Heat today, gone tomorrow

The heat wave has about 12 hours to live before a cool front brooms it away in favor of a spell of generic late-April dreariness.

Before then, though, odds are that it will hit 90 one more time, tying a record for the longest-lasting April heat wave, and this could be a miserable day for some allergy-sufferers.

The air is going to be quite dry this afternoon with a stiff southwest wind, prompting the National Weather Service to issue a "red flag" fire-danger warning that will be in effect for the entire region until 8 p.m.

Those also are ideal conditions for pollen travel, and this morning the oak-tree pollen is off the charts, said Dr. Donald J. Dvorin, the region's official counter for the American Academy of Allergy, Asthma and Immunology. That explains that green sheen on your car.

At 8 a.m., the 24-hour counts were "extreme," he said. "I think this will be the peak day," he added. Some of those sufferers may even mistake their sneezing and assorted discomforts for swine-flu symptoms, he said. But, no, swine flu and pollen allergies have nothing in common.

Any rain would help shoot down the pollen flight, but it now appears that the precipitation will hold off until late tonight or the early morning hours, and it should have no effect on tonight's Phillies-Nationals game at Citizens Bank Park.

"Unless they go 14 innings, they should be safe," said Tony Gigi, a meteorologist with the National Weather Service in Mount Holly.

If the temperature does reach 90 for a fourth consecutive day, it would match the four-day stretch of 90-plus April days in 1976.

April heat waves are unusual, and only four other ones have occurred in the period of record, dating to 1874 - 1896, 1976, 1990 and 2002.

What does an April heat wave portend about the summer? Based on such limited experience, it is impossible to say.

The weather service points out that the last time it happened, in 2002, the summer was a hot one. In two other instances, 1976 and 1990, the following seasons were ever so slightly on the cool side. The summer of 1896 was cool.

The government's Climate Prediction Center is on the fence, but Weather Services, Inc., a commercial forecasting company in Massachusetts, thinks the Philadelphia summer will end with temperatures below normal.

WSI meteorologist Todd Crawford says that for the first time in 15 years, both the North Atlantic and North Pacific Oceans are cooler than normal. "This fact, along with a lack of any significant drought conditions in the eastern United States, should result in a relatively cool summer east of the Rockies," he said.

One thing to watch, however, is the general dryness in the Philadelphia region, a damp April notwithstanding.

Soil moisture is a key factor in summer heat, the experts say. The less moisture in the soil, the easier it is for the sun to heat the surface since it doesn't have to spend energy on evaporating water.

As of last week, the entire region is in the "abnormally dry" category on the U.S. Drought Monitor map.

April will end up being the first month of the year in which rainfall has averaged above normal in Philadelphia.

For the last 90 days overall, precipitation is below normal in every county in New Jersey and all but one in Pennsylvania. Rain totals are off by a third or more in Philadelphia and the Pennsylvania suburbs. The Jersey counties are in a little better shape, but only Cape May County is close to normal.

The rain the next few days isn't expected to amount to much, maybe a quarter of an inch, the weather service says.

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