Why is Philly at times warmer than much of the East Coast?

[CyphaPSU]

39.9526° N, 75.1652° W, the lat/long of Center City.  Climatically, Philly is warmer than most places at the same latitude within the continental United States during the cooler months.  Geography plays a big role here, our close proximity to the moderating waters of the Atlantic Ocean can explain that, particularly in the wintertime.  That makes sense.

However, I've been pondering a question for a couple of years now that deals with Philly's temperatures compared to some other locations, locations with a more fuzzy answer.  The question comes from comparing Philly's daytime high temps, not inland areas, but to other East Coast areas; and not during winter, but rather during spring and summer. 

Often during warm air advection events on the East Coast during the spring and summer, a cursory look at sfc map temps reveals that the Philly area often matches and sometimes surpasses the temperatures of locations along the East Coast far to the south, and yet areas not nearly as far north of Philly remain significantly cooler.  Just north of New York City seems to be the line at which the temperature gradient dramatically decreases during such events.  As far as those areas north of NYC, such as Connecticut and up, I think geography here again plays a large role with how given the change in shape of the coastline and raised terrain (and at times backdoor cold fronts can come into play during the spring).  But, what about times when Philly has a warmer day than--let's say--Richmond, VA and Raleigh, NC while any difference in cloud cover is negligible?  The temp difference can be even more pronounced comparing Philly to rural areas in eastern VA and NC, which lack the urban heat island effect Philly experiences.

The only causes I can think of as to why Philadelphia can achieve such high temperatures during warming events is because of the urban heat island effect and compressional warming.  With the large metropolitan area Philly is complete with its low albedo surfaces, combined with its low-lying elevation and its somewhat inland location from the coastline, the urban heat island effect (UHIE) I think has quite an impact on Delaware Valley temps.  I think compressional warming also comes into play given how the terrain slopes down from the Appalachians and Piedmont toward the Delaware Valley.  But, I don't believe these reasons are sufficient to explain all the data.

There are times when Philly's temps can rival urban centers far to the south, even places such as Atlanta, which are also affected by the UHIE.  Mostly rural areas in the Delaware Valley region can at times be on par with their southern counterparts many hours away down the map.  All the locations I have discussed so far are set east of the Appalachians, which theoretically should also experience some compressional warming as a result of downloping air movement.  So, my point is that I do not think that these two reasons alone can explain everything.  What is it about the geography of the Delaware Valley that can make it so warm at times?

I have searched the Interwebs more than once seeing if my specific question regarding Philly's temps during warm periods versus other East Coast areas has been addressed, but I've never found anything.  It's not like this is an unknown phenomenon, models accurately project this.  So, I'm wondering what the thoughts are of anyone on the board about this matter.  Please chime in!

[Rainshadow]

5 minutes ago, CyphaPSU said:

39.9526° N, 75.1652° W, the lat/long of Center City.  Climatically, Philly is warmer than most places at the same latitude within the continental United States during the cooler months.  Geography plays a big role here, our close proximity to the moderating waters of the Atlantic Ocean can explain that, particularly in the wintertime.  That makes sense.

However, I've been pondering a question for a couple of years now that deals with Philly's temperatures compared to some other locations, locations with a more fuzzy answer.  The question comes from comparing Philly's daytime high temps, not inland areas, but to other East Coast areas; and not during winter, but rather during spring and summer. 

Often during warm air advection events on the East Coast during the spring and summer, a cursory look at sfc map temps reveals that the Philly area often matches and sometimes surpasses the temperatures of locations along the East Coast far to the south, and yet areas not nearly as far north of Philly remain significantly cooler.  Just north of New York City seems to be the line at which the temperature gradient dramatically decreases during such events.  As far as those areas north of NYC, such as Connecticut and up, I think geography here again plays a large role with how given the change in shape of the coastline and raised terrain (and at times backdoor cold fronts can come into play during the spring).  But, what about times when Philly has a warmer day than--let's say--Richmond, VA and Raleigh, NC while any difference in cloud cover is negligible?  The temp difference can be even more pronounced comparing Philly to rural areas in eastern VA and NC, which lack the urban heat island effect Philly experiences.

The only causes I can think of as to why Philadelphia can achieve such high temperatures during warming events is because of the urban heat island effect and compressional warming.  With the large metropolitan area Philly is complete with its low albedo surfaces, combined with its low-lying elevation and its somewhat inland location from the coastline, the urban heat island effect (UHIE) I think has quite an impact on Delaware Valley temps.  I think compressional warming also comes into play given how the terrain slopes down from the Appalachians and Piedmont toward the Delaware Valley.  But, I don't believe these reasons are sufficient to explain all the data.

There are times when Philly's temps can rival urban centers far to the south, even places such as Atlanta, which are also affected by the UHIE.  All the locations I have discussed so far are set east of the Appalachians, which theoretically should also experience some compressional warming as a result of downloping air movement.  So, my point is that I do not think that these two reasons alone can explain everything.  What is it about the geography of the Delaware Valley that can make it so warm at times?

I have searched the Interwebs more than once seeing if my specific question regarding Philly's temps during warm periods versus other East Coast areas has been addressed, but I've never found anything.  It's not like this is an unknown phenomenon, models accurately project this.  So, I'm wondering what the thoughts are of anyone on the board about this matter.  Please chime in!

Sometimes it is because the heat that is manufactured in the central U.S. wraps around the ridge and comes thru our area vs going thru Richmond, Atlanta, etc.  Also lower dew points will make it easier for temps to get higher (here).  I find Newark & Teterboro get even hotter than Philly in these scenarios. The 1000-850mb thickness charts show the former pretty well. But the freebie sites that I have seen have such a coarse gradient with this feature that it masks the thermal heat core.

[Rainshadow]

As you posted, the compressional warming because the prevailing wind direction is usually west or southwest by this time of year does not hurt the heat cause either.  NYC itself gets some half baked sea/sound/river breeze by mid afternoon on weak gradient days and I am still not used to max temps at Philly occurring later in the day than what I remember from growing up in NYC.

[tombo82685]

I would definitely say the biggest issue to us is the Downsloping flow over the APPS. Point to ponder though, imagine if those mtns weren't there how much cooler do you think philly's avg temp would be? I would also say DC is much warmer than Philly usually, and in terms of overnight lows, nyc area is worse than philly. Really holds on to the urban heat in the summer time. In terms of places further north like nyc and sne, the water plays a huge role there. The only way they get warm is with a west flow or southwest flow. Any southerly flow or southeast flow is going to be cooler for them because it's a flow right off the ocean while philly being inland doesn't get that benefit. 

[famartin]

A few things to consider when comparing PHL to elsewhere:

1 - PHL is the lowest location at its lattitude.  Its just above sea level, but relatively protected from sea breeze because its on a river instead of on the ocean. Points west are all several hundred to several thousand feet above sea level. Many locations in the south that are not on the coast are also considerably elevated.  Would it surprise you to know that ATL, for example, is 1,000 feet higher than PHL is?  This plays a factor.

2 - Sometimes, individual cases are a result of weather and not climate.  Here are PHL's average high temps from 1988-2017 vs. ATL's:

MON MAY JUN JUL AUG

PHL 74.3 83.3 87.7 85.5

ATL 80.6 87.0 89.8 88.7

Here are the highest temps observed in the listed calendar month  at PHL vs ATL from 1988-2017:

MON MAY JUN JUL AUG

PHL 97 100 103 101

ATL 95 106 105 104

Here are the average number of 90-degree days in the listed calendar months at PHL vs ATL from 1988-2017:

MON MAY JUN JUL AUG

PHL 2 6 12 7

ATL 2 11 16 14

[CyphaPSU]

On 5/7/2018 at 10:47 PM, famartin said:

A few things to consider when comparing PHL to elsewhere:

1 - PHL is the lowest location at its lattitude.  Its just above sea level, but relatively protected from sea breeze because its on a river instead of on the ocean. Points west are all several hundred to several thousand feet above sea level. Many locations in the south that are not on the coast are also considerably elevated.  Would it surprise you to know that ATL, for example, is 1,000 feet higher than PHL is?  This plays a factor.

2 - Sometimes, individual cases are a result of weather and not climate.  Here are PHL's average high temps from 1988-2017 vs. ATL's:

MON MAY JUN JUL AUG

PHL 74.3 83.3 87.7 85.5

ATL 80.6 87.0 89.8 88.7

Here are the highest temps observed in the listed calendar month  at PHL vs ATL from 1988-2017:

MON MAY JUN JUL AUG

PHL 97 100 103 101

ATL 95 106 105 104

Here are the average number of 90-degree days in the listed calendar months at PHL vs ATL from 1988-2017:

MON MAY JUN JUL AUG

PHL 2 6 12 7

ATL 2 11 16 14

Thanks to the responses, everyone.

famartin, thanks for the input and I agree that elevation makes a big difference.  It's the very reason why Tussey Ridge would often have snow on it while Happy Valley was snow free for days, and the difference in elev is only about a 1,000 feet or so.  The problem is that there is no nice geographically analogous major city to Philly in the South, no big metro area that is 50 or so miles inland from the coast, but still at sea level.  My question has always revolved around specific weather events--especially during warm air advection events in the spring months--not averages or climate.  It is interesting how Philly can at times reach higher temps than various coastal plain regions up and down the coast, and I've been trying to explore the reasons for a while now.  North of NYC the differences are pretty straight forward, geography of the coastline and terrain has a lot to do with it.  It is the lower Mid-Atlantic coastal plain and even further south where the temp comparisons become interesting to ponder at times during certain warming events.

[Rainshadow]

Our current weather (7/1/18) is an example of how this can happen from time to time.  Notice the heat that is manufactured in Kansas and Nebraska takes a route via Michigan to arrive in our area today while places to the south are bypassed by it.