South Florida Seabreeze/Outflow Boundary Tornadoes
Russell L. Pfost, Pablo
Santos, Jr., and Thomas E. Warner
National Weather Service/Weather
Forecast Office
Miami, Florida
Wednesday, August 3 2005
revised form
Friday, September 9 2005
ABSTRACT
In the late afternoon and evening of 7
August 2003 two tornadoes produced significant damage across parts of
metropolitan Palm Beach County, Florida. These tornadoes were
produced as a strengthening updraft encountered cyclonic shear
along an enhanced east/west sea breeze
convergence line meeting a southward moving outflow boundary from the
north. The
second tornado in particular produced substantial damage to a trailer
park and industrial areas in both Palm Beach Gardens and Riviera Beach
and crossed a major interstate highway (Interstate 95). Detection
and warning of the tornadoes was a challenge for National Weather
Service (NWS) forecasters at the Weather Forecast Office (WFO) in Miami
due to the distance from both the Miami (KAMX) Weather Service
Doppler radar (WSR-88D) and the Melbourne (KMLB) WSR-88D resulting in
beam elevation and sampling issues.
1. Introduction
Florida ranks as the
top state in number of tornadoes per 10,000 square miles from 1953 to
2003 (DOC, 2003). South Florida's
peak months for tornadoes are June and August (Gregoria,
2005), reflecting both the convective "rainy" season from late May
through mid October as well as a tropical cyclone influence which peaks
around mid August through the end of September each year. Because
convection in South Florida during rainy season is almost always
related in some way to sea and/or lake breeze convergence, the
development of tornadoes is also greatly affected by the boundaries
created as the sea and lake breezes begin their diurnal migration.
While the scope of this study is not to address
tornadogenesis theory, the paper by Collins et al.,
2000 is an
excellent source relating to South Florida tornado events. The data
shows
non-supercell tornadoes can develop when pre-existing near-surface
vertical vorticity is stretched, or when local near-surface horizontal
streamwise vorticity is tilted and stretched, provided certain
conditions are met. Sources of
vorticity can be the sea breeze convergence boundary or a thunderstorm
outflow boundary, both common events in South Florida during the
convective season (Collins et al., 2000).
On 7 August 2003 beginning about 2110 UTC two
tornadoes produced significant damage across parts of metropolitan Palm
Beach County, Florida. The first tornado caused scattered F0
(Fujita, 1981) damage in southeast Jupiter along
U.S.
Highway 1 and in The Falls
subdivision. The path length of this tornado was only
about 1.5 miles and only about 70 yards wide (Fig.
1). Most of the
damage was
limited to trees and shrubbery (Fig.
2) although some screens and metal roofing of a shopping center
along U.S. 1 (Fig. 3) were
damaged.
The second tornado produced damage of F1 (nearly F2)
magnitude in Palm Beach Gardens and Riviera Beach (PBG-RB) beginning
around 2125 UTC and lasting for about 15
minutes over a path of 4 miles (Fig. 4).
An
exceptional
picture of the second tornado (Fig.
5) was taken by Jeff Hooper of
the South Florida Sun-Sentinel.
Fortunately, no one was
killed, and only 28 minor injuries were reported along with 58 (mostly
mobile) homes destroyed, 21 homes sustained major damage, and 150 homes
had minor damage (Table 1). In addition, several
cars and a few tractor-trailers
were moved or tipped over, light poles were snapped, and many roofs in
the tornado's path were damaged or torn off. A tornado causing F1
damage produces winds of approximately 73 to 112 mph (Fujita,
1981). Pictures and first-hand surveys by
National
Weather Service (NWS) meteorologists suggest the worst damage was
likely
caused by winds in the higher part of that range, i.e., more than 100
mph.
Table 1 - PBG-RB
Tornado Damages
| Deaths |
0 |
| Injuries |
28 |
| Homes destroyed |
58 |
| Homes with major damage |
21 |
| Homes with minor damage |
150 |
The PBG-RB tornado first began causing damage
near Northlake Boulevard and Military Trail and moved southeast to 'A
Garden Walk'
mobile home park
where the worst damage occurred (Fig. 6).
The tornado
crossed I-95 just north of Blue Heron Boulevard. As it did so, it
turned over a truck on I-95 and then moved in a more easterly direction
into Riviera
Beach just north of Blue Heron Boulevard. Along the way it severely
damaged the roof of a Pepsi-Cola plant (Fig.
7).
It continued
through
neighborhoods in Riviera Beach just north of Blue Heron Boulevard but
the damage swath was narrower and less severe until it lifted or
dissipated
near the intersections of Blue Heron Boulevard and Old Dixie Highway.
The tornadoes that struck Palm Beach County were
unusual in that they occurred in August with low environmental shear
but large instability. They were also associated with an atypical
weather pattern in South Florida in which
thunderstorms move into the metro Atlantic coast area in the afternoons
(a more normal pattern with prevailing easterly flow is for showers and
thunderstorms to occur along the metro Atlantic coast in the mornings
and very early afternoon before moving across the Everglades areas
toward the Gulf of Mexico coast in the afternoon and evening).
The tornadoes occurred with thunderstorms building toward the south
along a sea-breeze convergence
line (locally known as the "zipper effect" because the development of
thunderstorms toward the south along the sea breeze convergence line
has the appearance of a zipper on satellite imagery time lapse loops)
reinforced by
thunderstorm
outflow boundaries
from earlier thunderstorms to the southwest and west of the
metropolitan area. The tornadoes also occurred in a very low
shear environment typical of South Florida in summer.
The tornadoes occurred at significant distances from
both
the Miami (KAMX) WSR-88D and the Melbourne (KMLB) WSR-88D. Even
at such significant
distances from data acquisition (RDA) sites at KAMX and KMLB,
analysis in real time of rotational velocities, reflectivities, and
vertically integrated liquid (VIL) from both sites provided key
information to the fact that tornadoes were forming. In addition,
realtime mesoscale analysis of surface data provided key information
that local convergence was further enhanced by the interaction of the
seabreeze with outflow boundaries of earlier
thunderstorms north and west. The boundaries provided sufficient
cyclonic
shear which could then be stretched
by strong updrafts associated with the southward propagating "zipper"
convection.
2. Overview
a. Synoptic conditions
On 7 August 2003 a
positively tilted long wave trough extended over the eastern half of
North
America, roughly from Quebec, Canada south southwest to Alabama, with a
series
of short waves rotating through the base of the trough (Fig. 8A).
The
subtropical ridge, a semi-permanent synoptic feature for South Florida
in summer and early fall, extended across the central Bahamas and South
Florida into the southeast Gulf of Mexico (Fig.
8B). The prevailing flow
across South Florida was south to southwesterly as the axis of the
surface ridge was across the extreme south tip of the peninsula
including the
Florida Keys. Upper flow across South Florida was light,
generally less than 10 knots.
Moisture was plentiful with precipitable water
around 2.3 inches on the Cape
Canaveral
10 UTC sounding (Fig. 9) and 1.9 inches
on the Miami
12 UTC sounding (Fig. 10). Average
precipitable water for
August in Miami is near 1.9 inches (Gregoria, 2005).
Deep layer instability was
present on both soundings, with afternoon temperatures
forecast to rise well into the 90s, maximum surface based convective
available potential energy (SBCAPE) (Fig. 10) was
as high as 5000
J/kg. With such light winds, calculated helicity values were very
small for both upper air locations, and local circulations including
thunderstorm outflow boundaries and sea breezes were expected.
Convective inhibition was non-existent and lapse rates were
close to adiabatic through the boundary layer.
.
b. Mesoscale conditions
Palm Beach County is uniquely situated in southeast
Florida between Lake Okeechobee to the west and the Atlantic Ocean to
the east (Fig. 11). This area is favorable
for
thunderstorm development on days with weak environmental flow as the
lake breeze from Lake Okeechobee and the sea breeze from the
Atlantic generate significant low level/surface convergence. In
August, the temperature of Lake
Okeechobee is normally around the mid 80s to 90 F while the temperature
of the Atlantic Ocean is usually in the lower 80s. On 7 August
2003 air temperatures over land reached 96 F at Belle Glade, a town
near the southeast coast of Lake Okeechobee, with mid 90s common over
interior South Florida away from the coasts.
By 15 UTC on 7 August thunderstorms had developed
along the Gulf coast of southwest Florida and had produced an outflow
boundary extending north-northwest to south-southeast roughly from
Arcadia to Immokalee to Everglades City (Fig. 12).
By
17 UTC, the outflow
boundary was halfway across the peninsula, bisecting Lake Okeechobee
and extending south to Flamingo on the north coast of Florida
Bay (Fig. 13).
By 18 UTC, the primary area of severe activity was from Lake Okeechobee
north to Cape
Canaveral, and the outflow boundary had propagated as far east as Fort
Pierce to Belle Glade to western Broward county to Flamingo.
At 1930 UTC the temperature reached 94 F at the
University of Florida's Florida Agriculture Weather Network (FAWN) site
at Belle Glade with a dewpoint reported at 80 F (Fig.
14).
On satellite and
radar, thunderstorms were developing on the old outflow boundary in the
vicinity of Belle Glade while a sea breeze convergence line was evident
on visible satellite imagery extending from Jupiter to West Palm Beach
to just west of Boca Raton in Palm Beach County (Fig.
15).
By 20 UTC
the old outflow boundary had moved well off the east central Florida
Atlantic coast, but the south end of the old outflow boundary
interacted with the sea breeze convergence line to start new
thunderstorms over the Atlantic east of Stuart (time
lapse Fig.
16).
These
thunderstorms propagated southward along the sea breeze convergence
line, and by 21 UTC visible satellite imagery clearly showed the
'zipper' thunderstorm development effect (Fig. 17).
At the same time, the
thunderstorms over western Palm Beach County southeast of Belle Glade
had produced another outflow boundary which was propagating
east-northeast while the sea breeze convergence line was continuing to
advance westward (Fig.
18). This helped enhance the updraft along the seabreeze
convergence line.
Maximum observed temperature and dewpoints of the
three identifiable air masses in Palm Beach County at 21 UTC were as
follows:
| Air Mass |
Temperature (F) |
Dewpoint (F) |
| Marine layer air behind sea breeze convergence line (Lake
Worth C-MAN LKWF1) |
85 |
75 |
| Land area air before sea breeze and outflow boundaries (Belle
Glade FAWN) |
94 |
80 |
| Thunderstorm cooled air behind outflow boundary (Belle Glade
FAWN) |
83 |
79 |
c. Vorticity
Calculations
At 21 UTC, the Lake Worth (LKWF1) Coastal Marine
Automated Network (CMAN)
station was reporting sustained
south to southeast winds of 15 to 20 knots (8 to 10 m/s), which can be
considered representative of the sea breeze. At 22 UTC, the Palm
Beach International
Airport (PBI) Automated Surface Observing System (ASOS) station
reported northeast
winds of 10 knots (5 m/s), which was after the converging boundaries
had moved
through the site. Assuming a 40 to 45 degree angle between the sea
breeze wind
and the convergence boundaries and assuming an approximate boundary
width of 1
to 2 km (from the WSR-88D lowest level 8bit reflectivity data), the pre-existing local vertical vorticity
across the boundary can be estimated as follows (using winds in m/s):
(1)
In the presence of (1) a strong updraft, (2) a long residence
time of the
convection along the boundary that represents the source of the
vorticity, (3) no
convective inhibition (CIN), and (4) steep lapse rates from the surface
to the
level of free convection (LFC) (Fig. 9), this estimation of vertical
vorticity should
be sufficient for tornadogenesis (Weisman and Klemp, 1982, 1984).
Given the weak environmental shear, an additional
source of vorticity (horizontal in this case) could have been the
buoyancy
gradient across the boundaries (Weisman and Klemp, 1982, 1984). However, the convergent boundaries generated
by thunderstorm outflows and the sea breeze both have a cooling effect
along
their path. In fact, available observations do not support significant
thermodynamic differences across these boundaries.
To summarize, very low shear and
weak environmental flow combined with abundant low level moisture and
thermodynamic instability to produce severe convection in South
Florida
with numerous storm scale effects (including sea breezes, lake breezes,
and
thunderstorm
outflow boundaries). An outflow boundary from earlier midday storms across central
Florida
interacted with
a sea breeze convergence line to produce a 'zipper effect' of
thunderstorm
development offshore Martin
County and
northeast Palm Beach
County.
Thunderstorms that
developed along the lake breeze convergence line near Belle Glade
produced
another outflow boundary that propagated eastward, ultimately enhancing
the
convergence along the sea breeze convergence line in northeast Palm
Beach
County and promoting the 'zipper effect'. The combination of
these factors ultimately led to tornadogenesis by stretching
pre-existing vertical vorticity from colliding boundaries.
3. Analysis of Radar Data
a. Radar Characteristics and Beam Geometry
Palm Beach County is within the coverage areas of
both the KAMX and KMLB WSR-88Ds.
However, because of the distance (Palm Beach Gardens is about 75
statute miles from KAMX and about 83 statute miles from KMLB), storms
occurring over the northern half of Palm Beach County are sampled with
a KAMX radar beam width greater than 7000 feet and more than 6400 feet
above the surface. From KMLB, the radar beam width is more than
8700 feet and more than 8900 feet above the surface over northern Palm
Beach County. Thus,
warning forecasters at Miami can only observe broad rotational features
of thunderstorm associated mid-level mesocyclones over northern Palm
Beach County and must use spotter reports and surface
observations in real time to provide accurate and timely
warnings. Because of the beam elevation and width at such
distances, radar observation of the actual tornado itself or any other
low
level thunderstorm feature is impossible. Thus, this case is a
prime example of the problems faced by operational WFO warning
forecasters at long ranges from WSR-88D RDA sites.
b. Radar Product Analysis
However, radar velocity products (created using high
resolution Level II archive data and WSR-88D Algorithm Testing and
Display Software (WATADS)) clearly
showed a
mid-level mesocyclone and reflectivity products (including Vertically
Integrated Liquid or VIL) clearly showed a high probability of large
hail. Figures 19 and 20
show the 0.5 and 1.5 degree
elevation angle rotational velocities from KAMX and KMLB and times
associated with the tornadic thunderstorm as well as approximate times
of tornado touchdowns indicated by the tornado symbols. Note
especially the two relative peaks in midlevel rotational velocity
immediately before the first report of each tornado, and the subsequent
drop in rotational velocity thereafter. The implied
tornadogenesis threshold
value for this single case could be estimated around 25 knots.
The velocity data is impressive not only for the relatively high
rotational velocity values (Fig. 21A) but also
the depth of the
rotation.
A velocity cross section from KMLB at the time of the tornado (Fig. 21B, 2129
UTC) shows a circulation extending at least through 20,000 feet and
probably more (radar beam height prevents determination at low levels).
Thus the thunderstorm responsible for the tornado
displayed classic severe weather signatures of rotational velocity in
excess of 25 knots, a deep circulation indicating a mesocyclone
extending through at least 20,000 feet of the atmosphere, high VILs
indicative of severe size hail, and maximum reflectivities at heights
far exceeding the -20C level, indicative of severe size hail (0.75
inches/19 mm) and
possibly severe level wind gusts (58 MPH or greater).
The first tornado possibly began as a waterspout, coming onshore in
extreme northeast Palm Beach County but causing damage over land for
only a short time (Fig. 1).
The second, more damaging tornado path was initially south-southwest,
following the best convergence and resulting convection between the sea
breeze and the surging outflow boundaries from earlier convection north
and west of Palm Beach County (Fig.
4). Ultimately, the outflow boundary surge
from the west pushed the convergence line back to the east, and the
second tornado
path in
the last stages paralleled that push, turning eastward across I-95 and
into Riviera Beach.
The vertically integrated liquid (VIL) provided excellent indication of
severe size hail (greater than 3/4 inch in diameter). Fig. 22 is
a VIL product from the KMLB radar at 2119 UTC, clearly showing a
maximum grid
based VIL of 79 g/kg. With the freezing level near 15,000 feet
and the height of the -20C near 27,000 feet, Fig.
23 is a
reflectivity cross sections from 2109 UTC clearly showing the maximum
reflectivity well above levels necessary for severe size hail (greater
than 3/4 inch in diameter). Although not classifiable as a weak
echo region, the suspension of such high reflectivity values above a
gradient of reflectivity is an indication of the strength of the
updraft associated with this severe thunderstorm.
4. Summary
It is unusual to have a significant
non-tropical-cyclone-associated tornado in South Florida in
August. However, in special circumstances like those on 7 August
2003, the combination of outflow boundaries, sea breeze
convergence and high surface based instability can overcome the lack of
sufficient shear, producing significant tornadoes. The PBG-RB
tornado
was unusually strong (on the high end of F1) with a long damage
path. It occurred
at the edge of radar coverage by the KAMX and KMLB radars where the
center of the radar beam was 6400 and 8900 feet above the surface of
the earth respectively. Thus, it was impossible to observe any
low level features in making warning decisions for this tornado event.
WFO Miami meteorologists issued a Special Marine Warning at 2122 UTC
until 2315 UTC for the thunderstorms that spawned the first tornado
that tore through parts of Jupiter and Juno Beach. NWS Skywarn
spotters called in reports of a tornado as early as 2213 UTC 10
miles north of PGA Boulevard in Palm Beach County. The warning
meteorologist was unsure of the report because the location provided
was not near
any current thunderstorm activity (the spotter provided a location near
Interstate 95 rather than accurately estimating the position of the
tornado several miles
east of Interstate 95). WFO Miami issued a Tornado Warning at
2230 UTC. At 2235 UTC another Skywarn spotter reported tornado debris
visible in Palm Beach Gardens at Interstate 95. At 2237
UTC the tornado was reported at Gardens Road and Blue Heron Boulevard,
with damage to the Pepsi-cola plant roof. WFO Miami issued a
Severe Weather Statement at 2244 UTC updating the Tornado Warning and
providing the latest damage information. The tornado dissipated
by 2255 UTC and WFO Miami issued a final Severe Weather Statement at
2305 UTC.
In spite of the distances involved from the NWS network of WSR-88D
radars, there were signatures of severe weather in products
from both KAMX and KMLB. Large VILs, elevated maximum
reflectivities, and rotational velocities near 30 knots all were key
indications
of impending severe weather. Skywarn spotters played a
critical role in providing information that tornadoes were
forming, helping to fill the data void created by the distances
involved.
5. References
Collins, W. G., C. H. Paxton, and J. H. Golden,
2000: The 12 July 1995 Pinellas County, Florida, Tornado/Waterspout. Wea.
Fcstg:, 15, 122–134.
Fujita, T. T., 1981: Tornadoes and Downbursts in
the Context of Generalized Planetary Scales. J. Atmos. Sci.,
38, 1511–1534.
Gregoria, D. F., 2005: NWS WFO Miami Severe
Weather Climatology. Unpublished study awaiting review.
U.S. Department of Commerce, National
Oceanic and Atmospheric Administration, 2003: Storm Data.
Annual Summary. National Climatic Data Center.
[Available from National Climatic Data Center, Federal Building, 151
Patton Ave., Asheville, NC 28801.
WATADS (WSR-88D Algorithm Testing and Display
System) 2000: Reference Guide for Version 10.2 [Available from Storm
Scale Applications Division, National Severe Storms Laboratory, 1313
Halley Circle, Norman, OK 73069].
Weisman,
M. L., and J. B. Klemp, 1982: The dependence of numerically simulated
convective storms on vertical shear and buoyancy. Mon. Wea. Rev., 110,
504-520.
Weisman,
M. L., and J. B. Klemp, 1984: The structure and classification of
numerically
simulated convective storms in directionally varying shears. Mon. Wea. Rev., 112,
2479-2498.
6. Figures
Figure 1
- Location map
of the Jupiter/Juno Beach tornado.
Figure 2 -
Damage to screen on back of home from
Jupiter tornado.
Figure 3 - Damage to
roofing at Jupiter shopping center on
U.S. Highway A1A.
Figure 4 - Location
map of Palm Beach Gardens - Riviera
Beach tornado.
Figure 5 - Palm Beach
Gardens - Riviera Beach tornado
(credit Hooper, South Florida Sun-Sentinel).
Figure 6 - Tornado
damage in A Garden Walk mobile home
park, Palm Beach Gardens.
Figure 7 - Tornado
damage to the Pepsi-Cola Plant, Riviera
Beach.
Figure 8A - 500 mb observations and analyzed heights and 250 mb wind
speed at 12 UTC 7 August 2003.
Figure 8B - Surface observations and mean sea level pressure analysis
from MSAS at 15 UTC 7 August 2003
Figure 9 - Cape
Canaveral Sounding, 10 UTC, 7 August, 2003.
Figure 10 - Miami sounding, 12 UTC 7 August, 2003, modified for an
afternoon maximum temperature of 91 F.
Figure 11 - Palm
Beach County
location map.
Figure 12 - Satellite visible imagery 15 UTC 7 August
2003.
Figure 13 - Satellite visible imagery 17 UTC 7 August
2003.
Figure 14 - Satellite visible imagery 1930 UTC 7 August 2003.
Figure 15 - KAMX 0.5 WSR-88D radar reflectivity 1930
UTC 7 August 2003.
Figure 16 - Time lapse of Satellite visible imagery 1301 to 2145 UTC 7
August
2003 (large file - 4 MB)
Figure 17 - Satellite visible imagery 2115 UTC 7 August 2003.
Figure 18 - Satellite visible imagery 22 UTC 7 August
2003.
Figure 19 - Hand calculated
rotational velocity (RV) values for 0.5 and
1.5 degree elevation slices from the KAMX velocity products.
Figure 20 - Hand
calculated rotational velocity (RV) values for 0.5 and
1.5 degree elevation slices from the KMLB velocity products.
Figure 21A -
4 Panel Storm Relative Velocity from KAMX at 2129 UTC 7 August 2003
Figure 21B -
Velocity cross section from KMLB at 2129 UTC 7 August 2003.
Figure 22 - VIL
product from KMLB at 2119 UTC 7 August 2003 showing maximum just west
of Palm Beach Gardens at 79 g/kg.
Figure 23 -
Reflectivity cross section from KMLB at 2109 UTC 7 August 2003 through
the
thunderstorms from northeast at left to southwest at right.