Flood watch: Here's why

A flash flood watch now is in effect until 4 p.m. for Philadelphia and surrounding counties on both sides of the river.

Should it hold together a substantial slug of rain should cross the region around lunchtime and continue until mid-afternoon. Here is the latest radar image from Intellicast.com.

What happened yesterday showed how quickly creeks can respond to rainfall. The flow of Pennypack Creek, the site of a dramatic rescue and a drowning, became dangerous in a hurry.

It was running at 52 cubic feet per second at 8:30, and it jumped to 2,560 cubic feet per second by 11:15 a.m.

This morning, it will be starting at a higher base. It was at 218 cubic feet per second as of 11:15.

The National Weather Service, which issued the flood watch, says up to 2 inches could fall in some places in the next round of rain.