The problem with these forecasts is that the listed "low" generally occurs at about 6:00 AM to 7:00 AM on the next day, and not on the day under which it is listed.
Therefore my improvement for deriving more accurate daily HDD's is to take the high temperature listed for the current day, and sum it with the low temperature listed for the previous day, and divide the sum by 2 to get the average temperature for the current day. From there it is a matter of subtracting this average from 65 to get the days HDD value.
For example, using the weather forecast I've attached, to get the HDD's for Thursday 4/16, take 43 degrees from Thursday 4/16, and take 24 degrees from Wednesday 4/15 and sum them to get 67 degrees. Divide by 2 and you have 33.5 degrees as the average temperature for Thursday. Lastly, 65 - 33.5 = 31.5 HDD's for Thursday 4/16.
