An April Heat Wave

Normally speaking, when those outside the profession misquote or misinterpret a scientific (or accepted) weather definition, meteorologists are driven crazy.  I bring this up because I have been hearing many referring to our recent warm spell here in April as a bonafide heat wave.  The half of me that thinks as a scientist recalls the World Meteorological Organization’s official definition of a heat wave:  “A spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90°.”  However I must concede that the nonscientific, pragmatic side of me screamed “who cares,” especially after I ventured upstairs and the thermometer read 80° … indoors … at Midnight!

It’s no surprise that this warm weather is unusual, and in some cases unprecedented for the date, given that we are only seven days into April.  “IT’S GLOBAL WARMING!” I heard today.  Without opening up that can of worms, let me offer a smaller-scale explanation, at least for this “heat wave.”  A broad area of high pressure, the magnitude of which we don’t normally see for a few at least a few more weeks, has set up shop well off the East coast.  If you’ve paid attention during your earth science classes, air flows clockwise around a high pressure, thus our southwesterly flow which is pumping in the warm and slightly humid air from the Southeast.

Having said that I am willing to give in to the pragmatic half of my brain, I must satisfy the scientific half by defining an “April heat wave.”  For this time of year, let’s say a heat wave can be any period of unusually warm weather defined as three or more days with temperatures of 80° or above.  If that’s true, many of us aren’t even there yet, but should be by Wednesday or Thursday.  Let’s take a look at some local official observing stations and what they’ve measured recently:

Observing Site	Tuesday's High	Monday's High
Pottstown-Limerick Airport	89°	81°
Blue Bell (Wings Field)	87°	79°
Willow Grove Naval Air Station	86°	78°
Doylestown Airport	84°	77°
Northeast Philadelphia Airport	87°	78°

The quiet passage of a warm front last night clearly has shown up in the record books. Keep in mind that our normal high for this time of year (at least at the closest climate station, Philadelphia) is 59°. Many locations won't even get that low the next few nights (as I write this the thermometer outside is still above 70°). At the International Airport, a record was indeed established on Tuesday as the mercury hit 87°, nearly 30° above normal, and tying the old records from 1929 and 1942.

So when does relief get here? Not after many locations record an "official April heat wave." Wednesday will likely be the warmest day of the spell, with some locations likely to hit 90°. With the southwesterly flow expected to increase Wednesday into Thursday, you may also start to notice an old friend: humidity. Enjoy the sunshine and warmth while it lasts. Despite continued warmth into the 80s, clouds will begin to increase on Thursday ahead of a strong cold front which will knock us all on our butts for the weekend: 50s during the day, 30s at night. That's more typical for April, much to the chagrin of warm-weather bums.

There I said it. It's a heat wave.
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