PHILADELPHIA (CBS) – After we had a March to forget in the Philly area with temperatures that ended almost 4 degrees below normal on average and what ended up being the second snowiest March on record, we could use a dose of some spring weather for the month of April.
Unfortunately, it doesn’t look like we are going to get that true spring thaw until at least the middle of the month and while some snow has already fallen across the area, another round of snow showers could be possible before the first full week of the month is over. This blog post will dive into what we can expect temperature wise through the month and if/when the warmer weather finally arrives.
Let’s begin by looking into what usually happens with our temperatures during the month of April. As we might expect we slowly warm throughout the month, with the average high on the first being 59 degrees but by April 30 our average high has climbed all the way to 69 degrees.
Also, the average high temperature for the month sits at 62.3 degrees, so not too bad all things considered. The warmest average high temperature we ever had for Philly came in 1941 when our average high for the month was 70.7 degrees.
However, when looking into the record books we use the average temperature for the month, not the average of the highs for the month to decide how conditions as a whole compare to those prior. In that case, Philly in April has an average temperature of 52.8 degrees, vastly cooler than the average high temperature for the month. Just last year we had the warmest April on record in Philadelphia at 59.5 degrees, barely, and I mean barely, beating out 1994 by 0.01 of a degree for the warmest average of all time.
Based on model outlooks and projections for 2018 it does not look as though we are going to be coming close to breaking the record set last year. If we break things down even further, we see that on average April averages 27 days where the high temperature is warmer than 50 degrees, 18 days where the high is greater than 60, and eight days where we max out more than 70. There has even been 18 times since record keeping began for Philly (1874 for those keeping score at home) when every day of the month had a high temperature that stayed above 50, the last time being just last year. Interesting enough, there has only been one time since record keeping began where we didn’t have a single temperature in the 70s, and that was the very first year of the records in 1874. Now that the record and typical April temperature patterns are out of the way, let’s look into what this year looks like for April in Philly.
There are a couple different ways to go about peeking into the future, in the long-range. There are long-range weather models that are put out, and then are also atmospheric patterns that you can watch to give you an idea of how the overall month will play out — we will dive into both. Let’s start with the what the models are turning out for the month. Both the GFS and the European Model have mid- to long-range predictions for the month. The newest runs of both of those models show us staying cool heading through most of next week, before there is consensus that we get a brief warm up toward the end of next week. The best chance for some warmer than normal temperatures this month right now looks like it should be between April 13-16.
After that brief warm up, though, both the models show another colder than normal end to the month. Overall, the models are showing a colder than normal month as a whole by anywhere from 2 to 5 degrees. Another way to get an idea of some longer range patterns is to use what is called the Arctic and North Atlantic oscillations.
Both of these oscillations are related but not the same and are officially patterns that emerge in atmospheric pressure in the mid-latitudes as well as the Arctic regions of the globe. Both of these oscillations run in phases that we call positive and negative, with each phase corresponding to certain behavior in the atmosphere. When it comes to the Arctic Oscillation (AO), a positive phase usually corresponds to warmer temperatures across the mid-latitudes, while the negative phase is the opposite, usually leading to colder trend of temperatures across the U.S. The North Atlantic Oscillation (NAO) is related to the AO and there is much discussion about which is a better representation of the overall atmospheric pattern, but a consensus has not been come to over which to put more weight on.
That being said we will look at both for the month of April. The overall flow of the NAO and AO match up pretty well with the thinking of a mid-month quick warm up before another cool down in the second half of the month. There are some discrepancies, though, about just how positive and negative the phasing will be throughout the month but again the overall trend, unfortunately, is showing that we are likely to end up with a cooler than average month.
So what does all this mean for the month of April? Well, right now we will be colder than normal for the first two weeks of the month it looks like before a brief warm up to end the second week and start the third week of the month, but another cool down comes for the end of the third week of April, persisting possibly all the way into the beginning of May. Overall, right now, I think we will end up somewhere between 2 to 4 degrees below normal for the month on average.
Check back at the end of the month to see how this outlook for the month compares to what actually happens!