The languid atmosphere over the Philadelphia region really hasn’t been locked in a perpetual state of steam bath for the last month. It only feels like it.
So observes Sarah Johnson, the warning coordination meteorologist at the National Weather Service office in Mount Holly, which yet again has posted a flash flood watch, this one in effect until 2 a.m Tuesday.
Heavy rains began oozing into the region between 5 and 6 p.m. Wednesday, setting off flood warnings in parts of Chester and Montgomery Counties, in effect until 9:45 p.m. Downpours also generated warnings in parts of northern Delaware.
The weather service says “localized” rain amounts of up to 5 inches are possible during the evening. (No, this is not the same story you read last week.)
By now the forecast language may evoke a certain familiarity: “Excessive runoff may result in flooding of rivers, creeks, streams, … etc. etc. etc.“
The atmosphere was is stocked with levels of “precipitable water” — that’s the measure of the rainfall potential — as it ever gets, the weather service warns.
Conditions were similar last week, so downpours in some areas were all but a certainty.
As usual, don’t be surprised if the final totals are all over the place.
What time will the heaviest showers arrive in Philly?
Timing is always a problem with showers, said Johnson, noting that some had popped up during the morning.
As with the storms last week, these would be associated with an approaching “cold” front, but it is forecast to slow to a glacial pace as it nears the Philly region.
Then, rather than brooming away the rain-forest conditions, it is forecast to stall nearby. Precisely where remained uncertain.
The front will be a triggering mechanism for shooting the sun-warmed, vapor- filled air skyward, setting off the storms.
“With very high moisture available and relatively slow movement, flash flooding is a big concern,” the weather service said in its morning discussion.
Why has it been so muggy?
“Slow movement” has fairly well characterized the behavior of the atmosphere in the last month.
“We had some brief breaks,” said Johnson. True. The July 4th weekend, for example, was splendid.
That said, “The really moist air masses have been here more often than not since mid-June,” she added.
That consistently high water-vapor content of the air has been evident in the nighttime temperatures, which have averaged more than 2.5 degrees Fahrenheit above normal since June 1.
Moisture in the atmosphere inhibits daytime heating from escaping into space, thus keeping the temperatures higher from sunset to sunrise.
This time of year, what goes around tends not to go anywhere in a hurry. The temperature contrasts that drive the movement of weather systems weaken in summer as the sun’s heating spreads more evenly across the northern hemisphere.
Why is so hard to say when and where storms will form?
When storms are brewing, and the air is this full of moisture, the slow movements can be especially dangerous. Sometimes, storms will “train,” as in keep hitting he same places.
Forecasting summer storms is especially problematic, said Adam Clark, a specialist with the National Severe Storms Laboratory in Norman, Okla.
With the weaker flow, the systems “tend to smaller and more challenging to pinpoint,” he said. Computer models have trouble pinpointing where and when they might pop, he said, in part because they often develop in areas above the surface that are poorly observed.
What is a safe bet is that the word “showers” will appear in forecasts the rest of the week.
Is this ever going to end?
The mugginess will be with us all week.
But only 160 days until the winter solstice.
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