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Wednesday, July 10, 2013
Rivers don’t flood around here in July.
But the Roanoke River did on Thursday, its 14.38-foot crest the 10th highest since 1900 and only the second time it has gone out of its banks in July, the last time in 1905.
The Roanoke and New River valleys experienced their rainiest first week of July on record, with more than 6 inches at Roanoke and nearly 5 inches at Blacksburg, just a year after the worst heat wave since 1930 — augmented by derecho-caused power outages — occupied the same time frame.
Rain totals for the year — 33.47 inches at Roanoke and 34.65 inches at Blacksburg — are running so far above normal, about a foot, that it would take totally dry weather into October to level the year off again with the 1981-2010 average that is considered “normal.”
It’s just plain soggy out there, at a time of year when the grass is often starting to turn brown and bare ground is turning into powder.
More rain, possibly heavy, is expected in the next couple of days as a cold front moves southeastward. Meanwhile, Tropical Storm Chantal lurks in the Atlantic, about a week from possible effects on the eastern U.S.
The bad news is that it is already this wet in July, with more than four months left in hurricane season and its potential to deliver drenching remnants of tropical systems into our steep terrain.
“If the pattern persists with heavy rain every few days and we get a tropical system on top of it, it does really set us up for something significant,” said Peter Corrigan, hydrologist at the National Weather Service in Blacksburg.
As Corrigan explains, most of the worst floods, locally and worldwide, occur because of extreme heavy rain that follows a prolonged wet period.
Total rainfall for 2013 is running 125 percent to 150 percent of normal throughout the 40-county forecast area of the Blacksburg weather service office, Corrigan said. The Blacksburg office covers much of Southwest Virginia and parts of southeast West Virginia and northwest North Carolina.
The “J” months — January, June and July — have been particularly soggy in Southwest Virginia. February has been the only month so far this year somewhat drier than normal.
“It primes you for a potentially big flood,” Corrigan said.
The good news is that, well, it’s July. The high sun angle, likelihood of some hot days , and fully leafed trees sucking water out of the ground and releasing it into the air can quickly dry the surface out.
“If we get a week, even, of fairly dry weather we can pull a lot of moisture out of the ground, and return to fairly normal conditions rather quickly,” Corrigan said.
The upper air pattern got stuck in an unusual manner over the United States on the week of its 237th birthday. Two large high pressure systems, one over the western U.S. and one over the Atlantic off the East Coast, trapped an upper-level low in the middle for several days.
The desert Southwest experienced record heat — and that’s saying a lot, with Death Valley tying the nation’s hottest June temperature of 129 and Las Vegas tying it s all-time hottest temperature of 117. Meanwhile, record cool temperatures dipped deep into the heart of Texas, with unprecedented fall-like 50s in July.
We got stuck with the firehose — a southerly flow of dense tropical moisture from the Gulf of Mexico, Caribbean and western Atlantic. Corrigan calls it an “infinite supply” of moisture.
Last week’s atmospheric pattern was the latest variation of what has been a recurring theme since mid-January. “Blocking” patterns, involving strong high pressure north or east of the continental U.S., have caused far southward jet stream dips that have resulted in unusual periods of cool and wet weather in parts of the central and southern U.S.
Locally, there was heavy rain, wet snow and flooding in mid- to late January and two rounds of accumulating snow in spring. Nationally, snow fell and accumulated as far south as Arkansas in early May, and cold weather kept tornado season almost entirely at bay until an extremely destructive burst in Oklahoma in late May.
Meanwhile, the West, the Northeast and parts of eastern Canada have had periods of unusually warm, dry weather.
Getting the weather pattern stuck in a way that keeps dry weather over us for several days and/or tropical systems away will be the key to reducing the potential for major flooding as we move through late summer and fall.
Just six months ago, the concern was worsening drought through much of Virginia. It’s not unreasonable to think the weather pattern might flip again … or, at least, turn the firehose a notch or two below full blast for a while.
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