Heavy rain recap: Saturday soaker leads to flash flooding
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Why did it rain so much Saturday in Central Jersey, and what does the wet weather outlook look like for Sunday?
Wow, what a storm. Especially in the Princeton-West Windsor-Plainsboro area, where up to 6 inches of rain fell in a short period of time.
I could come up with a bunch of fancy meteorological jargon to try to describe why this localized heavy rain event happened. The reality is that Mercer/Middlesex counties were just extremely unlucky.
Thanks to our incredibly humid atmosphere, we knew as early as Thursday that this weekend's rains had the potential to be torrential. Round one of rain on Friday saturated the ground. So with a very wet antecedent soil moisture condition and continuous bands of heavy rain training over the area Saturday, it's no surprise that flash flooding inundated streets and that rivers, streams, creeks, and canals overran their banks.
Such heavy rain spawned a "Flash Flood Emergency" by the National Weather Service. This extreme version of a Flash Flood Warning is a rarely used, strongly worded message cautioning of "an extremely dangerous and life threatening situation".
The highest reported rainfall total I've seen was an incredible 5.75" at Plainsboro, reported to the National Weather Service through social media. (Given radar estimated rainfall, that is a reasonable report.) According to extreme precipitation statistics published in NOAA Atlas 14, this rainfall amount exceeds the 10-year 24-hour storm for that location (5.03"). I've seen reports that the vast majority of this rain fell within 3 hours - if that's true, this storm would have exceeded the 200-year storm for such a short duration.
Sunday's rainfall is expected to be more scattered, as hit-or-miss, on-and-off showers and thunderstorms remain possible through Sunday night. Overall, a much better situation. However, while the rain will not be as steady and unrelenting, the ground is still soaked and waterways are still running high. So it won't take much new rain to have flooding issues once again.
Was this a drought-busting rain? Sort of. I am sure we'll see some positive changes to the U.S. Drought Monitor map next week thanks to the abundant rainfall. While a good portion of New Jersey's drought-stricken area saw over an inch of rain, we're still running a sizable rainfall deficit for the year.
As I often remind you, flooding is one of the most dangerous effects of weather around the world. It can happen quickly and without warning. Never attempt to drive, walk, or swim through flooded areas. Water is an incredibly powerful force and only a few inches can knock you off your feet or sweep your car away.
Dan Zarrow is the Chief Meteorologist for Townsquare Media New Jersey. Follow him on Facebook or Twitter for the latest forecast and realtime weather updates.