With cold air on the way and the potential for a few snowflakes late this week and into the weekend, now is a good time to reacquaint yourselves with the Capital Weather Gang’s tools for predicting snow and other types of winter storms.
We’re fast approaching the average first date of measurable snow (Dec. 10-15 around the region). Moreover, Dec. 5 (today) happens to be a particularly snowy date in recent Washington weather history. In the eight years from 2002 to 2009, Washington recorded snow five times on this date.
We employ three primary tools to communicate the risk of snowfall: the Snow Potential Index, the Snow Lover’s Crystal Ball and the Winter Storm Impact Scale.
Snow Potential Index (SPI)
The SPI is a 0-to-10 meter that conveys the likelihood of at least one inch of snowfall in the general D.C. area over the next week. An SPI of 1 signals a 10 percent chance of an inch of snow over the next seven days, while an SPI of 10 means at least an inch is virtually guaranteed.
The SPI is released every day in the 5 a.m. forecast (and published on Twitter and Facebook the evening before). We use arrows to indicate the tendency of the SPI compared with the previous day. An up arrow (↑) means the SPI has increased, and a down arrow (↓) notes a decrease. A sideways arrow (→) means it has stayed the same.
If there is no chance of snow, the SPI drops to 0. When the SPI remains at 0 for longer than a day, we stop publishing it until it comes back up to at least a 1.
Here is today’s edition:
SNOW POTENTIAL INDEX
A daily assessment of the potential for at least one inch of snow in the next week, on a 0-to-10 scale.
1/10 (→): Still low chances, but activity Friday night, at times this weekend, and maybe toward Tuesday of next week is worth watching.
Snow Lover’s Crystal Ball
The Snow Lover’s Crystal Ball appears when there is at least a 30 percent chance of an inch or more of snow two to seven days into the future. The SLCB is accompanied by commentary that examines the different snow scenarios that might occur.
Once a snow threat is within two days, we no longer employ the SLCB and focus on forecasting how much snow will occur, the timing and the effects.
CWG Winter Storm Impact Scale
The Winter Storm Impact Scale, introduced in January, classifies the predicted impact of winter storms using a 1-to-5 rating:
We usually release such ratings one day before a storm is predicted to begin.
Last winter, after it was introduced, the immediate D.C. area witnessed five Level 1 “nuisance” events and one Level 2 “disruptive” event (March 13-14).
As we inch closer to this winter’s snow season, here are some other Capital Weather Gang resources that you may find helpful: