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Introducing Capital Weather Gang’s Winter Storm Impact Scale

Digging out from Snowmageddon in Oakton, Va., on Feb. 7, 2010. (Kevin Ambrose)

Two years ago, I proposed the idea of developing a scale for rating winter storms in Washington. Reader feedback on the concept was extremely positive.

Since then, I’ve been thinking about such a scale and began to design a model for it. Now, I’m pleased to roll out the first version of our Winter Storm Impact Scale.

Several of Capital Weather Gang’s expert contributors helped with its development.

CWG Winter Storm Impact Scale categories

Category 1 — Nuisance storm: Untreated roads are slick. Possibility of school closings and delays.

Category 2 — Disruptive storm: Slick roads. School closings and delays likely. Possible adjustments to government operations. Some flight delays.

Category 3 — Significant winter storm: Snow-covered or glazed roads, some impassable. School closings very likely. Government operations likely altered. Flight delays and some cancellations.

Category 4 — Major winter storm: Many roads impassable or hazardous. School closings virtually certain. Altered or closed government operations very likely. Significant airport and mass transit disruptions.

Category 5 — Crippling/historic winter storm: Travel difficult to impossible for long duration. Near certainty of schools closed for multiple days. Near certainty of government shutdown for at least a day. Airports and most mass transit very likely shut down.

How are the winter storm categories computed?

We all know the impact of a winter storm in the D.C. region is not just about the amount of snow or ice. It’s also about when the storm hits; how cold it is before, during and after the storm; how long the storm lasts; and how hard the precipitation falls. This scale takes all of these variables into account.

For every winter storm, we will calculate a score of up to 120 points based on the following variables:

  • Total accumulation of snow and ice.
  • Duration of the storm.
  • Precipitation intensity.
  • Average temperature the day before precipitation begins.
  • Average temperature during precipitation.
  • Average temperature the day after storm ends.
  • Wind speed.

We have developed weights for these variables such that the precipitation amounts, duration and intensity matter most (accounting for about two-thirds of possible points) and that temperature and wind are of secondary importance (accounting for a third of possible points).

But there are also many more factors that influence a storm’s effect in our region.

A storm can earn bonus points, potentially increasing its impact rating, if:

  • It is the first snow or ice event of the season (when people are not yet used to wintry precipitation)
  • Snow or ice coincides with a commute.
  • Snow or ice coincides with a big travel day (such as the day before Thanksgiving).
  • A flash freeze occurs after precipitation ends.
  • Strong winds appear after precipitation ends (causing blowing and drifting snow or power outages).
  • Snow is on the ground before the storm begins.
  • Snow is heavy and wet, or if ice accretion is significant, potentially stressing power lines.
  • Subfreezing temperatures persist for days after the storm, so the precipitation does not melt.

A storm can earn deductions, potentially decreasing its impact, if:

  • The sun angle is high (such as when precipitation falls during the day in March).
  • A majority of precipitation falls on a weekend or holiday.
  • There is a pause or lull in the precipitation for several hours during the storm.
  • The snow changes to sleet for several hours during the storm.
  • There is a temperature spike after the precipitation ends and it is well above freezing.
  • Temperatures in the days after the storm are well above freezing.
  • Frozen precipitation changes to rain.

For every storm, we will take all of this into account to develop the storm category based on the following point totals:

  • Category 1: 20-39
  • Category 2: 40-59
  • Category 3: 60-79
  • Category 4: 80-99
  • Category 5: 100+

We have calibrated and tested this scale using past winter weather events. For example, the Snowzilla blizzard, Jan. 22 and 23, was a Category 5. So was Snowmageddon, from Feb. 5 to 6, 2010.

The little snow event two days before Snowzilla, Jan. 20, rated a Category 2, even though it dropped just an inch of snow. The rating was boosted by the fact that temperatures were very cold before and during the snow and that it coincided with the evening commute.

This rating scale is a work in progress, and we will fine-tune it over time, learning from each storm.

It’s important to note, because of the large amount of weather variability in our region, not one storm rating will fit all areas. We will need to, in many cases, develop a different rating depending on whether you live in, say, Frederick, Md., or Fredericksburg, Va.

We welcome feedback on this initial scale concept. If there are variables or circumstances not mentioned above that you think should be considered, please provide a comment.

Also, although we are initially calling this the Capital Weather Gang’s Winter Storm Impact Scale, we’re looking for a catchy acronym for this scale.

A leading candidate is WISE: Winter Storm Impact Scale for Everyone

Here are some others:

Washington-Area Winter Storm Index (WAWSI)
D.C.-Area Winter Storm Index (DAWSI)
Washington-Area Winter Storm Impact Scale (WAWSIS)
Capital Area Winter Storm Impact Scale — (CAWSIS)

Fire away with your ideas … and thank you.