Flash Flood and Heavy Rain in Pennsylvania, May 8 to 12, 1998

A series of low pressure systems that passed through the Middle Atlantic States and then stalled offshore gave Eastern Pennsylvania nearly five days of continuous rain from when it started Friday morning the 8th until it exited the state from north to south on Tuesday the 12th. Storm totals ranged from 2.1 to 5.1 inches. The highest storm totals were in Bucks and Monroe Counties. The lowest occurred in Chester County. Overall there were few periods of heavy downpours and there were sufficient lulls of light rain or drizzle to keep most small streams within bankfull. The only gaged streams that briefly exceeded bankfull were the Wissahickon within Philadelphia and the Perkiomen Creek in Schwenksville (Montgomery County). Flooding problems were confined to flooded basements, backyards and some poor drainage flooding. The heaviest rain for any given day during this event was mainly Friday the 8th. The axis of heavy rain slowly shifted northward over the weekend. The heaviest rain on Saturday the 9th through Monday the 11th fell across sections of the Lehigh Valley and the Poconos. Precipitation totals on the 12th were generally under a quarter of an inch throughout the region. Tuesday May 12th also marked the 12th consecutive day that any precipitation (trace or greater) fell at the Philadelphia International Airport. This tied the monthly record for the month of May previously set in 1894.Heavy rain the evening of the 8th caused some minor flooding along the Perkiomen Creek in Schwenksville. The creek crested at about 7.5 feet. Flood stage is estimated at 7 feet. Heavy rain the evening of the 8th also caused the Wissahickon Creek in Philadelphia to crest at its flood stage of 5.0 feet at about 10:45 p.m. EDT.Storm totals included 5.07 inches in Springtown (Bucks County), 4.70 inches in Tobyhanna (Monroe County), 4.65 inches in Chadds Ford (Delaware County), 4.44 inches in Mount Pocono (Monroe County), 4.06 inches in Perkasie (Bucks County), 3.83 inches at the Willow Grove NAS (Montgomery County), 3.77 inches in Crum Creek (Delaware County), 3.63 inches in Valley Forge (Montgomery County), 3.45 inches in Easton (Northampton County), 3.44 inches in Palm (Montgomery County) and the Northeast Philadelphia Airport, 3.26 inches in Glenmoore (Chester County), 3.13 inches at the Lehigh Valley International Airport, 3.05 inches at the Philadelphia International Airport, 2.86 inches in Hamburg (Berks County), 2.76 inches in Reading (Berks County) and 2.24 inches in Lehighton (Carbon County). A low pressure system heading toward the Middle Atlantic States was in the lower Ohio Valley the morning of the 7th. By the morning of the 8th, it moved into northern Indiana as a secondary was forming in Virginia. On the morning of the 9th, this secondary low was located about 325 miles southeast of Philadelphia. But, it became trapped by a low at upper levels of our atmosphere and drifted slowly south through the morning of the 10th. Meanwhile, a large high pressure system started building into Northern New England. Yet another low pressure system developed over the Southern Plains on the 9th and was in the Tennessee Valley the morning of the 10th. On the morning of the 11th, two low pressure systems, one trapped under the closed low in the western Atlantic and the other just east of Charleston, South Carolina continued the wet onshore flow. These two lows merged into one by the morning of the 12th about 275 miles east of Cape Hatteras, North Carolina and remained nearly stationary through the 14th. The high pressure system over Northern New England slowly built south and brought drier air into the region and gave the area its first rain-free days of the month on the 13th and 14th.