I’m a numbers guy and find scales that give me a quick sense for how severe a hazard or weather event might be very useful. That motivated me to come up with a concept for a winter storm impact scale designed for the Washington, D.C. metro region.
- Precipitation intensity: This is perhaps the most important element in the local impact of snow events. Heavy precipitation reduces visibility. Extremely heavy snowfall rates can lead to gridlock and stranded motorists. Heavy snow may accumulate even when temperatures are above freezing and ordinarily would melt.
- Temperatures during the storm: This impacts whether and how quickly frozen precipitation accumulates. It is among the most important factors in determining the impact of a local storm. Snow or mixed precipitation occurring at temperatures of 31-34 tends to have much less of an effect than at temperatures of 21-24.
- Precipitation timing: A storm that occurs during the week and during commuting hours tends to have more impacts than one on the weekend. Storm impacts may be reduced during holiday periods and when schools are closed. Storms occurring later in winter may not have as big of an effect as storms early in the season since people are used to them and have gained experience in responding-coping.
- Precipitation amount: Obviously, this is what people pay the most attention to, but it’s often not as important as precipitation intensity, timing and temperature.
- Precipitation type: Is it snow, sleet, freezing rain, rain or a mix? Snow and freezing rain tend to cause the most problems.
- Precipitation composition: In other words – is it a light, powdery snow which can be swept away. or a heavy, wet snow – which can cause power outages?
- Precipitation duration: A long-lived storm can tax residents and officials responsible for responding to it – stressing resources.
- Temperatures before the storm: This plays a big role in how long it takes for frozen precipitation to accumulate
- Temperatures after the storm: Will crashing temperatures behind a storm freeze everything rock solid making snow and ice removal more challenging? Or will temperatures moderate and everything quickly melt?
- Wind: This can cause blowing and drifting snow and reduce visibility. Severe wind can cause power outages. Wind can be particularly problematic towards the end of ice storms leading to mass outages.
- Geographic coverage: Is the storm affecting just a small part of the D.C. metro region or its entirety?
The Capital Weather Gang would subjectively come up with a number after analyzing these factors through the lens of the forecast. Here’s a draft version of what the scale would look like:
Category 1: Nuisance storm – Some frozen precipitation may affect a small part of the region but will pose minimal disruption. Such a storm is no more hazardous than a typical plain rain storm. No school or government delays. Traffic may be a bit worse than normal.
Category 2: Minor storm – Frozen precipitation is likely to affect at least half of the region. Some areas experience hazardous conditions and some disruption, some do not. Some school, government, and travel delays. Typical snow accumulation: 1-2 inches.
Category 3: Moderate storm – Frozen precipitation is likely to affect most, but not necessarily all, of the region. Many areas significantly disrupted; some areas may have just minor disruptions. Widespread school, government, and travel delays. Some schools and governments close. Typical snow accumulation: 2-6 inches.
Category 4: Major storm – Frozen precipitation of either heavy intensity or long duration affects much of the region. Many areas significantly disrupted. Significant travel delays. Many schools, governments, and businesses close at least for a short duration. Typical snow accumulation: 6-12 inches.
Category 5: Extreme storm – Frozen precipitation of heavy intensity and long duration affects much of the region. The region is severely disrupted if not shut down. Travel is difficult or impossible. Most schools, governments, and businesses close, possibly for a long duration. Typical snow accumulation: Over 12 inches.
Note that the timing of a storm could influence its classification by one category. In other words, what would ordinarily be a category 3 storm – should it occur at rush hour – would get bumped up to a category 4 storm.
Assuming we move forward with this scale, we’d post the definitions on a static page where folks could refer to it and it could include examples of past storms with various ratings as a reference. We could even classify some past storms retroactively. For example, Snowmageddon would no doubt be a Category 5. (And yes, we might reclassify a storm after the fact if our pre-storm rating erred).
I welcome your feedback on this classification scale and criteria. Also, if anyone has a good idea for a name for the scale and complementing acronym, extra points.
Lastly, for fun – using the proposed criteria above, vote on a rating for last Tuesday’s storm, in the poll below: