The NFL season ended last month, but I couldn't resist writing about a post on the University of Washington website from the Washington State climatologist, concerning the Seattle Seahawks apparent liking for rain and snow. The posting uses game day weather information to demonstrate that if it is raining or snowing the Seahawks perform noticeably better than if it is sunny or just overcast.
Since 2002 their record with precipitation is 17-4 for a winning percentage of 0.81 and a positive points differential of 12. Without rain or snow their record is a near palindromic 42-25 for 0.63 and a points differential of 5.
On the face of it the evidence is persuasive. However, two problems aren't really addressed. The sample with precipitation is very small, just 21 games and small samples can very easily lead to extreme results that aren't indicative of performance over a much larger number of matches. Also there is no account for team strength of the opposition. In 2003 there was intermittent rain for the visit of the then 3-6 Detroit Lions.
Extending the sample size isn't an option, although you could include the two seasons between the demolition of the indoor Kingdome and the move to Qwest Field when the Seahawks played at Husky Stadium. On the 28th of October 2001 the Miami Dolphins visited Husky Stadium, the game was played outdoor, the weather was sunny, temperatures hit 43 degrees F, humidity was 82% and an 8 mph north easterly wind blew. Shaun Alexander led the running game and Matt Hasselbeck was under centre. The Dolphins won 24-20 in regulation.
So there is a large quantity of available information concerning the prevailing conditions. I've therefore sorted all home games by precipitation. The original survey sticks to just home games, although logically the 'hawks should also play above their usual level of road form if they arrive in say Green Bay and are surprised, but presumably delighted to see snow falling.
I found 22 home games since 2000 where precipitation is mentioned on the game log. The majority description was rain, with a couple of showers or intermittent showers and two games were played in snow. The 'hawks won 16 and lost 6 (73%), slightly different from the quoted 17-4, although the phrase precipitation "at or in the immediate vicinity" of the stadium is used on the Washington site. In dry or overcast weather, the win loss record was 57-31 (65%). So slightly higher for the larger "dry" weather sample and a fair bit lower for the smaller "wet" games. Note how a couple of wins or losses one way or another can really bounce around the % figures if you are used limited sample sizes.
If we look at margin of victory, the strength of schedule issue is still unresolved. In my sample, Seattle win by and average of 8.5 points in the wet and 5 in the dry, compared to 12 and 5 respectively in the study. However, all we really have is slight discrepancies to eyeball. If we look at individual games, one, Arizona 0 Seattle 58 stands out. The Cardinals quarterback in week 14 was John Skelton, he threw four interceptions, took one sack, fumbled the ball twice and lost it once. The game day weather was rain. His only other NFL game when the match saw rain was in week 11 of 2011 at San Francisco, when he completed just 30% of his 19 passes, was intercepted three times, fumbled once and was sacked once, before he was mercifully replaced by Richard Bartel.
Such an outlier skews the margin of victory for Seattle in wet weather, but the narrative could just as easily be that John Skelton is a liability in the wet rather than Seattle, one of his beneficiaries, being a margin of victory revelation.
To insert context and team strength into the study we must look at each individual matchup. I could run my NFL model for the early seasons, but the results would be very close to the Vegas line for Seattle's games, so I will use the Vegas figures. Over the 13 seasons, Seattle beat the estimated margin of victory or defeat set by Vegas 109 times and failed 109 times, so the Vegas estimations of the quality of Seattle and their opponents would appear accurate over a large sample size. The handicap given or received by Seattle has now effectively turned each game into a coin toss and we can compare the success rate of Seattle against the handicap to see if either "dry" weather Seattle or "wet" weather Seattle produce against the spread results that are statistically different from the 50% overall rate that the Vegas line aims for. And neither are.
Seattle's results vary by weather, but by no more than you would expect from random chance once you account for the strength of their opponents and sample size. There is no evidence that they are either a wet or dry weather team. The omission of additional weather related factors, such as wind strength, possibly combined with evidence that Seattle are reliant on passing the ball would also help to eliminate other possible causes if the difference was significant. The story is interesting, but poorly constructed and certainly unproven.