* April synthetic $6.75/bbl over WTI * Canadian Natural reports Horizon maintenance * April WCS quoted at $25.25 under WTI CALGARY, Alberta, March 5 (Reuters) - Canadian synthetic crude prices jumped to a five-month high on Tuesday on word of a further tightening in supplies from some of the largest oil sands mining and processing plants, industry sources said. Light synthetic for April delivery was quoted at $6.75 a barrel over the benchmark West Texas Intermediate contract, up $1.15 from Monday's settlement, according to Shorcan Energy Brokers. That was its highest premium since October 4. Canadian Natural Resources Ltd said it is conducting routine maintenance at its 115,000 barrel a day Horizon oil sands project in northern Alberta, but did not give details of the work or the impact on production levels. The work is in advance of a planned 18-day maintenance shutdown of the facility this spring, which is shaping up to be a busy period for maintenance. Maintenance on a hydrogen plant at Suncor Energy Inc's Northern Alberta oil sands facility is set to start by the end of this month, market sources said. The planned 14-week shutdown will lower output from Upgrader 1, one of two plants that convert mined bitumen into synthetic crude. The company also expects to start a full shutdown of Upgrader 1 in the second quarter. Lasting seven weeks, the maintenance will curtail the unit's 100,000 bpd of synthetic output. Last week, Canadian Oil Sands Ltd, the largest interest owner of Syncrude Canada Ltd joint venture, said production fell 13.5 percent to 240,000 barrels a day in February. That is down from capacity rates of about 350,000 bpd. Heavy crude prices also rose, and traders said they have been surprised by the strength, which arose with little change in fundamentals that include growing production and tight pipeline capacity to move it to market. April Western Canada Select last sold for $25.25 a barrel under WTI, compared with Monday's settlement of $26.30 under.