Yes, the forecast can be very unreliable at this point. The
2017 forecast shows the unreliability quite well. With 1 week to go before the HM, we had no indication of any severe weather -- only the possibility of showers. And, despite the 1 week prediction of 55 (or 52) degrees as the low temperature, the 2017 FM had an official race start temperature of 37 degrees with strong 30+ mph wind gusts.
(Full credit to
@rteetz for the following weather info; I copied it from his 2017 Weather Watch thread)
Weather.com forecast on 12/31/2016
1/4- High 77 Low 54
1/5- High 77 Low 56
1/6- High 74 Low 56
1/7- High 75 Low 55 showers at night
1/8- High 70 Low 52 shower in the morning
Then, the predicted temperatures dropped significantly in just 1 day... but still about 10+ degrees away from the actual temps.
Weather.com forecast on 1/1/2017: