What low forecast confidence means

By Niki Morock, meteorologist

If you read this morning’s forecast discussion from the Raleigh National Weather Service Office, it actually says that the forecasters have low confidence in the forecast “from Friday night onward.” I noticed because I also have low confidence in the forecast, and I was looking to them for their take on what the models are predicting. Does this mean that we aren’t good at what we do? No, on the contrary, it means that we are good enough to know when to admit that the computer models are giving us contradictory information.

GFS dominant p-type
This map shows the dominant precipitation type for Saturday evening according to this morning’s GFS model run. The GFS is just one model ofseveral that meteorologists consider when making a weather forecast. In this map, blue = snow, purple = sleet, red = freezing rain, and green = rain.

Meteorologists do not look at just one computer model when making a forecast because that would be foolish. There are numerous weather prediction models, and each has its own strengths and weaknesses.

These models are created by taking observations from current weather, trends from those observations, complex mathematical equations, and some basic assumptions about the atmosphere and using high speed computers to calculate the most likely outcomes. The current weather observations and the math equations may be basically the same for each model, but the remainder and the resolution can vary, which is why meteorologists look at each one, study each one, and do their best to understand each one’s strengths and weakness.

When all the models show basically the same potential outcome – in other words, their prediction – it is easy for meteorologists to have high confidence in our forecasts.  When the models vary greatly in their outcomes for the same time period, then we are vexed – sometimes horribly. Those are the times when some forecasters lean on intuition and past experience and others just shake their heads.  Most of us will be intellectually honest with you and say that we are not totally sure what will happen and when.

In the case of this coming weekend, our confidence is low because one model keeps our atmosphere pretty dry until late Saturday while the other has precipitation starting Friday night.  Both have the temperature pretty close to freezing, and that makes deciding whether to mention snow or stick with rain difficult when one degree in either direction makes a huge difference in what type of precipitation will fall.

Until the models start to show some agreement, the smartest thing we can say is that we could see wintry weather this weekend, but we aren’t yet sure when or how much. It’s smart because it’s honest, but honesty doesn’t necessarily sell advertising. Flashy, grocery-run inducing headlines do.

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