It was supposed to have snowed last week! What happened?
To state the obvious, in order for it to snow, it needs to be cold, and there needs to be precipitation. In making their predictions, forecasters rely on experience as well as computer models of how the atmosphere will evolve given the observed starting conditions. The computer models used for the east coast include short and long range NWS models, as well as Canadian, British, and European models. Each model has a different understanding of the atmospheric physics involved, as well as a different view of the starting point. The observations that make up the initial conditions for the models can include satellite measurements, airport observations, and a variety of other inputs like roadside sensors. Even if this data is shared by weather services (some of it is not), it’s not all used because a bad reading of a temperature used can make a model too warm or too cold; some quality control needs to be performed.
Last week’s problem was that the models were giving widely varying answers as to where the precipitation was going to be, how much was to fall, and how cold the atmosphere was going to be. As you can see from last Friday's snowfall map, the snow ended up south of Lorton. The other problem we had in Lorton was that it was too warm to snow - not just on the ground but in the air above us. If it’s warm aloft, even if it’s snowing in the clouds, if it melts before it reaches the ground it’s just rain to us down here!
We don’t understand completely the physics of the atmosphere. We don’t have accurate measurement of the weather everywhere. As a result, sometimes the forecasts, which are approximations at best, end up being wrong. Because in the end there are people involved in forecasting, my judgement is we’ll not be surprised by a snowfall - our problem is that it’s going to be forecast more often than it occurs.
My rain gauge in Lorton recorded the following precipitation at 6am on the day indicated for the previous 24 hours:
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