Westmoreland took over top spot late in the session, his time of a minute 30.346 seconds just 0.017s quicker than Samsung Honda's Michael Laverty.
Australian Josh Brookes took third ahead of Westmoreland's team-mate Alex Lowes, who had led the way early in the session and holds the final Showdown spot - sixth - in the standings.
Brookes and Laverty are second and fourth. Top rider and defending champion Tommy Hill was eighth after a crash at Coppice.
Alastair Seeley put his WFR Honda fifth ahead of the Padgetts Honda of Luca Scassa and Buildbase BMW's Barry Burrell.
Michael Rutter and Ian Lowry rounded out the top 10 while Noriyuki Haga crashed at Redgate to finish 16th, but like Hill avoided injury.
The Japanese's fellow Showdown contenders Stuart Easton and Chris Walker were 15th and 22nd respectively.
1. James Westmoreland (WFR Honda) 1m:30.346s
2. Michael Laverty (Samsung Honda) +0.017s
3. Josh Brookes (Tyco Suzuki) +0.071s
4. Alex Lowes (WFR Honda) +0.105s
5. Alastair Seeley (WFR Honda) +0.348s
6. Luca Scassa (Padgetts Honda) +0.482s
7. Barry Burrell (Buildbase BMW) +0.571s
8. Tommy Hill (Swan Yamaha) +0.714s
9. Michael Rutter (MSS Bathams Kawasaki) +0.742s
10. Ian Lowry (Padgetts Honda) +0.778s