Canada crude-Synthetic premium hits 13-month high

* Synthetic last quoted at $14.75/bbl over WTI

* Suncor in early stage of upgrader turnaround

* WCS quoted at $9.50 under WTI

CALGARY, Alberta, Sept 12 (Reuters) - Canadian synthetic crude premiums climbed on Wednesday to their highest in 13 months as supplies remained tight with a processing unit at Suncor Energy Inc's oil sands facility down for planned maintenance.

Light synthetic for October delivery last sold for $14.75 a barrel over benchmark West Texas Intermediate, up $1 from the day before, its widest premium since August 19, 2011, according to Shorcan Energy Brokers.

The strength of the oil sands-derived crude has surprised some market players, who point out that the Suncor work had long been scheduled.

Suncor said last week that the upkeep on a vacuum tower at one of its upgraders will include shutting a coker unit at the Northern Alberta site. The turnaround is scheduled to last six weeks.

Fat premiums come after both Suncor and Syncrude Canadian Ltd pumped out big jumps in volume in August. Suncor produced a company-record 373,000 barrels a day, up from 351,000 the month before.

Syncrude produced 359,500 bpd, up from 262,000 in July and the most since December 2009, according to Canadian Oil Sands Ltd, which has the biggest ownership stake in the joint venture.

Heavy crude prices also strengthened on Wednesday. Western Canada Select heavy blend was last quoted at $9.50 a barrel under WTI, compared with $10.75 a barrel under on Tuesday.