* Bombardier desires "optimal weather" for inaugural flight
* Environment Canada forecasts rain all week in Quebec town
* Plane performed high-speed taxi runs on Monday morning
By Solarina Ho
TORONTO, Sept 9 (Reuters) - Rainy, cloudy weather is forecast for Bombardier Inc's flight test site in Quebec for much of this week, adding to suspense over the unspecified date for the maiden flight of the Canadian company's all-new CSeries jetliner.
The 100- to 149-seat CSeries is the first completely new narrow-body aircraft in its class in decades, and is Bombardier's attempt to break into a market dominated by Airbus and Boeing Co.
The first flight is an important milestone for the CSeries, and the $3.4 billion aircraft is already nine months behind its original schedule.
Bombardier, the world's fourth-largest plane maker, has said the CSeries will be more fuel-efficient and economical than other planes in its class.
The Montreal-based plane and train-maker said on Monday it was "getting close" to first flight, the start of the next phase of the program that will determine whether the CSeries can live up to its performance and efficiency promises.
"Optimal weather is preferred. It's just one less obstacle to have to worry about when you're going up for the first time," said Bombardier spokeswoman Marianella de la Barrera, adding that she did not expect first flight to occur in rainy weather.
"At this point ... we're at the mercy of Mother Nature."
The company declined to give a date for the first flight, although a source familiar with the situation has tipped Sept. 17 as a likely deadline.
In a sign that the first flight might be only days away, Bombardier on Friday finally began high-speed taxi trials at its test site at Mirabel, Quebec.
"The wind speeds that we've been clocking in Mirabel have not been optimal for us, so they have hampered us somewhat," de la Barrera said, explaining that Bombardier sought to avoid high winds during taxi tests to prevent an inadvertent take-off.
"But having said that, we're looking good. It's taking a little longer than we had anticipated only because of the wind speeds."
The trials on the test plane, known as FTV1, continued on Monday morning under mostly cloudy but rain-free skies with low winds. High speed taxi runs of 120 knots were reached, de la Barrera confirmed.
Wind speeds at the airport were under 10km/h prior to mid-morning, according to Environment Canada.
Plane-tracking website planefinder.net as well as live broadcasts of air-traffic control communications on liveatc.net offer the public - which includes a handful of people gathering daily in Mirabel to observe the runway progress - a glimpse into the jet's daily activities.
Late on Monday, Environment Canada forecast rain at Mirabel, some 50 km (30 miles) north of Montreal, all week. The weather is expected to turn sunny by Saturday.