Typical DC-to-AC capacity factor for flat roof PV in Massachusetts

I get much different production/capacity factor numbers for Massachusetts than the Mass Clean Energy Center says are typical, and I'm wondering if I'm doing something wrong in my simulation. The Mass CEC says 12.8% is typical and I know a number of veteran installers that use 1,100 - 1,200 as a rule of thumb for first year kWAC/kWDC for flat roof systems, which matches. Under a typical commercial flat roof system design I am getting 14.4%, which is significantly different. Which seems more appropriate to others? I want to be conservative, but realistic.

Here are my relevant settings:
System degradation: 0.5%
Availability: 99%
mismatch: 98%
Diodes and connections: 99.5%
DC Wiring: 98%
Soiling: 95%
Nameplate: 100% (only use panels with positive tolerance)
AC wiring 99%
Azimuth: 0
Tilt: 10 degrees
Inverter: Satcon PVS-110-S-MT 240V
Shading: none

I'm wondering if I should put some shading in the winter months to account for snow. Anything else seem out of line?

Thanks!

SAM does not account for lost production due to snow covering modules. That is one possible source of difference between the capacity factors in SAM and the typical values you are expecting. One simple way to account for the difference would be to adjust the availability factor on the System Performance page. That approach is only an approximation because it decreases the total annual production.

Best regards,
Paul.