Outside of Wilmington and a couple of other minor climate sites, the region has yet to experience its first 90 degree day. The best Philadelphia did last month was 89 on April 10th, very similar to last year when a few places hit 90 before Philadelphia finally did on May 28th. The region's first 90 degree day more often than not occurs in May -- in the last 16 years it has not occurred in May on only seven occasions. Over the last three years, the last week of May has been our first 90 degree day. Prior to that, we had a 90 in April followed by waiting until June for our first 90 in 2008.
Our latest first 90 in recent years was July 12th, 1979...however, in 1976 and 1977 we had 90 degree days in April. Our earliest ever first 90 was April 7th, 1929...back when the climate site was in Center City. In the "Airport era" (since 1940), the earliest first 90 was April 12th, 1977.
While the frequency of 90 degree days has increased in recent years, its first occurrence has not become substantively earlier, still more often than not occurring in May. Playing the percentages, odds are that we will likely see a run at 90 at some point before the end of the month. Looking out at the uber long range modeling, the bulk of guidance keeps a ridge of warmth over the Rockies and Plains while keeping us "normal" or close to it through the middle of the month. Any 90 degree day would probably occur after May 20th given our current and anticipated weather patterns.