The forecast was wrong 55% because of the weather models and 45% because of wishcasting, groupthink, and human error.
As I cautioned last week, the weather models were showing a pretty big storm hitting the Washington DC area, but it also showed temperatures sitting above freezing for almost the entire storm. You can't get a foot of snow when it's 37 degrees. Forecasters were aware of this but they were banking on two things:
1) Evaporative cooling. When rain falls through dry air, it evaporates. This evaporation cools the air while it raises the amount of water vapor. Meteorologists were expecting that the rain/snow would fall into the very dry airmass sitting over the DC area, evaporate, and bring temperatures down below freezing. That didn't happen. Rain fell and saturated the air well before temperatures got out of the mid-30s, so the precipitation kept falling as rain.
2) The models were calling for the low pressure system to wrap cold air into the area from the northwest, which never materialized.
Folks who live in the DC area are very well acquainted with the rain/snow line. The freezing line often hovers parallel to I-95 somewhere in the DC area due to their proximity to the ocean. One side (usually to the east in this case) is all rain, and the other side (to the west) is all snow. Since meteorologists expected the air to cool below freezing, they didn't expect much of a rain snow line west of the Delmarva Peninsula.
They were wrong. The weather models were also wrong. They were showing massive accumulations even with temperatures well above freezing -- a combination forecasters wishfully ignored because of the expected evaporative cooling. They wanted to predict snow and have it not happen instead of predicting nothing and having it snow a ton.
The forecast was rather accurate west of I-95 where it didn't rain very much, but right along the transition zone and east (which happens to be where DC was), it was a huge fail.
It was a major, embarrassing bust for everyone, myself included, and especially the people who hyped this up so much that their entire credibility was on the line. I held off until Sunday until I was literally the only person saying there wouldn't be much snow in/around DC, which highlights the groupthink aspect. One news station doesn't want to be the only one saying it's not going to snow.
Weather is an inexact science. We need to save our wolf cries for when we're absolutely sure things are going to get bad. The next time the models are calling for snow while simultaneously saying it's going to be 36 degrees, maybe believe it.