Ray Martin and a colleague have generously extended the ISP data record back to 1963:
I've captured all of this and generated a snowfall record for ISP from 1963 through this year. It is complete from the perspective of the record that it was extracted from, but it is not complete:
http://www.northshorewx.com/ClimateData/IslipDailySnowfall.pdf
Prior to June 1999 and since January 2004 the record looks reasonable. It contains no snowfall for the 4 seasons from 1999 - 2000 throug