PITTSBURGH, Pa. — Western Pennsylvania is expected to see repeated rounds of rain and thunderstorms Sunday, with forecasters warning that some downpours could be heavy enough to produce localized flooding.
The threat is developing rather than an active region-wide flooding emergency. Forecasts Saturday showed rain arriving overnight and continuing through much of Sunday, with the most persistent rainfall expected during the morning and early afternoon.
The National Weather Service forecast for Pittsburgh calls for a 100% chance of precipitation Sunday, with showers and thunderstorms expected and a high near 78 degrees. The forecast calls for one-quarter to one-half inch of new rainfall at the Pittsburgh point forecast, although thunderstorms could produce greater totals in individual locations.
The amount measured at any one location will depend heavily on where repeated bands of rain develop. That distinction matters because localized flooding can occur even when the average forecast rainfall across a broader area appears relatively modest.
When the heaviest rain is expected
WTAE designated Sunday an Impact Day because of the expected rounds of rain. Its Saturday forecast said showers would begin after midnight and that most of the day's rainfall would fall through early Sunday afternoon. The station said some of the heaviest rain could occur around midday before precipitation begins to decrease Sunday evening.
CBS Pittsburgh similarly forecast a soggy Sunday morning and early afternoon, with heavy rain and thunderstorms possible. Its meteorologists said some locations, particularly north of Pittsburgh, could receive more than an inch of rain.
Those local forecasts are broadly consistent with National Weather Service guidance showing an unsettled pattern moving into the region. The Pittsburgh weather office has warned that rain could be heavy at times and that isolated flooding and river rises are possible as repeated rain chances continue into the middle of next week.
The Weather Prediction Center also identified a risk of excessive rainfall reaching parts of the region. In its midday Saturday discussion, the center maintained a slight risk into far southwestern Pennsylvania for the period extending through Sunday morning. Forecasters cited the possibility of thunderstorms repeatedly moving across the same locations, a pattern that can quickly increase rainfall totals.
For the subsequent period beginning Sunday morning, the Weather Prediction Center kept a slight excessive-rainfall risk over portions of the Ohio Valley but emphasized uncertainty over exactly where the heaviest bands would develop. That uncertainty means the flooding threat is not expected to be uniform across Western Pennsylvania.
Risk extends beyond Pittsburgh
The concern is not limited to Allegheny County. The National Weather Service office in State College, which covers parts of western and central Pennsylvania including Cambria and Somerset counties, said Saturday afternoon that moderate to heavy rain could result in localized flooding Sunday.
The differing National Weather Service offices cover separate portions of Pennsylvania, but both identify heavy rainfall as a potential concern in areas west of the state's center.
As of Saturday morning, CBS Pittsburgh reported that no flood watches had been issued for Sunday in the Pittsburgh area. A flood watch, if eventually issued, would mean conditions are favorable for flooding but would not mean flooding is already occurring. Forecasts and watches can change as meteorologists gain a clearer picture of where the heaviest rain will set up.
What residents should watch
The practical concern Sunday will be whether repeated thunderstorms or heavy showers begin moving over the same communities. Those areas could accumulate substantially more rain than neighboring locations and could see water collect on roads, in low-lying areas and near streams.
Drivers should be especially alert during heavier downpours because flooding depth can be difficult to judge and water can cover road hazards. Residents near flood-prone streams or locations that experienced drainage problems during previous heavy rain events should monitor updated National Weather Service forecasts and any watches or warnings issued Sunday.
The forecast does not currently point to continuous heavy rain at every location throughout the entire day. Instead, multiple waves are expected, with periods of heavier rainfall separated by lighter precipitation or breaks.
Sunday also begins a cooler and unsettled stretch for the region. The Pittsburgh National Weather Service forecast shows additional shower chances Monday through Thursday, with daytime temperatures falling into the 60s for much of the week.
The most immediate period to watch remains Sunday morning into early afternoon. The precise placement of the heaviest rainfall remains uncertain, so the extent of any flooding will depend on where the strongest rain bands develop and how long they remain over the same areas.