Tip:
Highlight text to annotate it
X
Flood Awareness Week continues with a short discussion on warm season flooding resulting
from tropical rainfall.
Did you know that during the passage of a tropical cyclone or its remnants, both flash
flooding and river flooding are likely. In fact, the remnants of a tropical cyclone typically
affect our region every two to three years on average. As shown on the chart, most fatalities
that occur during tropical cyclones are attributable to inland freshwater flooding, not storm surge
or strong winds. This may be surprising, but reinforces the fact that flooding is a serious
threat to life and property. In fact, the top three weather related disasters in Vermont
history were a result of flooding from a tropical cyclone or its remnants.
There are several features which govern the distribution of heavy rainfall and potential
flooding during the passage of a tropical system. These are, but are not limited to:
The speed of the system, the interaction with mountainous terrain (if present), interaction
with other features such as a stalled surface front, and lastly, antecedent rainfall and
soil moisture conditions. Pictured here is a map of total rainfall that occurred during
Tropical Storm Irene. While most areas received very heavy rainfall, the map clearly shows
that the heaviest totals occurred in the higher terrain of the Green and Adirondack Mountains.
This is quite common as warm moist air is driven upward by the mountains where locally
heavy precipitation then occurs. However, antecedent rainfall and soil moisture conditions
are perhaps the most important factors that influence the potential for flooding during
a tropical cyclone, or any other heavy rainfall event for that matter. Wet or saturated soil
cannot hold as much moisture as dry soil. Thus when heavy rain falls on saturated soils,
water will not be absorbed and rapidly run off into area streams, rivers, and in some
cases overland areas. This was the case during Tropical Storm Irene, and the Great Flood
of 1927. The map shown here represents soil moisture levels immediately after the passage
of Tropical Storm Irene. Note how the soils in the North Country are ranked as very wet.
Tropical Storm Irene lashed the region on August 27-28, 2011, and was among the costliest
weather disasters in North Country history. Irene was a large West Indian Hurricane, taking
a classic re-curving path from the tropical Atlantic through the Bahamas and northward
before slowly weakening to tropical storm strength across New England. What made Irene's
path unusual however was the fact that the re-curving process did not take the storm
out to sea, but due northward along the eastern seaboard and into western New England, including
Vermont. Widespread heavy rainfall on the order of 3 to 7 inches occurred on top of
antecedent wet ground conditions, leading to devastating flooding in eastern and southern
Vermont, and in portions of the Adirondack Mountains of eastern New York.
Irene caused extreme and in some cases record flooding. As a result, a Federal Disaster
was declared for most of the state on September 1, 2011...only 3 days after the event. Total
dollar damages to North Country infrastructure was estimated in the $1 to $2 billion dollar
range. Shown here is the hydrograph trace for the East Branch of the Ausable River at
Ausable Forks, NY showing the record crest in excess of 18 feet observed during Irene.
Devastating, life threatening flooding occurred throughout the Ausable river valley on the
evening of August the 28th into the early morning hours of the 29th.
Hurricane Floyd took a path similar to Irene, tracking along the eastern seaboard before
weakening to a tropical storm as it passed through New England on September 17, 1999.
A Federal Disaster was declared for 10 Vermont counties as a result of wind and flood damage.
The magnitude of Floyd's rainfall was similar and in some cases greater than that of Irene,
but flooding was less due to antecedent dry weather. This emphasizes the importance of
soil moisture conditions prior to and during such events.
The Great New England Hurricane of September 21, 1938, nicknamed the Long Island Express
due to its fast movement, caused massive destruction to much of New England, including Vermont.
Similar to Floyd and Irene the 1938 storm was a large West Indian Hurricane with origins
in the tropical Atlantic. It is still regarded to this day as the most destructive Hurricane
to strike New England in the past 100 years, and the only storm to maintain hurricane strength
in Vermont since 1788. While flooding was noteworthy, particularly across southern Vermont,
destructive winds caused the most damage leveling large tracts of forest across the eastern
portions of the state as shown in this US Forest Service map.
The Great Flood of November 3-4, 1927 is still to this day regarded as the greatest natural
disaster in Vermont history. It resulted from the remnants of a tropical storm interacting
with a stalled surface front across the area. Antecedent conditions during October were
extremely wet, setting the stage for devastating flooding when 5 to 9 inches of tropical rainfall
pummeled the state on November 3-4. With runoff exacerbated by the near saturated ground conditions,
and without the beneficial help of vegetation taking up a portion of the water, the stage
was set for devastating flooding. Countless homes and roads were destroyed, along with
1285 bridges. Unfortunately 84 Vermonters lost their lives, mainly due to the extreme
flooding. This event, along with the Great Hurricane of 1938 helped heighten public awareness
to the threat of flooding in New England, and ultimately laid the groundwork for the
creation of many dams and other flood control measures now in existence across our region.
In review, some of the top weather disasters that have occurred in Vermont history were
due to tropical cyclones or their remnants. Surprisingly, inland flooding remains the
primary threat to lives and property across the nation and the North Country during such
events, eclipsing storm surge and wind by a large margin, though wind did play a substantial
role in the case of the 1938 Hurricane. Thus it is paramount to review all flood safety
rules and take appropriate actions as necessary the next time a tropical cyclone affects our
region.
Thank you for viewing this presentation on warm season flooding from tropical rainfall.
For more information on flood facts, flood awareness and preparation, visit us on the
web at www.weather.gov/Burlington, like us on Facebook, or follow us on Twitter.