In Philly’s stormy week, how much rain has fallen — or hasn’t — near you has varied widely

End-of-the-world phone alerts, tornado warnings, severe storm advisories, a flood here, a flood there. Just another July week in Philly.

For the meteorologists at the National Weather Service, the atmosphere’s behavior has precipitated a flood of storm reports about closed roads, stuck cars, downed trees, and sloshed-over creek banks.

Heavy rains pounded parts of South Jersey Thursday, with significant flooding reportedon Long Beach Island Thursday. Shower chances look to persist through the weekend — timing elusive — which could present more threats given the antecedent wet conditions.

Yet a whole lot of people in the Philly region may be wondering what all the fuss is about.

As so often happens with the storms of summer, the ones this week exhibited a characteristic caprice, hammering some areas with several inches of rain, and snubbing others or spraying relatively benign tropical rains.

With thunderstorms, said Robert Shedd, chief hydrologist at the weather service’s Middle Atlantic River Forecast Center, “you can get 2 inches in one place, and two miles away, you could have a quarter inch.”

Chester County was an extreme version of exhibit A this week.

By the river center’s calculation, portions of the southern end of the county were inundated with as much as 5 inches Wednesday morning and Thursday morning

That was as much as 25 times more than the 0.2 inches measured in a section of the northern end of the county.

On Monday, just about all the rain action was in the southwestern corner of Chesco, with the rest of the region receiving paltry amounts, including just 0.01 inches at Philadelphia International Airport.

The rains were more widespread Tuesday and Wednesday, but still the disparities among the county totals were striking, based on the river center’s analysis, which uses a sampling of reliable stations.

At the low end, averaged out among the stations, about a third of an inch fell across Montgomery and Bucks Counties; a portion of Upper Bucks, however, received up to an inch. By comparison, Chester County weighed in with 1.29 inches, a total depressed by the low totals in the north. Delaware County, where Chester Creek sloshed over in Chester, averaged 1.79 inches.

Wilmington airport measured 1.84 inches, more than twice what fell up the river at PHL.

Your local meteorologists, and their computers, would love to know.

Summer presents all kinds of forecast problems, said Robert Trapp, atmospheric science professor at University of Illinois Urbana-Champaign and storm specialist.

As opposed to well-organized winter storms that are driven by sharp temperature contrasts that cover thousands of miles, summer storms “tend to be on the small scale.” Like, sometimes, a few miles.

The sun’s power weakens the contrasts and, thus, the winds, so any storms that form tend to show little interest in moving, said Trapp, who is director of the university’s School of Earth, Society and Environment.

Sometimes, they basically keep raining on the same places, and while they do they may have drying effects on surrounding areas.

Thunderstorms form when warm moist air is forced to rise over cooler air and condenses. Think of a basketball released from the bottom of a swimming pool, the weather service says. It rises quickly because it is so much less dense than the water.

In this week’s case, said Nick Guzzo, a weather service meteorologist in the Mount Holly area, the triggering mechanism has been a nearby insipid, meandering front that has shown little interest in moving.

When storms form, they generate drying descending air around their perimeters that keeps it from raining in areas nearby.

All this happens on scales that are difficult to predict, meteorologists say.

“The skill of the models during the warm season tends to go down,” Trapp said.

And the larger problem is that observations are lacking. “We just don’t sample the atmosphere very well,” he said.

Said Adam Clark, with the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory in Norman, Okla., the “disturbances that trigger storms during the summer often originate above the surface, where we don’t have as many weather observations.” Plus, he said, “the thunderstorms themselves occur on small scales not represented well by models.”

Increasing the nation’s weather balloon network would help provide valuable data for computer models, Trapp said. The weather service has only 92 balloon sites across the nation.

The balloons, which can climb as high as 20 miles into the atmosphere, are equipped with instrument packs that take key measurements in the mayhem brewhouses. In addition to providing data for models, they serve as a ground-truth check on satellite observations.

Trapp, however, points out that the balloons are expensive. Each release costs $200, the weather service says, and typically they are sent up twice a day.

Said Clark: “No researcher would turn down the opportunity to have more data, but there are many logistical challenges with creating a denser balloon network.” The storm laboratory is looking at “other observing platforms that can be automated,” he said.

Said the river center’s Shedd: “We’ve come a long ways in the last 20 years, but still have a long way to go.”

The Pennsylvania Department of Environmental Protection last week quietly ended the region’s drought watch.

A watch is still in effect for areas near the Jersey Shore, which was classified as “abnormally dry” in Thursday’s weekly U.S. Drought Monitor update.

That may change next Thursday.