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

1
Case Review 15-17 September 1999.Heavy Rainfall
from Landfalling Hurricane

• Summary of Hurricane Floyd Rainfall
• 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 greater than 6
  inches in 24 hours occurred, 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.
• Matt Kelsch UCAR/COMET

2
Case Review 15-17 September 1999.Heavy Rainfall
from Landfalling Hurricane

• Summary of Hurricane Floyd Flooding Impact
• More than 60 people lost their lives primarily
  from inland flooding, and mainly in the
  Carolinas.
• Historic flooding of main stem rivers occurred in
  much of eastern North Carolina, with some notable
  flooding all the way up 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.

3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
Tar River near Tarboro
9
HPC Day 1 Verification valid 1200 UTC 17 Sep 99
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
Case Review 15-17 September 1999.Heavy Rainfall
from Landfalling Hurricane

• Summary of Hurricane Floyd Rainfall Production
• Floyd was an 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
• Abundant maritime tropical moisture in the low
  levels,
• Frontogenesis enhancing the low-level lift, which
  is where rainfall production is concentrated in
  maritime tropical air masses,
• Significant isentropic lift of moisture laden air
  in the low 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.

14
Frontal positions at various times, 24-h
accumulation (gt5 dashed red, 7 dashed orange).
15
ETA 1200 UTC 16Sep99 analysis 925 mb Wind,
Omega, and Frontogenesis Image
16
ETA 6-h Fcst valid 1800 UTC 16Sep99 925 mb Wind,
Omega, and Frontogenesis Image
17
ETA 1200 UTC 16Sep99 analysis 300-K sfc Wind and
Pressure, with PW image.
18
Case Review 15-17 September 1999.Heavy Rainfall
from Landfalling Hurricane

• Summary of Hurricane Floyd Rainfall, cont.
• The low-level precipitation production was
  typical of maritime tropical air masses. Storm
  cells were generally low-centroid compared to
  typical thunderstorms in the mid-latitudes.
  Thus, the radars were under sampling the
  precipitation particles at distant ranges,
  resulting in underestimation of rainfall in those
  areas.
• Further inland in the polar air mass, the
  freezing level was lower causing a prominent
  bright band in the radar-derived precipitation
  (KENX).

19
(No Transcript)
20
Albany NY WSR-88D Strom Total Accum.
21
Case Review 15-17 September 1999.Heavy Rainfall
from Landfalling Hurricane

• Summary of Hurricane Floyd disaster
• 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 of inland
  flooding, sometimes long after the threat of
  coastal damage is over. This is true even for
  major hurricanes like Floyd.
• Camille 1969
• Agnes 1972
• Alberto 1994
• Fran 1996
• Mitch 1998

22
Case Review 15-16 September 1999.Comparison of
Hurricane Floyd and Fran

• Similarity
• Both involved enhanced rainfall production with a
  deep above-freezing cloud layer (warm-rain
  process) and maritime tropical moisture, although
  Fran retained those characteristics much longer.
• Differences
• Floyd was interacting 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, while
  warm advection precipitation associated with a
  significant polar front determined much of
  Floyds rainfall distribution. Much of Floyds
  precipitation occurred just on the cool side of
  the front, near the coast.

23
(No Transcript)
24
Storm Total From Hurricane Fran 6sep96 (KAKQ
radar). Notice how accumulation are concentrated
toward the west along the upslope of the
Appalachians.