Transcript Presentation/Poster - National Weather Association

Probability of Precipitation: POP
What is the Chance That People Get it?
© Alan Sealls, WKRG-TV, Mobile, AL
NWA 40th Anniversary Meeting
October 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
15% Retired
25% Private
Sector
13% Student
41% Gov
34% Bcast
72% Work
Not all survey-takers answered all questions.
© 2015, Alan Sealls
Where do you live?
What region do you forecast for?
(meteorologist)
(public)
SE
NE
SC
NW
NC
NE
WC
C
EC
SW
SC
SE
AK
HI
SE
SC
C
Regional boundaries are not universally defined.
© 2015, Alan Sealls
What is the single most important
factor you account for in your PoP?
What does the chance for rain mean
to you?
(meteorologist)
(public)
Statistical
Probability
Areal Coverage
Impact
Duration
Area
Quantity
Area
Probability
Intensity
Probability
Impact to User
Climatology
Other (please
specify)
© 2015, Alan Sealls
Which of these factors do you account for in your POP?
Choose as many answers as you wish.
meteorologist
100%
public
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
© 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
1.
Statistical Probability
2.
Areal Coverage
2.
Areal Coverage
3.
Duration
3.
Duration
4.
Impact
4.
Quantity
5.
Quantity
5.
Intensity
6.
Intensity
6.
Impact to User
7.
Climatology
7.
Climatology
* Likely bias in answers to match the fixed order in which variables were presented.
© 2015, Alan Sealls
Forecast scenario
survey question #4
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?
Public Version
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?
© 2015, Alan Sealls
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.
551 Meteorologists
35%
30%
25%
20%
15%
10%
5%
0%
PoP 0%
10%
20%
30%
40%
50%
60%
70%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
50%
45%
40%
901 Public
35%
30%
25%
20%
15%
10%
5%
0%
PoP 0%
80%
90%
100%
© 2015, Alan Sealls
Forecast scenario
survey question #5
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?
Public Version
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?
© 2015, Alan Sealls
548 meteorologists
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.
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
PoP 0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
20%
30%
40%
50%
60%
70%
80%
90%
100%
50%
45%
896 Public
40%
35%
30%
25%
20%
15%
10%
5%
0%
PoP 0%
10%
© 2015, Alan Sealls
Forecast scenario
survey question #6
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?
Public Version
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?
© 2015, Alan Sealls
546 Meteorologists
Q 6. Totally sunny for 90% of area. Thunderstorms deliver 2" of rain over 10% of area.
35%
30%
25%
20%
15%
10%
5%
0%
PoP 0%
10%
20%
30%
0%
PoP 0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
50%
45%
40%
890 Public
35%
30%
25%
20%
15%
10%
5%
40%
50%
60%
70%
80%
90%
100%
© 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
I think…
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
Probability of Precipitation: POP
What is the Chance That People Get it?
Details posted at weatherthings.com
© Alan Sealls, WKRG-TV, Mobile, AL
NWA 40th Anniversary Meeting
October 18, 2015