The Devil is in the Details

1
The Devil is in the Details

• When the Forecast is Very Good
• but the Results are Very Bad!

Greg Fishel CBM Nate Johnson CBM WRAL-TV
Raleigh, NC
2
Case 2
Gridlock! January 19, 2005
3
Event Setup

• Unusually cold prior to event
• Air temperatures near or below freezing
• Highs in 20s and 30s
• Lows in 0s and 10s
• Dewpoint depressions 10-20
• Ground temperatures unusually cold
• 4 soil temperatures in 40s before event
• Dropped to below 33 during precipitation

4
Forecasts

• Clipper-style system in northwest flow aloft
• Typically produce little, if any precip
• Precip even less likely with very dry air mass
• NWS Zone Forecasts
• First mention of snow - 18/17Z
• A chance of snow in the afternoon. Highs in the
  lower 30s. Chance of snow 30 percent.
• No accumulation expected
• Upstream areas reported only a trace

5
Observations - KRDU
6
Only 21 Minutes of SN
7
Total Liquid Equivalent 0.04
8
Storm Total Accumulations
9
Results
10
The Forecast Wasnt Bad

• Forecast High lower 30s
• Actual High 30
• Chance of snow forecast nearly 24 hours out
• POP 30
• Forecast time frame of late morning to early
  afternoon was very accurate
• Accumulation forecast off by 0.04 liquid
• With air temps well below freezing and very cold
  ground temperatures, no reason to worry about
  melting

11
but the Results Were!

• If the forecast was good,
• why were the results so bad?
• We tend to focus on the sensible weather side of
  the forecast
• How cloudy/cold/windy will it be?
• P-type? When and how much?
• However, other peripheral factors at play
• Affect reactions to forecasts
• Affect perceptions of forecast accuracy

12
Peripheral Factors

• Meteorological
• Unusual snow-to-liquid ratios
• Possibility of intense but brief burst of snow
• Psychological/Sociological
• Basic Southern reaction to snow
• Ghosts of busted forecasts past
• Non-Meteorological
• Interaction of cars, roads, snow, and cold
• ((tire snow melting) cold ice!)

13
Psych/Soc Factors

• No such thing as easy snow forecasts
• Cold air, moisture, and lift rarely in sync
• Clueless models short leads, low confidence
• Results very track-dependent and often messy
• Winter weather often means ice
• Power outages, impassable roads, etc.
• Runs on the grocery store!
• Ghosts of busted forecasts past
• January 24-25, 2000

14
What Happened?

• Precip was brief but relatively intense
• Snow-to-liquid ratios of 171 to 231 (!)
• Snow started, people reacted
• Schools offices closed rush hour, early
• Interaction between warm cars, tires, and road
  caused snow to melt
• Extremely cold air and ground temperatures caused
  melted snow to refreeze into ice
• Gridlock! 2nd Worst Person in the World

15
Observations Conclusions

• Forecast would have been considered good from a
  purely meteorological view
• Peripheral factors played a role in what
  happened after the meteorology
• Dramatically affected perception of the
  forecaster and the quality of the forecast
• Some peripheral factors are well understood,
  others are not
• Understand those that can be understood, be
  mindful of those that cannot