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Probability of Precipitation: POPWhat is the Chance That People Get it?
© Alan Sealls, WKRG-TV, Mobile, ALNWA 40th Anniversary MeetingOctober 18, 2015
Do all of these percentages mean the same thing?
Does the public interpret the percentages in the same way?
© 2015, Alan Sealls
Parallel online surveys conducted through “Survey Monkey” to professional meteorologists and to general public to determine
what goes into PoPs, and what people think PoPs mean.
627 meteorologists 1088 “normal” people
41% Gov
34% Bcast
25% PrivateSector
72% Work
15% Retired 13% Student
Not all survey-takers answered all questions.© 2015, Alan Sealls
What region do you forecast for?(meteorologist)
Where do you live?(public)
Regional boundaries are not universally defined.
SENE
SC
SC
C
SE
© 2015, Alan Sealls
What is the single most important factor you account for in your PoP?
(meteorologist)
What does the chance for rain mean to you?
(public)
ProbabilityArea
AreaImpact
© 2015, Alan Sealls
Probability
meteorologist
public
Which of these factors do you account for in your POP? Choose as many answers as you wish.
© 2015, Alan Sealls
Rank variables used to devise PoP, if you had to use all of them.
(meteorologist)
Rank importance of variables in the rain chance, to you.
(public)
1. Statistical Probability
2. Areal Coverage
3. Duration
4. Impact
5. Quantity
6. Intensity
7. Climatology
* Likely bias in answers to match the fixed order in which variables were presented.
© 2015, Alan Sealls
1. Statistical Probability
2. Areal Coverage
3. Duration
4. Quantity
5. Intensity
6. Impact to User
7. Climatology
Forecast scenario survey question #4
© 2015, Alan Sealls
You are forecasting rain for the next day. The day will be dry except for a thin line of showers that moves across your entire forecast zone or area to produce 0.5" of rain, lasting 30 minutes at all locations. Otherwise, the sky is totally sunny before and after the rain. What percentage would you put on that forecast?
You are forecasting rain for the next day. The day will be dry except for a thin line of showers that moves across your entire area to produce a half inch of rain, lasting 30 minutes everywhere. Otherwise, the sky is totally sunny before and after the rain. What rain percentage would you put on that forecast?
Public Version
Q4. Dry day except for thin line of showers producing 0.5" of rain, lasting 30 minutes at all locations. Otherwise, sky is totally sunny.
PoP
901
Publ
ic
PoP
© 2015, Alan Sealls
Forecast scenario survey question #5
© 2015, Alan Sealls
The next day will be overcast with stratus. Multiple times during that day, for every location in your forecast zone or area, there is drizzle or light rain. By the end of the day the rain total is 0.1". What percentage would you put on that next day forecast?
The next day will be totally gray and cloudy. At many times during the day, for every location in the area, there is drizzle or light rain. By the end of the day the rain total is only a tenth of an inch. What rain percentage would you put on that next day forecast?
Public Version
Q5. Overcast with stratus. Multiple times during day, every location gets drizzle or light rain. By end of day the rain total is 0.1“ everywhere.
548
met
eoro
logi
sts
PoP
896
Publ
ic
PoP
© 2015, Alan Sealls
Forecast scenario survey question #6
© 2015, Alan Sealls
The next day will be totally sunny for 90% of your forecast zone or area. A stationary cluster of thunderstorms delivers 2" of rain over 10% of your forecast zone or area. What percentage would you put on the forecast for that zone or area?
The next day will be totally sunny for 90% of your area. A stationary group of thunderstorms drops 2" of rain over 10% of your forecast area. What rain percentage would you put on that forecast for your area?
Public Version
Q 6. Totally sunny for 90% of area. Thunderstorms deliver 2" of rain over 10% of area.
546
Met
eoro
logi
sts
PoP
890
Publ
ic
PoP
© 2015, Alan Sealls
© 2015, Alan Sealls
Survey Limits/Issues/Flaws
1. Survey questions on ranking should have been in random order.
2. Surveys should have referred to “percentage rain”, rather than
“rain chance” or POP, since those imply statistics.
3. Purposely disregarded any “official” definition of POP.
4. Purposely did not include national forecasters or US territories.
5. Purposely limited data collected on survey-takers.
6. Some “public” respondents seemed to be more weather-savy
than a typical person.
© 2015, Alan Sealls
Survey Summary/Results
1. No single set of ingredients universally makes up PoPs
2. Forecasters use probability and coverage as main factors.
3. The public sees PoPs mainly as a probability.
4. Many forecasters prefer word descriptions over PoPs.
5. 31% (=194) of meteorologists left comments at survey end!
6. 13% (=138) of public left comments at survey end.
7. Read enlightening comments at weatherthings.com!
© 2015, Alan Sealls
Opinion/Future
1. Forecast companies/units should use consistent ingredients.
2. Ingredients will vary by climate zone and weather pattern.
3. Graphic-only PoPs suffer from lack of explanation.
4. No “solution” to dilemma of multiple rain scenarios with
multiple percentages, or even words.
5. We must explain and define “PoPs” whenever possible.
© 2015, Alan Sealls
I think…
Probability of Precipitation: POPWhat is the Chance That People Get it?Details posted at weatherthings.com
© Alan Sealls, WKRG-TV, Mobile, ALNWA 40th Anniversary MeetingOctober 18, 2015
