Snow has never been measured in Philly after April 28. Here’s what went wrong with this winter’s forecasts.

The region has just passed a mile marker that defines the true end of the road for the snow season.

Snowflakes were sighted this year as late as April 16, and Monday marked the 15th consecutive day of below-normal temperatures, but never in the 136-year period of record has measurable snow fallen in Philadelphia from April 28 through Oct. 9. The latest-ever snowfall was the 0.1 inches of April 27, 1967.

Thus it should be reasonably safe to assume that the snow book for the winter of 2019-20 is firmly closed. So how did the forecasts fare?

The pre-season outlooks offered quite a range of potential outcomes, but the most conservative estimates still ended up being roughly 60 times higher than the actual official seasonal total — 0.3 inches.

» READ MORE: Feds favor warm winter for Philly and much of the rest of the nation — with caveats

In fairness, the consensus in the early going was that nature was offering almost no clues about the coming winter, save for a robust Northern Hemisphere snow cover in the fall.

In her outlook, Cecily Tynan at 6ABC said the region could expect mixes of rain and snow but opined that enough cold air would be available to generate snowfall within the range of normal, which would be 22.3 inches for the season. Kathy Orr and the forecast team at Fox29 came in with a similar estimate.

» READ MORE: Why forecast snow often ends up as rain in Philly

Glenn Schwartz at NBC10, who called for 30 to 35 inches, said a factor in his outlook was a lull in solar storms, “sunspots,” which correlated well with a pattern in the North Atlantic that favored snow.

That pattern did develop — in April.

Schwartz was referring to the North Atlantic Oscillation, which is measured by an index of pressure differences between the Arctic and North Atlantic Ocean at a latitude near Philadelphia’s.

Generally, so goes the index, so goes the snow in Philadelphia, says Louis Uccellini, who runs the National Weather Service and is an expert on coastal snowstorms.

When pressures in the Arctic are higher and the air heavier, cold air tends to pour into the Northeast and the pattern favors nor’easters. The opposite is true when the pressures at the lower latitudes are higher.

Guess what was the case this winter?

» READ MORE: When winter doesn’t come: Here are the winners and losers of Philly’s nearly snowless season

Unfortunately the oscillation isn’t predictable beyond 7 to 10 days. One reason, Uccellini says, is that it’s driven by forces in the high atmosphere where data are lacking.

Yet another reason why the art of seasonal forecasting remains a work-in-progress.

For those wondering whatever happened to spring, temperatures could get into the 70s Sunday.

» READ MORE: Philly’s gloomy weather has been matching the national mood