Coming tomorrow: High heat; high drama?

Posted: July 18, 2012

If you haven't felt heat like this in awhile, perhaps you've been in Anchorage the last few weeks.

For the 15th time this month, and the 24th time this year, the official temperature hit 97 at 3:59 p.m. today at Philadelphia International Airport. That's well shy of the record for the date, 102, set in 1988, but this 90-plus business is getting monotonous, if not expensive. And, yes, we know it's been hot all over the country.

For the entire year, the long-term normal for 90-and-above days in Philadelphia is 26, with 17 of those having occurred by the end of July.

Tomorrow, the 2012 number almost certainly bumps to 25. It is going to be one of the steamiest of an uncomfortable month, with highs aiming for the high 90s, and the heat index, for the low 100s. Wednesday night won't be a tip in the Arctic, either, and the excessive heat warning in effect until 6 a.m. Thursday. The Philadelphia Corporation for Aging will have its heatline, 215-765-9040, open until midnight, and again from 8:30 a.m. to midnight tomorrow.

But significant change is on the way, and later tomorrow afternoon watch out for a potentially dramatic atmospheric confrontation as a glacially moving front sags southward and interacts with our hot and waver-vapor-laden air mass.

The government's Storm Prediction Center has placed all of Pennsylvania and all but a fingertip of New Jersey in the "slight risk" zone for severe thunderstorms tomorrow night, defined as storms with gusts of at least 58 m.p.h.

Even if they technically fall short of that severe criterion, some strong storms are likely, and the National Weather Service is warning of the potential for downpours and frequent lightning.

The shower possibilities continue on Thursday, and the rainfall totals could approach an inch. On Friday, a drier and far-more pleasant air mass should build in for the weekend.

As it turned out, as hot as it got today, the heat indexes weren't all that bad.

During the day, the relative humidity dropped to near 30 percent; the dewpoint, close to 60, and the heat indexes tracked closely with the temperatures.

Tomorrow, the approaching front and an area of heat-pumping high pressure over the Atlantic will combine to brew a water-vapor broth over the region.

Heat indexes could hit 103 tomorrow afternoon, thus the continuation of the excessive-heat warning, even though it seems as though one has been in effect since the end of June.

The folks in Anchorage, Alaska, probably wouldn't mind a little heat about now. Through yesterday, July temperatures were running 5.4 degrees Fahrehnheit below normal, the mirror opposite of Philadelphia's 5.2 above normal.

The July temperature had yet to climb above 63 this month in Anchorage, where it can get quite warm, by the way; the temperature has been up as high as 86.

The calls for the mid-80s on Friday, and after three weeks of the hot-coal treatment, the mid-80s might feel Anchorage-like.


Contact Anthony R. Wood at 610-313-8210 or twood@phillynews.com

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