To follow up on my blog from yesterday, here's a bit of info on how to assess local river levels and then how meteorologists can conclude what the risk of flooding would be during heavy rain or snowmelt scenarios.
The US Geological Survey (USGS) is largely responsible for the maintenance and operation of various river gages across the country. These gages monitor the water level and streamflow for over 3,000 rivers in the contiguous US. This information can be found online (links below) and is very useful in monitoring the current river levels without being anywhere near them (for instance, in a weather forecast office or news studio). Monitoring the river levels becomes especially important to assess the risk of flooding potential, which is discussed further below.
First, click on any one of these links to follow along in the blog...
With once glance at any of these web pages, you will see color coded dots. This color coding is a good "one stop glance" at any rivers that are running high. The blue colors show rivers running much above normal streamflow (generally 90th percentile or higher). If heavy rain or lots of snowmelt (warm temps) is in the forecast, it would be these rivers that are the first concern for flooding.
So let's talk a little more about flooding. What types are flooding can happen on rivers? Really, there are 2 types that we are concerned about here in the North Country…
1) Flash flooding (basic overflow of rivers due to too much water)
2) Ice jam flooding (caused by large chunks of ice that block river water flow and force water out of banks)
Flash flooding on rivers is typically more "forecastable" than ice jam flooding..to an extent. The reason is because we often know when heavy rain events are coming, and also because there are people who keep track of this threat all the time. For our region, the Northeast River Forecast Center (a branch of NOAA and the National Weather Service) monitors the flash flood "threshold", if you will. This threshold can tabulate how much rain or moisture the ground can handle, before flooding becomes a problem. This assessment takes current soil moisture, and forecast precipitation into account. Taking these variables into account, as well as current river flow, a forecast can be made for the forecast river rise level. As a forecaster myself, the best tool to monitor this before a heavy rainfall event, the River Forecast Center/NWS provide a "flash flood guidance" product to help anticipate flooding on area rivers. The flash flood guidance(FFG) is an estimate of the amount of rainfall required over a given area during a given duration to cause small streams to flood. If you click on this link, you will see the local FFG by county for our region. Each county is color coded to indicate about how much precipitation can be tolerated before flash flooding becomes an issue. As of February 13, most of our region is shaded dark green, indicating that we can handle 2" or more of rainfall before flooding becomes an issue.
While at the Weather Service on Tuesday (again, refer to my blog from yesterday), Greg also showed me a neat program that they use internally at the NWS that allows them to input a rainfall amount forecast, and that will pump out an algorithm to conclude how high the water level would rise (again this is based on initial conditions of riverflow and soil moisture). The crew at our local NWS are able to use this for decision making processes to issue a flash flood watch in anticipation of the threat of flooding so people can become aware of the possibility of flooding.
As for ice jam flooding, this is, unfortunately, less predictable. Ice jam flooding becomes a problem when river ice melts or breaks up, and then clogs the river flow. We can know under what general circumstance this might happen (warm weather to cause ice to break up), but the actual location and timing of flooding is uncertain until an ice jam actually forms.
In tomorrow's blog, I'll share with you the latest spring hydrological outlook from our local National Weather Service office. Every other Thursday in late winter and spring, our local crew will asses the current status of our rivers, and take in to account any precipitation/temperature fluxes, and how much snowpack we currently have out there to forecast the threat for flooding as we progress into the Spring. I'll post the outlook Thursday! Be sure to check back in here with our weather blogs again for this, and many other topics of local interest.