Author: Anthony R. Wood

Just as the soils were beginning to take on that cracked and dried look and the vegetation was working up a thirst, the rains have returned to the Philly region, and showers and possible thunderstorms may bookend the week.

Rains Sunday ended the first seven-day period without measurable rain in Philly in two months. More showers were possible Monday and later in the week, forecasters said.

In all likelihood they won’t remotely approach the destructive deluges in the Midwest, said Brian Hurley, senior branch forecaster with the federal Weather Prediction Center, in College Park, Md.

And around here, despite the generous rains that have fallen upon the region since mid-June, additional rain wouldn’t be altogether unwelcome, said Hurley.

Precipitation in the last 60 days has been well above normal everywhere in the region save for Montgomery County, where they were just about normal, according to the weather service’s Middle Atlantic River Forecast Center, which uses a network of stations in the counties.

However, not so long ago, as we headed into June, Philadelphia had experienced 16 consecutive months of below-average precipitation totals.

And drier times may be coming, Hurley warned.

Drought conditions have eased, but not disappeared

New Jersey and Chester County remain under state-imposed drought warnings, with residents asked to go lighter on the water usage.

“We’ve been putting a dent in that drought,” said Hurley, but in the latest interagency U.S. Drought Monitor update, most of the region remains “abnormally dry,” with parts of Burlington, Bucks, and Montgomery Counties in “moderate drought.”

Thunderstorm activity has been well above normal this summer, with the weather service reporting 19 days officially with thunder detected at Philadelphia International Airport. The average for the entire June 1 to Aug. 31 period is about 15 days.

It’s also been quite warm. In 12 of the first 15 days of August, the temperature reached at least 90 at the airport, and this almost certainly is going to end up being one of the warmer summers on record.

The warmth and the rains are related.

Philadelphia has spent a whole lot of time on the western edge of the so-called Bermuda high, centered over the North Atlantic. Winds circulate clockwise around the center of high pressure, and areas to the west of the center (where we are) experience winds from the south and southwest.

Being on the edge also places the region in spraying distance of shower activity as fronts and minor disturbances approach along the western flank of the ridge, with daytime heating adding a kick to the storms.

Thunderstorms may become scarcer come September

Those convective thunderstorms, which can wring out rain in waterfall-like downpours, in all likelihood will become less numerous as September approaches and the days shorten, Hurley said.

“It does get a little harder to pop convection when the sun doesn’t do its thing as much,” said Hurley.

“In September we tend to dry out until we get tropical connections,” he said.

This year, however, those tropical connections may be scarce, according to the National Oceanic and Atmospheric Administration and other forecast services.

What could become the most intense El Niño — the anomalous warming of vast expanses of the tropical Pacific Ocean — in records dating to 1950 is rapidly gaining steam.

During El Niño, the interaction of the ocean and the overlying air generates powerful upper-level winds from the west that can shear apart tropical storms in the Atlantic Basin before they mature.

No Atlantic hurricanes have developed yet this season. Meanwhile, Hurricane Lala in the Pacific has caused extensive damage in Hawaii. Typically, when the Pacific is quiet, the Atlantic is active, and vice versa.

Lala is the 12th named tropical storm — those with winds of at least 39 mph — of the season to form in the Pacific. The average for the date is seven.

In the Philly region, historically tropical-storm remnants have been generous sources of rainfall.

Sometimes, however, they have been overly generous. (See Ida, 2021.)

The rapidly developing El Niño is likely to become a “historic event” that could become the most-intense in the period of record, the government’s Climate Prediction Center said Thursday.

The El Niño warming in the tropical Pacific, coupled with how it is affecting the atmosphere, is all but certain to have dampening effects on the Atlantic hurricane season, researchers said.

While it is likely to be disruptive and destructive during the next several months, during the tropical storm season it may mean fewer scares for East and Gulf Coast and Caribbean residents, and perhaps fewer flooding threats in the Philly region.

El Niño already is making its presence evident thousands of miles away in the Atlantic Basin, said Philip Klotzbach, tropical storm specialist at Colorado State University. Colorado State’s forecast methodology has been the prototype for the outlooks issued by the National Oceanic and Atmospheric Administration and commercial companies.

In his update last week, Klotzbach predicted the season would produce nine named storms, those with winds of at least 39 mph — down from 13 in the April outlook. NOAA also subtly trimmed its numbers. The long-term average is 14.

Even with above-normal temperatures that usually would be favorable for storms in the hurricane-development regions of the Atlantic, what’s going on in the Pacific is going to dominate, he said.

“El Niño is definitely the story of this season,” he said, and NOAA says the Pacific warming is likely to persist through April.

(Even The Farmers’ Almanac has noticed. The winter outlook it has cooked up appears to be right out of the El Niño recipe book, calling for wetness and storminess in the South, typical of El Nino. Helpfully, it said the forecast up our way is “tricky.”)

How does warmth in the Pacific affect Atlantic hurricanes?

El Niños, which occur every two to seven years as part of a natural process, agitate the overlying air and disrupt atmospheric circulation worldwide. It is part of the so-called El Niño Southern Oscillation, or ENSO, and since oscillations do oscillate, El Niño periodically shares the stage with La Niña, during which the waters are cooler than normal.

The climate center reported Monday that in the key El Niño region of the east-central Pacific, sea-surface temperatures over a 2.4 million-square-mile area were more than 3 degrees Fahrenheit above normal.

That air-ocean interaction generates strong west-to-east upper-level “shearing” winds that can keep tropical storms from maturing into hurricanes.

Klotzbach said that last week, the 30-day average of wind shear in the western Atlantic was higher than in any other 30-day period dating to 1979.

Other players in the atmosphere affect tropical storm activity, but El Niño “usually becomes the dominant factor,” said Matt Rosencrans, the weather service’s lead hurricane forecaster.

During the top six most-intense El Niños in records dating to 1950, tropical-storm numbers were significantly below normal.

Is this El Niño yet another symptom of a warming planet?

“There is still considerable debate in the scientific community on how warming may affect ENSO behavior,” said Michelle L’Heureux, physical scientist with NOAA’s Climate Prediction Center and the head of the ENSO team.

El Niños likely have been occurring for eons, but the period of record dates only to 1950 because “much fewer observations” are available before then, she said.

“It would take much more research to compare El Niño events in the older records,” she added.

El Niños in which the temperatures reached 3.6 degree Fahrenheit — averaged over three-month periods — occurred in 1965-66; 1972-73; 1982-83; 1991-92; 1997-98, and in 2015-16.

The warmest three-month average, 4.3 degrees Fahrenheit, was recorded in the 1982-83 event, and the climate center sees a 69% likelihood that this will exceed that level.

But L’Heureux cautioned that “intense El Niño events are not totally unexpected.” The top six list shows that they have reappeared every 10 to 15 years.

Thus, she said, “There’s a reasonable argument we were due.”

What might all this mean for the Philly region’s weather?

Tropical storms or their remnants “are especially notorious for flash-flooding impacts” in the region, said Sarah Johnson, warning coordination meteorologist in the weather service’s Mount Holly office.

In 2020 and 2021, the region was affected by six such storms or remnants, including the devastating Ida, in September 2021.

The 2020 season was quite active, and 2021’s hyperactive. Logically, fewer Atlantic storms would reduce the odds of remnants heading this way.

With the exception of 2012, the year of Sandy, Johnson said that all the years with below-average flash-flood warnings issued by the Mount Holly office coincided with seasons in which no tropical storms or remnants have affected the region.

We wouldn’t mind an encore.

Yet more rinse-and-repeat flood warnings were posted for Philadelphia and portions of adjacent counties Wednesday as more downpours oozed through the region.

And don’t be surprised if more pop up later in the week, as the atmosphere will remain thickened with so much skin-clinging water vapor that “it’s not going to take much” to turn on the faucets, said Michael Silva, a meteorologist at the National Weather Service in Mount Holly.

One indicator of the atmospheric saturation: The weather service has posted a heat advisory for Thursday as heat indexes could rise to 100 or better — even though temperatures won’t get much past 90. That is all about the humidity in what could be the start of a more muggy wave, as opposed to a traditional heat wave.

The moisture in the air on Wednesday was such that the rain, which began around lunchtime, ended up being more ferocious than expected.

“Before work, I was about to go out twice for walks,” Silva said. “Then I looked at the radar.”

The heavy rains were wrung out along a narrow corridor cutting through the city, and they generated widespread traffic disruptions during the afternoon.

The weather service said that 1.23 inches of rain had been measured at Philadelphia International Airport, just between noon and 1 p.m. More than 2 inches fell upon parts of Bucks County.

Road flooding was widespread in Philly, Bucks County, and South Jersey.

Among the highways affected were the Schuylkill Expressway in the Point Breeze area; I-95 in Eastwick, at the Platt Bridge, and in the Croydon, Bucks County, area, where the warning was extended until 6:15 p.m.; and Route 130 in Burlington County.

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The rains were getting juice from a rich pulse of moisture from the South, said Mike Gorse, a meteorologist at the weather service’s Mount Holly office.

What’s more, these types of threats may be a “repeated thing” later in the week, he added, echoing Silva’s comment, with showers in the forecast again for Thursday night and Friday as the region is stuck in another sultry air mass.

The recent rains represent quite a reversal of fortune for the region and the entire state of New Jersey.

After 16 consecutive months of below-normal precipitation in Philly, officially close to 10 inches of rain has been measured since July 1, and recent rains have set off rashes of flood warnings, including on Sunday.

July’s total, 7.03 inches, was 2.65 inches above normal.

Still, water levels along the Schuylkill in Norristown and the Delaware River at Trenton have remained relatively low, the result of all that antecedent dryness.

However, flashier creeks, such as the Neshaminy, in Bucks County, are prone to slosh over in downpours, said Sarah Johnson, the weather service’s warning coordination meteorologist in Mount Holly.

While Chester County and all of New Jersey remain under drought warnings, the recent turn to wetness may have some staying power, she said.

According to the weather service’s Middle Atlantic River Forecast Center, which relies on networks of observations in each county, rainfall in all 21 counties in New Jersey has been well above normal in the 60-day period that ended on Tuesday.

On the Pennsylvania side, rainfall was at or above normal in Philly and its neighboring counties — including Chester County, where the 9.17-inch total was 113% of normal.

The federal Climate Prediction Center’s longer-term outlooks see chances favoring above-normal precipitation from next Tuesday through Aug. 19.

Just hold the downpours.