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Overview

One of the biggest factors in determining weather impacts is probability precipitation.  It turns out that its also the most easily misunderstood metric in our weather suite.  In this article we examine the nature of the metric "Precipitation %" and discuss how best to use it and how to avoid making incorrect conclusions based on the data provided.  Read below to view the definition directly from the source. Once you understand the technicals of this information, we can discuss the practical impact of the information and how ultimately it affects consumer behavior.




Boy is Clint right.  In the context of forecasting precipitation's impact on sales and consumer behavior, its more about whether the consumer thinks it will rain than how much it actually did rain, when and where.  This is really important, so let me explain by example.  If you're a golfer - when you look to book a tee time you check the weather forecast.  If you're like me and you live in a hot humid summer area where it rains frequently, if you see anything less than 60-65% chance of rain on Saturday, you're gonna go golfing and take the risk.  but how about at 75% chance?  Again, if it were me, probably not - I'm going to plan something indoors or neutral instead.  These sort of consumer behavior risk calculations happen constantly as consumers evaluate near-future events.  If it turns out that it doesn't rain on Saturday, I may do something outside, but it has definitely altered my planning in a big way.  If it does rain, I also don't much care whether it was 0.1in or 1.1in of rainfall - either way I'm not golfing or mowing the lawn or planting flowers or anything else where I spend a lot of time outside.

So forecasts matter - they affect behavior.  This is basis for studying the level of impact on behavior (sales or not), not actual rainfall. But before we proceed, lets talk about one other snarly thing about rain - did it or didn't it rain?  We've had people say  "but I just need to know whether or not it actually rained".  This type of question brings up the highly localized nature of rain, geographically speaking, along with the fact that its locality moves - usually at between 4-15mph.  What I mean is that where it rains during a given day depends highly on where you are and what time of day to which you refer!  The nature of rain data collected is based on an area, usually a zip code.  Depending on where you are in the country, a zip code can be a rather small or quite large geographical area.  If a rain storm is moving through the area at 10mph and a zip code is 10 miles wide, if the rain passes over the zip code, its only going to be raining at this location for one hour out of a 24 hour day!  Then we have to know whether 100% of the area in the zip code was rained upon or only a portion of it.  Rainfall measurements by their nature are good for determining that it rained, but not when it rained and they're only able to say with 100% certainty that it rained at the point of measurement, not 5-10 miles North, South, East, or West, unless there is another reporting measuring device.

I hope all these details don't bore you - I'm mentioning them only to state that when it comes to whether or not it rained,  it doesn't matter! The biggest impacting factor is the % chance of precipitation.  Determining whether or not it rained can be calculated in surrogate by assessing something similar to "if the % chance of rain was above 80% for this zip code, its rainy or it rained".  The percentage you use will be based more on the region of the country - 50% chance in the southeast during the summer happens nearly every day, whereas 50% chance in AZ in the desert is extreme where 0-10% is the normal likelihood.

So, to sum it all up - precipitation % is a good surrogate and representative metric for rain and determining behavior because it incorporates geography and timing into a easily digestible metric that people base their actions upon. With these few key points kept in mind, you'll be well armed in not overstating or understating the impact of these values when combining them with sales.  


Precipitation % Visualized

Take a look at the two maps above.  The top left is out Year-Over-Year precipitation differential map.   The top right is our Days of Rain Map.  These are both two good examples of how we can use the metric of precipitation % to understand where its raining and how much for a give day or time period.  The first map indicates with red, where it is more rainy this year than the same period last year - in this case a day in June.  We can do this by aggregating up values at the zip code level to the county level and comparing to prior year before visualizing on the map.  A brief summary statement of this map would be "Its much rainier in the Deep South and Northeast / Midwest than it was on the prior year date 6/23/2016".  If you combine this with a sales heat map, for the same time period (assuming you have daily sales data at your disposal) you can see without any further deeper investigation whether sales were helped or hurt by the difference.  

Often times though you won't have daily sales data or perhaps just as likely - you're wanting to look at a sales impact over a longer time period.   The problem arises that you need to aggregate the data for rain each day into a longer running total.  This is what the Days of Rain Map above does.  It allows you to pick a longer time period - perhaps a week or two weeks (usually during peak season or weekends or holidays) perhaps and still evaluate the impact of the total number of rainy days and the affect it has on your sales performance.  Note: we've had customers (mis)use the Days of Rain map for a long period of time - 3 months or 6 months, for example, and come to some really erroneous conclusions.  

In general - weather needs to be evaluated against short term consumer behavioral adjustments.  If you live in Seattle, it rains all the time.  So much so, most people have umbrellas and rain gear.  So if you are an umbrella supplier, using the high number of days of rain over a 6mo period as a gauge for expected sales of umbrellas is fine, except where high rain amounts are common - and people have already adjusted to the weather pattern (they don't buy new umbrellas every week  for new rain).  It is something to be aware of - that, specifically for rain patterns, there are almost no non-recurring weather patterns that last more than 30 days - rainfall typically mean reverts over longer time periods.  The only known exception we've found to this is El Nino.  Most other rain patterns are short-lived - Hurricanes, Snowstorms, Blocking Low Pressure Systems - they all move in and out within a week and their behavioral impact typically is felt for a few weeks before and after at most.


Table of Contents

Learn About Popular Use Cases
  • Using Excel & Web Queries To Analyze Rain vs. Sales
    • Link to Article

Technical Definition of Precipitation % - Directly from the National Weather Service


Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".

EXAMPLE
ZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
119 PM EDT THU MAY 8 2008

GAZ021-022-032034-044046-055-057-090815-
CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-
ROCKDALE-SOUTH FULTON-
INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...
EAST POINT...LAWRENCEVILLE...MARIETTA
119 PM EDT THU MAY x 2008

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF
SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR
STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.
.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND
THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS
AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST
WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.







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