La Nina ruled the roost the past two summers, with plenty of heat and humidity along the East Coast. Two of the hottest summers on record in Philadelphia have occurred in the past two summers.
The good news? We probably don't have a third hot summer in a row in store for the region.
The bad news? Like most any summer, we will get heat, humidity, and rain.
In coming up with my summer 'projection', I've looked at a few factors in shaping this forecast. First, summers that are in a developing Nino pattern. This summer will likely fit that pattern as El Nino is starting to bubble in the tropical Pacific. Typically a Nino summer has at least one "cool" month tied to it...it rarely goes hot all the way through but 1991 was an exception...and that started out hot in May and kept on rolling along throughout the summer. Based on our pattern in May so far, a 1991 repeat is not likely.
Second, the rainfall patterns of the past couple of months. Spring until the last four weeks was pretty dry across the Eastern US...with recent rains helping to catch us up somewhat to normal. Had we continued in a dry pattern, we would lean a bit hotter in this forecast but with rainfall helping to moisten the ground up we might not have the propensity to torch as easily as we did in March and April, when the ground was pretty dry and we were in a very dry pattern, resulting in 80's both months and a near-record warmest March. Do Spring rains matter? Well, to an extent they do. Years where we were above average in rain in April and May were below average in summer temperature (see 2007, 2009). 2011 came in hot as a dry spell kicked in after a three inch rainfall in mid April and continued through July, with May rains coming in less than half of normal. It isn't a clear cut consideration but it can help or hinder the prospect of heat over the course of summer. If we stay damp/wet into June we will likely not have the same level of heat that we have had the past couple of summers.
In terms of rainfall, much of that is dependent on the strength of the summertime heat ridge that will build over the Southeast. The stronger the ridge, the higher the likelihood of a storm track to Philadelphia's north and the higher the likelihood of heat. Most of the heat in the US appears to set up in the Midwest, South, and Ohio Valley. This could yield a wetter than average pattern along the East Coast this summer although we hopefully won't see the levels of rain and flooding we saw last year. Rainfall could end up being higher than average in June and again in August, with July being near or slightly below average.