Rating Snowstorms Based on Travel Impacts
Ernie Ostuno
National Weather Service, GRR
Storm Impact: Travel Conditions
Recent survey found (not surprisingly) that travel impacts were the leading
cause of people altering their normal routines during snowstorms
Two Methods of Ranking Storms
• Pre-Storm:
– Saffir/Simpson scale: damage potential
• Post-Storm:
– EF scale: damage assessment
Winter Storms: Criteria vs. Impact
• NWS warning criteria based on amount and
rate of snow
• Impact: primarily the effect on travel
conditions, which we don’t explicitly forecast
• Zielinski (2002), Grumm and Hart (2001)
rated winter storms based on meteorology
• Kocin and Ucellini (2004) rated winter storms
based on impacts
• The impacts did not include taking into
account some important variables
Northeast Storms Severity Index
• Based on amount of snow that affects the greatest
•
•
•
area/most population
Somewhat analogous to the EF-scale; tornado will
likely get a higher rating if it hits a city
Ranks 11-12 Feb 1983 higher than 6-7 Feb 1978
Not accounted for: Timing, winds, temperatures,
local severity
Variables Affecting Travel Impacts
• Snow: Amount, Rate, Timing
• Wind: Speed and Duration
• Temperature: Hypothermia, Road Salt
• Icing: Freezing rain, rain to snow
Ranking Historic Northeast Snowstorms
• February 6-7, 1978
• January 7-8, 1996
• February 11-12, 1983 and 2006
• February 16-17, 2003
• April 1, 1997 and April 6, 1982
Ranking Factors: Amount of snow, winds, persistence of deep snow
Societal impacts to account for: Economic disruption, travel bans
Discussion point: which is more important, snow amount or wind?
Drifting snow and snowfall rates are important factors in affecting travel, as well as
timing
What makes a storm memorable enough to mark anniversaries of it?
Rankings by Snow, Wind*
Snow
17.7…2006
15.5…1983
15.4…1996
15.1…1978
13.1…2003
Wind
40…1978
37…1996
36…2006
33…2003
28…1983
Rank points**
2006…9.5
1978…9.5
1996…9
1983…5.5
2003…4
*Average of BDR and BDL snow totals and peak winds
** Top ranking for each category is given 5 points, next highest
given 4 points, etc., then added together. Wind is given a
weighting factor of 1.5
A Look at One Possibly Important Variable:
Snow Depth Following some of the Big Storms
40
35
30
25
19 7 8
19 8 3
20
19 9 6
2003
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
Snow depth in inches (y-axis) measured each day after the storm (x-axis)
Post-storm assessment ranking:
10 inches or more on the ground 5 days after = 1 point
10 inches or more on the ground 10 days after = 2 points
Final Ranking
Rank points
1978…11.5
1996…11
2006…10.5
1983…6.5
2003…4
“The Old 1 to 5 Scale”
Wind (mph)
>40… 5
30-40… 4
20-29… 3
10-19… 2
<10… 1
Snow (In)
>12… 5
8-12… 4
4-8… 3
2-4… 2
<2… 1
*Amounts can vary based on forecast zones*
Temp (F)
<15… 5
15-20… 4
20-25… 3
25-30… 2
>30… 1
Graphical Forecasts
Peak Wind
= Impact
+ Snow
+Temp
Impact Forecast generated from graphics that are
already being produced and calculated as follows:
45 % Wind + 40 % Snow + 15 % Temperature
Example: Wind = 35, Snow = 10 and Temp = 20
Impact = 4(.45) + 4 (.4) + 3 (.15) = 3.85
Let’s Look at Some Examples
Two recent cases show the difficulties in assessing impacts
Discussion points
• Severity vs. Impact
• Timing is everything?
• Can we forecast road conditions?
• What about icing?
Winter Storm of 3 Feb 2007
• High impact event with road closures,
blizzard conditions and extreme cold
• Impact mitigated since it hit on weekend
• Highly variable conditions, but frequent
whiteouts in falling and blowing snow
Lake Effect Blizzard with Whiteouts, Low Wind Chills
Winter Storm of 30 Jan 2008
• High Impact rain to snow event with
extremely poor travel conditions
• Quick temperature drop with a “flash freeze”
of rain into a sheet of snow covered ice
• Cold temperatures greatly reduced the
effectiveness of road salt
• Travel conditions were bad for several days
Not a lot of snow, but worst driving conditions EVER
Other high impact winter events:
• Black ice
• “Clear Air” drifting of snow
• Localized whiteouts
• Localized dense fog from snowmelt
• Dirt Roads icing over in thaw situations
• High winds after ice storms causing
continued power outages
These are high impact events
that we must anticipate and
communicate better!
Strategy for Communicating
Impacts of Winter Storms
• A rating system for winter storms based on impacts
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•
•
•
•
can help communicate how “bad” a storm will be
Existing forecast parameters can be used to
calculate impacts
No easy way to account for so many variables
Each storm is different; recognize the main impacts
Stress major impacts in briefings/NWSChat/AFDs
Choose wording that best conveys what the major
impacts will be
In Summary
Diagnose the Meteorology
Assess the Impacts
Describe the Event