You open SnowSense™ and see "67% Snow Day Probability." Now what? Do you pack your backpack? Set your alarm? Text your friends?
Here's the correct way to read that number — and why most people interpret probability scores wrong.
What "67% Probability" Actually Means
A 67% snow day probability means: if this exact weather pattern occurred 100 times in your region, school would be cancelled approximately 67 of those times.
It is not a personal prediction. It is not a guarantee. It is a statistical estimate based on current conditions compared to historical closure data.
Think of it like a weather forecast saying "70% chance of rain." That doesn't mean it will definitely rain. It means conditions strongly favor rain based on everything the model can measure.
The Three Probability Zones
Low: 0–35% — Go to School
Conditions don't favor a closure. Pack your bag and set your alarm. A probability this low means even if some snow falls, it's unlikely to reach the threshold your district uses for cancellation.
Middle: 36–65% — Prepare Either Way
This is the genuine uncertainty zone. Have your backpack ready. Check SnowSense™ again at 10 PM and again at 5 AM. This range is where storm track changes and overnight temperatures make the biggest difference.
High: 66–100% — Strong Cancellation Signal
Very Likely. Conditions strongly favor a closure. At 80%+, you can reasonably plan for a snow day while keeping in mind the superintendent still makes the final call.
What Drives the Percentage Up or Down
The four factors that control your probability score:
Snowfall — More accumulation = higher score, but rate matters more than total depth.
Ice Risk — Freezing rain or black ice potential pushes the score up faster than any other factor.
Storm Timing — Snow forecast for 2–5 AM dramatically increases the score. Same snow at 2 PM barely moves it.
Regional Calibration — Your city's infrastructure and historical tolerance modify the raw score. Boston needs higher raw scores to reach the same probability as Atlanta.
The Trench Truth:
The probability score is most useful as a trend rather than a static number. A score climbing from 30% at 6 PM to 65% at 10 PM to 82% at midnight is telling you the storm is developing faster and stronger than forecast. A score dropping from 70% to 40% overnight means conditions are improving. Watch the direction, not just the number.
Confidence Score vs. Probability Score
You'll notice SnowSense™ shows two numbers: Probability and Confidence.
- Probability: The estimated chance of cancellation
- Confidence: How decisive that estimate is — how far the conditions push the call away from a 50/50 coin flip
A 5% or 95% probability comes with high confidence: the conditions point clearly one way. A probability near 50% comes with low confidence, because the same weather could reasonably break either way. Confidence tells you how firmly the data is leaning, not how much snow is coming.
Check Your Current Score
Get your real-time probability and confidence score at the SnowSense™ calculator or browse by city at our snow day calculator by location.