PVC/MET/M1 - PWD data intermittently missing

Present weather detector was occasionally reporting a hardware problem during this time. 
The reported error did not affect visibility readings, but caused all other data to report 
as missing. Error resolved its self with no indication of the problem cause.

Use optical rain gauge for precipitation data

• pwd_pw_code_1_hr
• pwd_cumul_snow
• pwd_pw_code_15_min
• pwd_pw_code_inst
• pwd_avg_H2O_int_1_min
• pwd_cumul_water