apart, while in rural areas, they can be 50 or more miles apart.
Radar data can fill in the gaps between rain gauges by providing
rainfall estimates anywhere within the radar’s coverage area. To
determine whether the radar estimates are accurate for a given
event, compare in-situ measurements to the radar estimates at
rain gauge locations.
Rain gauges and weather radar can indicate how much it
rained but not whether that rainfall actually caused flooding.
Storm reports and damage surveys provide this additional information. The National Weather Service collects storm reports
for all tropical cyclones, and the U.S. Geological Survey conducts
damage surveys for most major flooding events. For prolonged,
widespread flooding events like Hurricane Harvey, satellite data
can also reveal the impacted areas.
Along the coast, storm surge is the greatest threat to life and
property. As with hurricane winds, the most severe storm surge
typically accompanies the right-front quadrant of the hurricane,
where the wind field and the storm’s forward motion act to push
water onshore. Unlike hurricane winds which tend to be similar
in nearby areas, storm surge can vary significantly even within
the span of a few miles.
The reason for this variability is that storm surge is a complex
phenomenon dependent on characteristics of both the hurricane and the local landscape. Among the local variables affecting
storm surge are the width and slope of the continental shelf as
well as the unique geometry of the coast.
Among the storm-related variables that impact storm surge are
hurricane intensity, size and forward speed, as well as its angle of approach to the coastline. The slightest change to any one of these variables can significantly increase or decrease storm surge potential.
In addition to storm surge – formally defined as the abnormal rise of water generated by a storm – one must also consider
the local astronomical tide. Storm surge rides atop the normal
tidal flow, creating a combined “storm tide” that ultimately determines the depth and extent of inundation. A storm that strikes at
high tide can result in inundation several feet deeper than if that
same storm strikes at low tide.
A final factor that influences damage along the immediate coast
is wave action, which can significantly increase the impact of
storm tide. Often, the height of waves riding atop the storm tide
can exceed the height of the storm tide itself. As a recent example,
during Hurricane Irma wave wash marks in the lower Florida Keys
were observed more than 10 feet above the storm tide.
Ahead of a hurricane, the United States Geological Survey
(USGS) typically deploys a network of temporary storm-tide
sensors along the immediate coast in the projected path of the
storm. These sensors record the depth of the storm tide through-
out the event, and comparison of nearby sensors provides insight
into the range of water levels experienced along a particular
stretch of coastline.
In those areas not covered by the storm tide sensor network, the
USGS often performs surveys of visible high-water marks in the im-
mediate aftermath of the storm. High-water marks are created when
small, light debris carried atop the water is deposited on vertical
surfaces like walls and doorways; as with storm tide sensors, they
provide data about the maximum water height at a given location.
The National Weather Service also performs post-storm dam-
age surveys that include findings regarding storm surge, maxi-
mum inundation, and wave height.
Each of these factors plays a key role in determining the full
impact of a storm on an area, the source of the damage and which
aspects may be covered by insurance. While the damage may be
obvious, the actual cause may take a little more investigation.
Megan D. Walker-Radtke, CCM (meganwr@
blueskiesmeteorology.com) is the chief meteorologist with Blue
Skies Meteorological Services in Gainesville, Fla.
TORNADOES REPORTED DURING
The locations of the 23 tornadoes (red triangles) reported
in association with Hurricane Irma. Irma’s track through
Florida is also marked, with cooler colors representing
weaker wind speeds as the storm moved over land. All
tornadoes occurred within the right-front quadrant of the
storm, and all but 4 occurred in coastal locations.