Tropical Storm Ernesto slid up the East Coast of the U.S. on Thursday (Aug 31st) and Friday (Sep 1st), losing it's tropical characteristics as it interacted with a stationary boundary in place over VA/MD. It also had to butt heads with a strong High pressure system in place over New England and Eastern Canada. This interaction brought a deep, Easterly flow of air off the Atlantic Ocean into Central PA. This is a usual set-up for heavy rainfall in Central PA.
The rain started Friday morning along the Maryland Border, and crept northward through the day, reaching the New York Border just after nightfall Friday Night (late on the 1st). You can see how slowly this event started by comparing the radar imagery during the day Friday. At 8:00 am Fri the leading edge of the radar echoes was along the PA/MD border. As the day progressed this leading edge migrated slowly north to central PA by 2:00 pm and just reached northern PA by 8:00 pm Friday night. This slow northward movement of the rain shield was a result of drying and evaporation due to a relatively deep dry layer of colder air to the north caused by a strong high pressure system anchored over New England.
The rain became heaviest overnight, and lasted into Saturday Afternoon and Evening (the 2nd). An interactive loop of radar reflectivity overnight through Saturday reveals many areas of moderate rainfall (yellow area) moving from south to north across central Pennsylvania. A static loop of radar estimated 3 hourly accumulation also shows areas of more concentrated rainfall moving across PA overnight.
Forecast Precipitation amounts (QPF) showed a general swath of 1-2 inches per 6 hour (06:00, 12:00, 18:00, 00:00 UTC) also moving from south to north across the area anticipating heavy rains from Ernesto to move into southern and central New York and then drier air in its wake. During the afternoon Saturday the bulk of the low level moisture remained over eastern PA and NJ where higher dewpoints indicated good on-shore low level moisture flow at 21:00 UTC. This flow of moisture was also revealed by the banded structure of radar reflectivity evident on the KDIX WSR 88-D radar. Water vapor imagery showed a deep layer of moist air flowing across PA as a persistant and strong east to southeast flow to the north of the low pressure center fed Atlantic moisture into PA. An animation of water vapor, surface pressure, and winds from 01:00 UTC to 21:00 UTC (9:00 pm Fri to 5:00 pm Sat) shows this deep layer moist air being replaced by drier air as the low moved north into central PA. An animation of the the RUC model upper air analysis for a point near State College for the period 18:00 UTC Sat to 06:00 UTC Sun morning showed this deep layer moisture eroding as drier air moved in aloft leaving only a shallow layer of moist air near the surface. The change in precipitatble water from 07:00 UTC to 12:00 UTC also shows this.
The remnants of Ernesto continued to weaken as it moved north into colder air and the surface low continued to fill and weaken during the day. However, light rain continued for most of the day Saturday despite mid level drying. A more comprehensive study by Rich Grumm and John LaCorte, explains why this occurred.
Before the rain from Ernesto was over it produced a widespread 1-3 inch rainfall over Central PA, with a few locations picking up over 4 inches - see text summary and images below. This pushed many area creeks and streams to bankfull, but only pushed one river gage over Flood Stage (Penns Creek at Penns Creek, PA). The 48 hour storm total total precipitation using a combination of COOP and rain gauge data (courtesy of the MARFC) details the 2 day rainfall amounts (from 7 am Fri to 7 am Sun) across PA.