WRAL WeatherCenter Blog

Inundated?

Posted June 23, 2008

That may seem an unlikely title as we find ourselves creeping back into moderate to severe drought designations around central North Carolina, but it comes up in relation to some viewer questions in the wake of all the flooding in the news across the Midwest and along the Mississippi River. The big question is, if we were to get swamped with too much rain over and over as the folks out there have, what levels of rising water would impact which locations?

That is a question that the NC Division of Emergency Management, the US Geological Survey and the National Weather Service have been working on for several years, and there are some useful results available to the rest of us on the web, with more to come in the future. One basic site you might find useful in times when river flooding is a threat is the Advanced Hydrologic Prediction Service, or AHPS. The address for our area is

http://newweb.erh.noaa.gov/ahps2/index.php?wfo=rah

Here you can click on a gage site for many communities across our viewing area, and for each site find a graph of observed water level, along with a map of the vicinity and a text listing of known flooding impacts of water reaching various levels at that site. This allows you to see an observed height, or a forecast crest level, for example, and get a sense of what flood conditions may result.

For some selected sites, an even more sophisticated combination of geographic information systems and hydrologic data has been created, in which you can see a map or an aerial photo of the vicinity of the community, with a range of water levels along the left side of the screen. Mousing over those levels will overlay an expanding or contracting area of water on the image, to show graphically the locations affected. There is also an option to mouse over the water for an estimate of depth at any location, and you can also check on or off boundaries for the "100-year" and (where available) "500-year" floods. I've attached screen shots showing a plain map of Louisburg with the Tar River overlaid at a "major flood" level of  24.2 feet and one of the Neuse at Smithfield using the aerial photo ("detailed map") option. On here, you'll see the boundaries of the 100-year flood in bluish-green, while the water boundaries for a "minor flood" river stage of 16.7 feet are shown in dark to light blue. To see these kinds of images for selected locations in eastern NC, choose from the list at

http://www.weather.gov/ahps/inundation.php

A few more locations with somewhat similar maps are available through a special USGS project site for the Tar River basin. For those maps go to

http://nc.water.usgs.gov/finmap/

These all relate principally to large-scale river flooding, something we haven't dealt with much given the continuing drought. Over the past few days, there have been some scattered, slow moving thunderstorms with localized very high rainfall rates. This is more prone to generate flash flooding, which is confined to smaller areas where urban drainage systems, small streams and creeks are unable to keep up with the deluge. When scattered storms of this type recur in varying locations over a series of days, it can help with drought conditions as it gradually evens out some of the "feast and famine" nature of convective rainfall, but with summertime heat ramping back up this week, along with less shower activity and lower humidity for Tuesday through the end of the week, we may end up just holding our own in regards to the drought this week.

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