Volume 56, Number 3 December 2012 Photo by Denice Drass

hurricane isaac
December 2012 ~ Mariners Weather Log
18
there were 3.5 ft. setups. Since the local
wind speed did not exceed 20 knots, the
over-water wind speed further offshore
must be higher.
Because of the storm track and the
geographic setting of Mississippi Sound
and surrounding waters, both Stations at
WYCM6 and SHBL1 experienced high
wind speeds and storm surges as shown
in Figs.6 through 9.
Fig.6 shows that there were
approximately 8 ft. positive surge and
10 ft. storm tides at Bay Waveland Yacht
Club, MS. At Shell Beach, LA, both
positive surge and storm tides were
11 ft. (Fig.8). During the period from
August 27 through 30 when Isaac was
over the Gulf of Mexico, there were no
set-down or negative surges measured
at WYCM6 and SHBL1. At WYCM6,
the wind speed and gust were over 45
and 55 knots, respectively (Fig.7), and
at SHBL1, they were 55 and 70 knots,
respectively (Fig.9). Note that these
positive surges can propagate into
nearby Lake Pontchartrain and Lake
Maurepas. They can also block or
prohibit the drainage of large runoff due
to heavy rainfall from many streams.
This phenomenon can cause widespread
coastal flooding.
Characteristics of storm surges, wind
speed and direction, wind gust, and
atmospheric pressure near the center of
Isaac are provided in Figs.10 through 12
for PSTL1 and Figs.13 through 15 for
GISL1, respectively. Notice that during
the passage of the center, wind direction
changed swiftly from northerly to
southerly. At the same time, wind
speed decreased and then increased
rapidly. Storm surge and storm tide
peaked approximately 3.8 ft. and 4.5 ft.,
respectively. Similar condition occurred
at Grand Isle, LA, where storm surge
and storm tide reached 4.2 and 5.7 ft.,
respectively. Note that the minimum
sea-level pressure was 970 millibar
(mb) at both locations. Note also that
at both locations, the sustained wind
speed and gust reached 60 and 80 knots,
respectively.
Fig.2. Storm surges at Naples, FL during isaac.
Fig.3.Wind direction, speed, and wind gust at Naples, FL during isaac.
Fig.4. Storm surges at Pensacola, FL during isaac.