There have been several interesting extreme "point rainfalls" this week in the Eastern U.S. A "point rainfall" is generally rain measured at a point on the earth over a certain period of time, as opposed to a radar estimate, which can provide a larger area of rainfall amounts that may not necessarily reflect what happened on the ground (radar beams are always slightly higher than the ground and get higher as you move out from their location).

1. From the Southeast Regional Climate Center SERCC on Twitter: "In a three hours from 1254 to 1554 on 8/12 At Cherry Point MCAS, NC a total of 8.63 in fell.. This would equate to close to a 500 yr event. Cherry Point MCAS in NC. Recorded 9.70 in. on 8/12/2009 That is the Highest one day rain on record. Previous record was 9.10 in on 9/20/1946"

Here's a map of the event (click to zoom in and see cities). In this case, the radar underestimated what the gauge showed (Cherry Point is shown by the plane icon - though by its estimates, amounts could have been even higher to the east).

This is pretty impressive - 8.6 inches in 3 hours. I assumed the previous daily record, the 1946 reading, was from a tropical storm, given the time of year, but our Interactive Hurricane Tracker showed this to not be the case. What they mean by "500-Year Event" is that the odds for this much of rain falling in one day at Cherry Point is once every 500 years.

2. Blog reader Dave left this comment on yesterday's blog entry: "2.75 in. in the gauge last night, near Cherryville in northwestern Northampton Co. Would be interested in seeing a precip map? The Lehigh River is swollen and brown..."

I responded to say that Dave actually got away lucky - although the radar estimate seemed spot on for him, it insinuated over 6 inches to his southeast! Dave responded back to say "Looks like Ballietsville was the bulls eye. (6.35 mi from our home) A very rare storm indeed." The LeHigh River at WalnutPort, PA reported over 4 inches of rain in less than 6 hours see below) and rose several feet afterwards.

3. We here in State College, Pennsylvania, home of AccuWeather HQ [Google Map] witnessed a heck of a thunderstorm Wednesday afternoon too, dropping 2-3 inches of rain in about a half hour. This caused flash flooding in typical areas in town (see all flooding pics from this storm, from all photographers):

COLLEGE AVENUE

RADIO PARK

Those last two events were due to being under an upper-level trough (which spawns storms underneath it due to instability, similar to an upper-level low which I have talked about before), and the low wasn't moving (upper-air movement generally moves storms) so the rain just sat in one place for a long time, which explains the Comment by blog reader Steve (also in yesterday's entry):

"These thunderstorms yesterday were weird. I work next door to Accuweather's headquarters and left work at 6:00, and the rain was coming down so hard that I had to drive 15 mph. But once I reached Circleville Road and started heading up the hill, the rain stopped, and just half a mile north the ground was dry, as if it had never rained the whole day. I looked in the rear-view mirror and couldn't see the bottom of the hill, and up ahead it was just a cloudy day. The same thing happened an hour later - I drove to the campus to pick something up, and then drove back, and halfway up Atherton St. the rain stopped. I was wondering why the whole time cops were screaming south - I don't think I've ever seen anything like it."

Some of the meteorologists here at AccuWeather had also commented on how unusual it was to see one part of the town inundated with water and other parts dry. Here's what the radar estimate looked like:

From some of our meteorologists' rain gauges, the radar estimate looked exactly right, but amounts varied greatly across small areas.

So how does this compare to all-time U.S. rainfall records? Well, 8.6" in 3 hours sounds pretty excessive but actually the record is 28.5 inches at Smethport, PA in 1942, according to Chris Burt [JessePedia], Author of "Extreme Weather" . His book also contains a U.S. map entitled "Estimate of Possible Maximum 24-Hour Rainfall Amount" which is crafted from an Army Corps of Engineers paper called "Storm Rainfall in the United States"). It shows that anywhere on the upper Texas an Louisiana coast could see as much as 38 inches of rain in a day (the all-time "point" record is 43" at Alvin, TX).

Posted by Jesse Ferrell on Friday, August 14, 2009 9:49 AM
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