Good flow comes with below freezing temperatures at night followed by bright sunshine and daytime temperatures well above freezing. Runs can occur with a falling barometer such as may occur before a wet March snowstorm. Flow does not occur if temperatures remain below freezing.
Flow ceases once overnight temperatures consistently remain above freezing.
Check the current satellite photo from the American Meterological Society. Now check the American Meterological Society's weather map . Remember, high pressure usually brings clear, sunny weather, low pressure brings clouds and precipitation. Winds flow clockwise around high pressure and counterclockwise around lows. Try your hand at predicting the sap flow based on weather conditions.