Lewistown nears historic August rainfall total

Localized storms soak Juniata River Valley

LEWISTOWN -- August has been unusually wet across the Juniata River Valley, but the heaviest rainfall has been concentrated in pockets, including around Lewistown, where precipitation is approaching levels rarely seen this time of year.

A Lewistown climate station recorded 8.93 inches of precipitation through Sunday, according to preliminary data compiled by the Iowa Environmental Mesonet. That includes an estimated 1.21 inches on Sunday. The National Oceanic and Atmospheric Administration's 1991-2020 climate normal for Lewistown is 3.62 inches for the entire month of August.

That means Lewistown has received almost 2 1/2 times its normal August rainfall before the month is finished.

Michael Colbert, a meteorologist with the National Weather Service in State College, said repeated slow-moving cold fronts helped turn August into an unusually wet month across parts of central Pennsylvania.

"There's just been a lot of cold fronts that have moved through very slowly," Colbert said. He described boundaries that stalled over the area, focusing repeated rounds of showers and thunderstorms over the same region.

The setup has changed in recent days. Colbert said an upper-level trough over eastern Canada has brought cooler air, lower humidity, and lighter showers. Unlike some earlier systems, it doesn't have a tropical moisture connection.

The heavy rainfall hasn't been spread evenly across Pennsylvania. Colbert said precipitation data for Aug. 1-15 showed a small area centered near Lewistown that received about twice its normal precipitation for that period.

Rainfall has also been well above normal in portions of Juniata County, although totals vary because thunderstorms have repeatedly produced narrow bands of heavy rain. The county doesn't have a current long-term climate station directly comparable to Lewistown, making it difficult to assign one month-to-date rainfall total to the entire county.

That variation is important. Summer thunderstorms can drop heavy rain over a narrow corridor while leaving a nearby community with considerably less. Lewistown's 8.93-inch preliminary total shouldn't be treated as the rainfall total for Mifflintown, Port Royal, McAlisterville or Juniata County.

Other parts of central Pennsylvania have also had wet stretches.

Colbert said Shippensburg experienced its third-wettest Aug. 9-23 period in a record covering about 93 years, though he cautioned that it isn't an official climate site and he couldn't verify whether the record contained gaps. Johnstown Airport had its second-wettest such period in 24 years.

At Harrisburg International Airport, which has a much longer record, the same period ranked 29th wettest in 137 years. Colbert said about 20% of the years in that record were wetter.

That comparison shows how localized the rainfall has been. Some communities have endured one of their wettest late-summer stretches in decades, while others have been wet without approaching records.

Lewistown could be one of the places worth watching.

Pennsylvania State Climatologist records list 9.66 inches in August 1955 as Lewistown's greatest August precipitation total in the older climate record. If the preliminary 8.93-inch figure for this month holds, Lewistown would be only 0.73 inches short of that mark.

The older Penn State climatology lists an August mean of 3.38 inches. The current NOAA normal is a little higher, but either comparison leaves this month far above what Lewistown typically receives.

Colbert stopped short of predicting a record.

"There’s no guarantee that will happen," he said.

But another weather system expected Thursday could make the final week interesting.

Colbert said humidity is expected to increase again, with dew points climbing into the upper 60s ahead of the system. The official rainfall forecast available as of now calls for less than a quarter-inch in Lewistown through 8 p.m. Thursday, but he said localized totals could be higher depending on where thunderstorms form.

"There's potential for some heavy rain, but I don't know exactly where that's going to be," Colbert said.

One concern is training precipitation, when showers or thunderstorms repeatedly move over the same locations. Colbert said south-to-north bands could develop and continue along the same axis for an extended period.

If thunderstorms line up correctly, he said, isolated locations could receive more than an inch.

"There could be some spotty amounts that go over an inch, maybe even an inch and a half, two inches," Colbert said. "That's not out of the question with that system given the ample humidity that's going to be surging back in."

Before then, the valley should get a break.

The National Weather Service's Monday forecast called for a trend toward warmer, drier weather with relatively low humidity Tuesday through Thursday, with showers and thunderstorms possible again mainly Thursday afternoon.

The rain has already separated August 2026 from an ordinary late summer.

The final number will depend on which gauge is used and what falls during the last seven days. But Colbert said the concentration of heavy rain around Lewistown is clear.

"Certainly, there's been some pockets of anomalously heavy rain," he said.

And with another humid system approaching, August may not be finished adding to the total.