rice following three strong typhoons in September and October, prompting it to go to the market early to boost its rice stocks.
Megi, the 10th and strongest tropical storm to hit the country this year, was about 390 km (240 miles) east of northern Luzon on Sunday afternoon and moving at 22 kph towards Cagayan, one of 20 provinces placed on alert, government forecasters said.
It was expected to make landfall by 8 a.m. (0000 GMT) Monday and cross north Luzon during the day. By evening it was expected to be over the South China Sea and heading towards China.
Megi, known locally as Juan, was estimated to have rainfall of about 20 mm (8 inches) per hour, the forecasters said. People living along the coast, near river banks and low-lying areas in the northern Cagayan and Isabela provinces have started moving to higher grounds, Benito Ramos, head of the national disaster agency, told reporters.
Trucks, rubber boats, amphibious vehicles and generators were sent to the north to help in the rescue and relief operations, he added. Schools in the north will be closed on Monday.
Graciano Yumul, head of the weather bureau, called on people to take precautions and stock up on food, fuel and water as well as emergency power because electricity services might be affected.
"This is a very strong typhoon and we're asking our people to be vigilant," he said, adding strong winds could uproot trees and damage houses made of light materials. "We don't want any casualty."
In July, a typhoon killed 102 people. It unexpectedly changed direction and sliced through Manila, cutting power across the sprawling metropolis of 12 million people, prompting President Benigno Aquino to sack the chief weather forecaster. ($1=43.2 pesos) (Editing by John Mair and Alex Richardson)
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