Case Review: 15-17 September 1999 Heavy Rainfall from a Landfalling Hurricane

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Case Review 15-17 September 1999Heavy Rainfall
from a Landfalling Hurricane

• Summary of the Rainfall with Hurricane Floyd
• Excessive rainfall associated with Hurricane
  Floyd occurred on 15-17 September along the
  eastern seaboard from coastal South Carolina to
  New England.
• Some of the greatest 24-h amounts occurred in the
  eastern Carolinas on 15 September, and from the
  Chesapeake Bay northward to western New England
  on 16 September. Large areas received more than
  6 inches in 24 hours, with local amounts
  exceeding 12 inches in the eastern Carolinas, and
  over 9 inches in northern New Jersey and the
  lower Hudson Valley of New York.

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Case Review 15-17 September 1999Heavy Rainfall
from a Landfalling Hurricane

• Impact of Flooding From Hurricane Floyd
• More than 60 people lost their lives primarily
  from inland flooding, and mainly in the
  Carolinas.
• Historic flooding of main stem rivers occurred
  across much of eastern North Carolina. Notable
  flooding occurred northward into New England.
• However, the region had been in a severe drought,
  and many watersheds and stream channels in the
  Northeast were able to handle the excessive
  rainfall and runoff.

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Note rainfall accumulation gt15 southwest of
ILM Mosaic of radar-estimated rainfall via RFC in
Atlanta
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Basin-averaged rainfall 24-hour period
ending 1200 UTC on 16 Sep 99 via RFC in Atlanta
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Simulated S and Observed O Hydrograph at
EFDN7. Lower dashed line is flood stage and
upper dashed line is flood-of-record.
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Tar River near Tarboro
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HPC Day 1 Verification valid at 1200 UTC on 17
Sep 99
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Storm-total rainfall as estimated by the WSR-88D
on Long Island Note bright-band effect to the NW
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New Brunswick, NJ
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New Brunswick, NJ
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Case Review 15-17 September 1999Heavy Rainfall
from a Landfalling Hurricane

• Summary of Rainfall Caused by Hurricane Floyd
• Floyd was a good example of a hurricane
  interacting with a mid-latitude baroclinic system
  to produce excessive rainfall north and west of
  its track. The primary precipitation processes
  were
• Abundant maritime tropical moisture in the lower
  levels.
• Frontogenesis enhancing the low-level lift, at
  the level where rainfall production is
  concentrated in maritime tropical air masses.
• Significant isentropic lift of moisture laden air
  from the lower levels.
• Topography likely played some role in the
  localized heavy rain in the Northeast, but the
  meteorological processes would have produced
  widespread heavy rain anyway. Meteorological
  processes associated with frontogenesis were most
  important as seen in the next image.

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Frontal positions at various times 24-hour
rainfall accumulation at 1200 UTC on 17 September
(gt5 dashed red 7 dashed orange).
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ETA analysis at 1200 UTC on 16Sep99 925 mb Wind,
Omega, and Frontogenesis Image
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ETA 6-h Fcst valid at 1800 UTC on 16Sep99 925
mb Wind, Omega, and Frontogenesis Image
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ETA analysis at 1200 UTC on 16Sep99 300-K
surface Wind and Pressure along with a PW image.
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Case Review 15-17 September 1999Heavy Rainfall
from a Landfalling Hurricane

• Summary of Rainfall from Hurricane Floyd
  Rainfall
• The low-level production of precipitation was
  typical of maritime tropical air masses. Storm
  cells were generally low-centroid compared to
  typical thunderstorms in the mid-latitudes.
  Thus, NWS radars under sampled precipitation
  particles at distant ranges, and hence,
  underestimated rainfall at those locations.
• Further inland within a polar air mass, the
  freezing level was closer to the ground. It
  created a prominent bright band in the
  radar-derived precipitation (KENX).

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North-South Vertical Cross-Section. Shaded values
represent vertical velocity while line contours
are isotherms in C.
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WSR-88D at Albany NY Storm Total Accumulation
Note bright-band to the NW
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Case Review 15-17 September 1999Heavy Rainfall
from a Landfalling Hurricane

• Summary of the Disaster from Hurricane Floyd
• Despite the media attention to the dangers of
  storm surge and high wind, Floyd had a much more
  severe impact from the inland floods.
• Recent hurricane and tropical storm disasters
  have demonstrated the serious dangers from inland
  flooding, sometimes long after the threat of
  coastal damage is over. This is true even for
  major hurricanes like Floyd and others in recent
  memory
• Camille 1969
• Agnes 1972
• Alberto 1994
• Fran 1996
• Mitch 1998

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Case Review 15-16 September 1999Comparison of
Hurricanes Floyd and Fran

• Similarity
• Both involved enhanced rainfall production within
  a deep above-freezing cloud layer (warm-rain
  processes) and maritime tropical moisture,
  although Fran retained those characteristics much
  longer.
• Differences
• Floyd interacted with a strong baroclinic system
  and developed more cool season precipitation
  character with time.
• Orography (in the Virginia Appalachians) and
  eyewall convection (near Raleigh) determined much
  of the rainfall distribution with Fran. However,
  the rainfall distribution with Floyd was
  associated with strong warm advection overrunning
  a significant outbreak of polar air. Much of
  Floyds precipitation occurred on the cool side
  of the front, near the coast.

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Radar-estimated storm-total rainfall created by
Hurricane Fran from 6Sep96 (KAKQ). Notice how the
accumulation is concentrated along the eastern
slopes of the Appalachians.