NWS Burlington Flood Safety Awareness: NWS Hydrologic Products


As part of Flood Safety Awareness Week, the
National Weather Service in Burlington, VT encourages everyone to be aware of the dangers
from flooding and take time during this week to review flood threats and flood safety practices.
In this presentation, we’ll review the hydrological and flooding related products that the National
Weather Service in Burlington, VT produce and issue.
  It’s important to be aware of these products
as flooding is one of the most deadly and damaging weather hazards to affects the US
every year. From 1995-2011, Flooding caused more fatalities than tornadoes. Of those fatalities,
the majority occurred while the person was driving into flooded areas. The northeast is vulnerable to flooding year
round from ice jam flooding in winter to flash flooding from thunderstorms in summer. It’s
important to know where flooding is likely to occur, what evacuation routes are available,
where to find the most up to date information and what it means. National flood fatality rates average about
94 fatalities per year. In Vermont alone, about one third of all weather related fatalities
are from flooding.  
Flooding also causes immense property damage, costing an average of 7.82 billion dollars
in the US.  
The National Weather Service offers several hydrological products from watches & warnings
to observations & outlooks. The NWS issues Flood Warnings for flooding
of smaller rivers and streams as a result of prolonged heavy rainfall and/or snowmelt.
Here is an excerpt of a Flood warning issued for an Ice Jam affecting the Malone, NY area.
There are 2 types of Flood Warnings: Areal and River
  Areal Flood warnings are issued when general
or areal flooding of streets, low-lying areas, urban storm drains, creeks, and small streams
is occurring, imminent, or highly likely. Flood warnings are issued for flooding that
occurs more than 6 hours after the excessive rainfall. These warnings are issued on a polygonal
basis and are generally in effect for 6 to 12 hours
  River Flood warnings are issued when the water
level at a river forecast point along a main stem river is expected to reach or exceed
flood stage.  
The warning will provide a brief description of what the flood stage means…such as roads
that will be covered or impassable due to flood waters.
  In the image here, you can see all the river
forecast points as the green and grey markers. Flash Flood Warnings are issued when rapid
rises in water levels, over a short period of time, will result in flooding. Below is
a excerpt of a Flash Flood warning issued for heavy rainfall near the Ticonderoga, NY
area.  
The causative event of Flash Flooding can be from very heavy rainfall over a period
of a few hours, dam breaks or ice jams. Within the NWS Burlington County warning area, most
Flash flood warnings occur in the warm season from heavy rainfall due to training thunderstorms.
  The top image shows the damage left behind
after a Flash flood occurred due to very heavy rainfall in July 2007 in the Barre-Stockbridge
area.  
The lower image shows an ice jam which has heaved large chunks of ice onto land, engulfing
and damaging trees. Other products to alert the public to flooding
potential are Flash flood watch, flood watch and flood or flash flood statement.
  A flood watch or flash flood watch is issued
to indicate current or developing hydrologic conditions that are favorable for flooding
or flash flooding in and close to the watch area, but the occurrence is neither certain
or imminent.  
At river forecast points, these watches indicated the possibility of flooding, typically within
a 6 to 48 hour time frame before the event, at specific forecast points along rivers and
streams. Here’s an example of a Flash Flood Warning
that was issued on May 29th, 2012. With some exceptions, most NWS severe weather/flood
warnings tend to follow a similar format. Warning Text will first contain information
on what type of warning is being issued and for which parts of counties are included in
the specific warning. What time the warning is in effect until will
follow. Next we have the Warning Basis portion. Generally
this contains the “what, when, from where and why” — in other words, information that
prompted the warning. You’ll also see some selected locations in
the warning, including cities, towns or other geographic references that may be familiar
or recognizable. Finally, you’ll see what are called “call
to action” statements — which are actions that should be taken to protect your life
and property. Along the main stem rivers such as the Lamoille,
Winooski, and Ausable rivers, there are river gauges which measure the water level. Based
on historical data, these points are designated different stages: Action Stage — this is the level which users
need to take some type of mitigation action in preparation for possible flooding
Minor Flood Stage — minimal or no property damage, but possibly some public threat…such
as field flooding Moderate flood stage — some inundation of
structures and roads near stream. Some evacuations may be required
Major Flood stage — extensive inundation of structures and roads. Significant evacuations
of people and or transfer of property to higher elevations. You can see current observations, river stages
and forecasts for points along our main stem rivers at our office’s Advanced Hydrologic
Prediction Service website, listed at the bottom of this slide. Simply point and click
on a colored icon to see more information about that particular river gauge. Every day the National Weather Service issues
the Daily River & Lake Summary, which contains the current/observed river stage, referenced
flood stage, and forecast river stages. To the right is an example, showing forecasts
for points along the Passumpsic, Winooski, Missisquoi, and other river or lake basins. For a further look into the future, the National
Weather Service issues a Winter/Spring Flood Potential Outlook. This product provides an
in depth look at long term forecasts for flooding…allowing for a long lead time. It gives users lead
time to consider response options, execute mitigation activities and plan reservoir operations.
Here’s an excerpt of one of our recent outlooks on the right. River forecast centers, a part of the National
Weather Service, are regionalized centers that specialize in hydrologic analysis and
forecasting and work with the Weather Forecasting Offices within its Hydrologic Service area
to accurately forecast precipitation amounts, runoff and changes in river and stream levels,
as well as the amount of precipitation it will take to cause flooding.
  Here in the North Country, we coordinate with
the Northeast River Forecast center located in Taunton, Massachusetts.  Here is a quick overview of the Northeast
River Forecast Center’s website. It has a plethora of information available. The URL
for their site is located on the bottom left of this slide. Some of this information follows:
The first tab contains information on Current river conditions and forecasts.  
On the second tab, you can view their quality controlled observed precipitation amounts. 
You can also see a 48-hour forecast of precipitation across the Northeastern states in the Forecast
Precipitation tab. And the Snow water equivalent tab includes
a map the observed amount of liquid in the snow on the ground. Thanks for watching this presentation of national
Weather Service Hydrologic products. For more information, visit us on the web at www.weather.gov/Burlington,
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