You can predict when lilacs will bloom by summing the squared mean daily temperatures (Celsius) since the last frost. Use an average of the past few years' daily temperatures to forecast the date when this sum will reach 4264. Although it was originally published in London, I've found that this algorithm works in the Pacific Northwest USA as well.
The algorithm was discovered by Belgian statistician Adolphe Quetelet who in 1835 invented demographic analysis as we know it today. His lilac law came to the attention of a novice botanist named Florence Nightingale, who established a lifelong friendship with Quetelet. The statistical methods he taught her enabled Nightingale to quantify and publicize the atrocious conditions in military hospitals during the Crimean War. The resultant public outcry led to a revolution in military medicine.
