This phrase goes back to Biblical times, but still holds meaning today with forecasters. Fly in the ointment refers to something in the forecast that is making the forecast more difficult, or more uncertain. The atmosphere is incredibly complex, and it doesn’t take much sometimes to throw off an entire forecast - which can be incredibly frustrating for meteorologists.

This is exceptionally common during the winter months, when certain variables make the winter forecast more difficult to make certain. For example, if the temperatures are right on the border of being snow vs rain, that would be a fly in the ointment. Other times, forecasters might struggle with the amount of ice that’s expected and whether it will be enough to cause travel disruptions and/or power outages. In this case you might hear this phrase.

The phrase goes back long before we were forecasting as a reference in the bible. Many believe the phrase originated from a verse in Ecclesiastes that says “Dead flies cause the ointment of the apothecary to send forth a stinking savour.”

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Since then, this phrase has been used by many, including forecasters, who use it to reference that one little thing (or fly) that can ruin an entire forecast.

This segment originally appeared in today's edition of the Morning Brief newsletter. Sign up here to get weekday updates from The Weather Channel and our meteorologists.

J​ennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.