Some folks may think getting to 70 in March is too warm too soon. Well, since 1874 it's happened 244 times in Philadelphia, an average of just under two 70 degree days per year. Warmth happens this early. What is rare is 80 degree warmth -- that's only happened 35 times in our recorded history. However, given our record high potential at this time of the year (mid 80's is our record high point for several days at this time of the year), warmth can and in fact often does happen for a day or two as warm winds of change try to surge into the region.
In fact, since 1990 only five years have not had a 70 degree high in the month of March: 1992, 1993, 1996, 2001, 2008.
Patterns at this time of the year are harder to establish and "lock in" compared to winter months or compared to summer months. The battle of the seasons often has airmasses in constant flux as warmth tries to push north and winter chill tries (some years, not this year) to hold on before its inevitable retreat. Blocking patterns of chill and warmth on either side don't often establish and hold in place to allow extremes of warmth or cold to last more than a couple of days. The "typical" setup with those three years is a ridge of high pressure aloft in the Southeast, some sort of trough in the West, and a storm system in the Plains to help funnel warmth northeast into the Ohio Valley and Northeast. The surface looks in each of the three years vary a little bit -- but the key in getting a lengthier warm wave is some sort of a blocking ridge of high pressure over the Southeastern US and some sort of a Plains storm system.
We mused a while back that March can be fickle and colder in extremely warm winters. However, March 1990, 1995, and 1998 all followed a warm (to very warm in '98's case) winter. March '98 was an odd case (which was discussed in comments in the linked post) -- temperatures for much of March '98 were running below average before the end-of-month warm wave boosted temperatures above normal for the month.
The one big caveat with each of these extreme patterns from the 1990's? Each of the patterns preceded or were succeeded by a chilly pattern. In 1990, it snowed a week after the last 70 degree day in the pattern. In 1995, the high was 37 degrees three days before the first 70 degree day. In 1998? Three consecutive days of highs in the 30's before the pattern ramped up two weeks later and snow fell five days before the first 80 in that 1998 warm wave. The most extreme example of meteorological yin and yang? March 1921. When March '21's heat wave ended on the 28th, thunderstorms crashed the party with temperatures in the lower 80's for highs. The next day, the high was just 44 and the morning low...25.