October was Philly’s first rainless month in 153 years of recordkeeping

Chances are that some parts of the region are going to see downpours on Friday — of leaves, that is.

After the first month without measurable rain in records that date to 1872 in Philadelphia and a record-tying high of 82 degrees on Halloween, branch-shaking winds gusting to 30 mph are likely to stir flurries of falling leaves. They would join their brittle brethren on ground that is about as dry as it gets around here.

The winds and relentless dryness have prompted the National Weather Service to issue a fire watch, warning that “any fire that develops will catch and spread quickly.”

No rain is in the forecast during the weekend, extending an extraordinary dry spell in the Philly region, the Northeast, and other parts of the nation. The U.S. Drought Monitor on Thursday classified more than 85% of the country as “abnormally dry,” the highest percentage since it began keeping track in 2000.

All 195 counties served by the weather service’s Middle Atlantic Forecast Center, from Virginia to Upstate New York, reported below-normal precipitation in the 30-day period that ended Wednesday.

The updated monthly forecast issued Thursday by the government’s Climate Prediction Center had the chances favoring continued warm and dry conditions in November.

What’s up with what’s not coming down?

Officially, Philadelphia finished October with a “trace” of rain, not even meeting the 0.01-inch threshold of “measurable.” This followed a September in which only 0.77 inches was observed at the Philadelphia International Airport station.

By a recount-worthy margin, the Sept. 1-Oct. 31 period just missed beating the record for two-calendar-month rain scarcity — 0.73 inches, set in May and June 1964.

But during this dry run, Philadelphia has set a record for consecutive rainless days — which hit 34 Thursday and is still counting — surpassing the 29-day streak that ended in November 1874.

The drought monitor, produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, has most of the Philly region in “severe drought” conditions.

While New Jersey has declared a drought “watch,” asking residents to conserve water, Pennsylvania has not yet followed suit. In addition to rainfall, Pennsylvania’s Department of Environmental Protection takes into account stream flow, groundwater level, and soil moisture.

In summary, it’s been “the general reloading and persistence of strong high pressure across much of the eastern half of the country,” said Allison Santorelli, the acting warning coordination meteorologist for the National Oceanic and Atmospheric Administration’s Weather Prediction Center, situated outside dry Washington.

The descending currents under a dome of high pressure, or heavier air, don’t give clouds and rain much of a chance. This particular dome has made moisture from the Gulf of Mexico off-limits to the Philly region.

“It’s quite boring,” said Stephen Morgan, a Fox Weather meteorologist based in rain-starved New York. “Sometimes the atmosphere just gets stagnant.”

The high pressure repelling moisture in the Northeast “has been pretty strong,” he said.

“We have to wait for something in the atmosphere” to disrupt the pattern, he said, and right now that “something” isn’t imminent.

Matthew Rosencrans, seasonal forecasting specialist at the Climate Prediction Center, says it would require “large-scale pattern changes” to douse the dry run. But those changes tend to occur slowly in autumn, he said, before the storm-spawning upper-air jet-stream winds move south and strengthen. Those winds are driven by strong temperature contrasts that have not yet matured.

Dryness begets dryness, says Fox Weather’s Morgan. Fronts continue to come through dry, having little moisture to work with, including the one due to breeze through Friday and kick off the winds and a weekend cooldown.

That dryness looks to continue, and no significant rain is in the forecasts through the first week in November, although the weather service sees a 30% chance of showers at midweek. In 1964, dry conditions persisted for months. In one 30-day period in that fall, a mere 0.07 inches was measured at the airport.

As for what it means for the winter, probably nothing, said Rosencrans. Dry soil isn’t much of a clue about what’s to come.

So expect a 100% chance of nonlinear chaos.

“The role of climate change in this persistent pattern is not something we can determine in real time,” said Santorelli, “but it is a fact that climate change may cause more weather extremes, including precipitation extremes in both directions.”