April 13, 2004 Peninsular Florida
Severe Wind Event
National Weather Service, Weather
Forecast Office, Miami, Florida
On Tuesday, April 13, 2004, a severe wind event
swept across parts of peninsular Florida in the wake of a mesoscale convective
system (MCS) of thunderstorms. This is unusual because the
winds, which gusted at times to about 70 mph in the vicinity of Lake
Okeechobee, were not associated with the thunderstorms themselves, but
were on the back side of the system. In fact, little or no rain
was falling when the wind event was occurring. This is event is
consistent
with a phenomenon called a 'wake low' pressure system. Wake lows
are sometimes associated with MCSs as a result of subsidence warming
that is maximized on the back edge of the trailing stratiform
precipitation area (Johnson and Hamilton, 1988). Another
excellent reference on this subject is a recent paper by Gaffin (1999).
Strong wind gusts were observed in the Fort Myers area around 08Z (4 AM
EDT) with a gust to 50 knots reported at Regional Southwest Airport
(KRSW) at 0817Z (417 AM) and 43 knots at Page Field in Fort Myers at
0800Z (4 AM). The strong winds were on the back edge of the light
rain shield, well behind the thunderstorms of the MCS as it was moving
across South Florida. A wind gust to 41 knots was recorded at the
Naples Regional Airport (KAPF) at 0841Z (441 AM EDT) coinciding with rapidly falling pressures. Strong
wind
gusts, some reaching as high as 70 mph across Lake Okeechobee, moved
steadily from west to east across peninsular Florida, reaching the
communities of South Bay, Belle Glade, and Pahokee around 1000-1100Z
(6-7 AM EDT). Several trailers were severely damaged in South Bay,
rendering
them uninhabitable. In Belle Glade, a shop window was blown out.
Fig. 1 - Lake Okeechobee elevation in feet NGVD (courtesy Susan
Sylvester, U.S. Army Corps of Engineers, Jacksonville District)
The wind event caused the water level of Lake Okeechobee
to slope steeply from south to north (Fig.1 courtesy U.S. Army Corps of
Engineers, Jacksonville District) in a "bathtub" type sloshing
effect. The strong south winds over 2-3 hours before dawn on
Tuesday blew the water from the south end of the lake (S310 near
Clewiston) toward the north end of the lake (S193 near Okeechobee). At
maximum, the difference between the lake elevation at S310 near
Clewiston (just over 12 feet msl) and S193 near Okeechobee (around 17.6
feet msl) was about 5.6 feet
near 5 AM EDT. You can easily see the sloshing that occurred at
S193 with five
separate peaks around midnight, 4 AM, 7 AM, 11 AM, and 3 PM EDT.
What caused this wind event?
Fig. 2 - Combined meteorological
parameter trace at SFWMD data platform LZ40 on Lake Okeechobee for
April 12-13, 2004. (Courtesy SFWMD Meteorologist Eric Swartz) (times
are EST, add 1 hour for EDT)
This 'wake low' wind event is more typical of mesoscale convective
systems (MCS) that occur across the Great Plains or the Southern
states. Conservation of mass requires that where air goes up,
someplace it must come down. Thunderstorms always mean air moving
upwards, sometimes at great speeds and with turbulent results. On
the back side of a line of thunderstorms, air is moving downward,
usually much more slowly than the rising air in the
thunderstorms. Sometimes, and probably in this case, downward
moving air is accelerated
by evaporative cooling of rainfall. Another
characteristic of downward moving air is adiabatic warming,
which can be seen in the temperature trace from South Florida Water
Management District (SFWMD) data collection platform LZ40 on Lake
Okeechobee just as the strong winds were beginning before 6 AM EDT
(Fig. 2). All of this dynamic movement of air creates a mesoscale
low pressure system called a 'wake low' and often results in very
strong winds.
KAMX Miami WSR-88D Radar Reflectivity Image as
strong winds occurred at Naples (0843Z or 443 AM EDT)
KAMX Miami WSR-88D Radar Reflectivity Image
as strong winds occurred at Lake Okeechobee (1015Z or 615 AM EDT)
The event is not
associated with the thunderstorms themselves, so therefore it cannot be called
a derecho,
even though it meets some of the criteria for duration and geographic
extent. Wake lows have resulted in heat bursts in some cases in
the midwestern states due to the adiabatic warming. In some
cases, wake lows combined with strong inversions (temperatures rising
with height) have resulted in gravity waves that have caused
complicated severe wind events.
Some good papers on this subject include (visit the web site of the American Meteorological Society for
online access):
Gaffin, D. M., 1999: Wake Low Severe Wind Events in the Mississippi
River Valley: A Case Study of Two Contrasting Events. Wea.
Fcstg., 14, 581-603.
Johnson, R. H., and P. J.
Hamilton, 1988: The relationship of surface pressure features to the
precipitation and airflow structure of an intense midlatitude squall
line. Mon. Wea. Rev., 116, 1444–1472.
Stumpf, G. J., R. H. Johnson,
and B. F. Smull, 1991: The wake low in a midlatitude mesoscale
convective system having complex convective organization. Mon. Wea.
Rev., 119, 134–158.
Pfost, WFO Miami, FL
