Originally posted March 10, 2011 at phfieldnotes.blogspot.com.
The New York and Philadelphia metro regions are experiencing two significant and consecutive rain events in a week and people are again concerned about how to assess vulnerability to flooding and how impending flooding compares to previous floods. The Advanced Hydrologic Prediction Service is a tool used by local and state offices of emergency management and provides past, current and updated river projections.
You can access this too; the website is http://water.weather.gov/ahps/. Click on an area of the country and you will be moved to a Weather Forecast Office region – for the New York and Philadelphia region, you want the Philadelphia/Mt. Holly NJ region. This screen shows the current status of gages by color. Next click on one of the forecast points and the individual gage will appear. You will observe the current stage of the river in blue, the predicted stage through time in green and, scrolling down, a list of past major events and corresponding flood elevation to place the forecast into perspective. As a citizen, this tool provides information so that you can take appropriate action in moving valuables to higher ground and implement your flood plan. For more on a flood plan, see the website Focus on Floods: http://focusonfloods.org/.
Advanced Hydrologic Prediction Service forecasts are updated as data becomes available and not before modeling is completed every six hours. At the bottom of the chart, the forecast time is posted and, unless an unusual situation develops, look for the next update six hours thereafter. It is important to realize that especially with early predictions, the modeling is performed on estimated rainfall. As rainfall is recorded by Doppler radar and rain gauges, and stream gages register the change in the stream stage, the model is refined. Times of peak stage and peak elevation will change over time and it is important to monitor. Also, you should heed the notices from your local emergency management coordinator as he or she is monitoring additional data and may have instructions for your safety – communities will have ways to communicate instructions though email, website and, if needing higher attention, reverse 911.
Risk awareness is your first step to prepare for a flood, and importantly, guides what mitigation or adaptation you can do to reduce your susceptibility to flooding in the future. Most often, the urge to improve resiliency is strongest immediately after a flood. Take advantage of this time, for in the next flood, you will be more confident in weathering the storm.
John A. Miller, PE, CFM, CSM
Water Resources Engineer