STORM2K

Assessing the regional risk

I've been recently researching and analyzing Florida severe wx climatology, and I've always wondered if a significant tornado outbreak could ever occur in southern Florida within the NWS Miami CWA. As a resident who has been uncomfortably close to some confirmed tornadoes and experienced several TOR Warnings over the years, I think the threat is underestimated; though an outbreak with more than six or seven confirmed tornadoes across south Florida within 24 hours is somewhat rare, the hazard is great because of several factors. 1) The dense population and infrastructure ensures that several areas and thousands of residents in the tri-county area of Palm Beach, Broward, and Miami-Dade counties will be affected. This was evidenced during the nocturnal February 2008 event in south Florida, which featured two confirmed (EF0) tornadoes in Everglades City and Fort Lauderdale. 2) Many of Florida's deadliest or most significant tornado events were nocturnal/overnight outbreaks, including February 2008, February 2007 (long tracked supercell on Groundhog Day in north-central FL), and the February 1998 outbreak in central Florida, the latter of which was Florida's deadliest tornado outbreak with three (F3) killer tornadoes. The nocturnal LLJ often aids overnight events, often in vulnerable locations such as rapidly growing central Florida, which features multiple trailer/RV parks, a concentration of older residents, and subdivisions; and southeastern Florida, which encompasses numerous schools and structures such as larger multi-story buildings. The fact that some moderate to strong tornadoes have been produced from "spin ups" near the BWER within squall lines also adds to the threat. Several examples of these cases have been observed along Florida's western (Gulf) coast. Embedded supercells are another significant concern as well, and many events have featured training supercells.

Although strong/violent tornadoes are infrequent in southern Florida, history indicates that it has occurred in previous decades. On April 5, 1925, a strong (F3) tornado touched down west of Hialeah, FL and moved northeast across Dade County. The parent supercell produced large baseball size hail to three inches (7.6 cm) in diameter, while the tornado caused significant damages to the west and north of the city of Miami. A large dairy farm and several residences were demolished, including those at Biscayne Park. Eyewitness accounts suggest the tornado became rain wrapped and eventually weakened as it approached and moved over Biscayne Bay as a waterspout prior to dissipating in far northeastern Dade County. This was the deadliest tornado to affect Dade County, resulting in five deaths. Although its intensity is uncertain, structural damages reflected in photographs strongly suggest a possible strong (F3) tornado. Additionally, a marginal (?) F3 tornado also affected northeastern Broward County from Oakland Park to Pompano Beach on March 1, 1980, which was very close to my old residence in SW Pompano Beach. Several intense tornadoes have also affected areas just north of the Miami CWA across central Florida, including April 4, 1966; April 15, 1958; February 1998; and February 2007. Two tornadoes from the 1966 and 1958 events are estimated to have been Florida's only F4 tornadoes based on damages, though these ratings in the official SPC database are difficult to verify and may have been too high, depending on the anchoring and types of structures affected.

Based on an exhaustive and objective review of some historical data, is it possible for a significant tornado outbreak with at least +7-10 tornadoes to affect the Miami CWA in south Florida? Personally, I believe it may be possible; I will even go on the record that it may be rare, but the potential hazards and death toll may be greater than those from strong tropical cyclones. A tropical cyclone with a central pressure below ~940 mb at landfall is rare, while tornado-related damages (including tornadoes via tropical cyclones) and flooding from heavy precip presents a far greater "risk potential" than portrayed in the media. I think the "nightmare" scenario is a long lasting event over 24 hours that features multiple strong tornadoes across south-central and south Florida, including tornadoes during the morning and evening hours. Although it would not approach the level of outbreaks in other regions of the Southeast and United States, it would affect commuters, schools, and residents throughout the day. Is there any synoptic setup and juxtaposition of thermodynamics/kinematics that would make this "nightmare" scenario possible?

Here is one hypothetical and more specific outcome of this broader scenario.

Total tornadoes: 10
EF1: 2 (one in Broward County; one in Lee County)
EF2: 2 (one in Lee County; one in Palm Beach County near Lake Okeechobee)
EF3: 5 (one in Collier County; one in Broward County; three in Miami-Dade County)
EF4: 1 (one long tracked event in Okeechobee/Saint Lucie counties)
EF5: 0

Do not focus on the specific counties involved; I was mainly presenting the question, "Would a similar number of tornadoes and ratio of strong/violent tornadoes be possible over 24 hours in the Miami and southern MLB CWAs?" Is it even remotely plausible for the timing and thermodynamics/kinematics to support a similar event so far south?

Your research is impressive. I just wanted to compliment your work. I am not familiar with local meteorological intricacies that I will leave to someone else to dissect.

When compared to the plains of the US, what Miami lacks are the strong jetstreams and arctic air that plagues that region. Therefore, it will be very rare for a severe outbreak of tornadoes to affect the Miami area. Nonetheless, it not impossible during a strong El Niño to have a minor outbreak. Still, I'm more worried about hurricanes than tornadoes not associated to hurricanes!

Thanks for bringing this up. Very interesting.

Here is the possible synoptic setup for the aforementioned hypothetical event. Please feel free to offer further education and corrections if necessary. Overall, is the following evolution possible?

The Warm Frontal Florida Outbreak

0600Z



A warm front is draped across central Georgia. A secondary warm front lifts northward over central Florida during the early morning hours. Moderate capping remains at the mid levels south of the warm front in southern Florida. The capping gradually erodes through the early morning as the first impulse of 40 kt 850-300 mb shear arrives over the west coast of the peninsula. Backed low level winds are widespread across the peninsula at this time, varying from SE along the southeast coast to S and SSE along the southwestern coast. A few areas of SSW winds are evident from locations north of the Tampa area to the Big Bend. At this time, a surface low is located over eastern Alabama, while a secondary surface low is forming over the eastern Gulf of Mexico with a cold front trailing SSW toward the northern Yucatan Peninsula. A third surface low is forming over the western Atlantic east of Charleston, SC, with a stationary front trailing shortly to the SW. Stationary front becomes aforementioned warm front further west over FL peninsula. Low over eastern Gulf of Mexico moves remains nearly stationary or moves very slowly northward, while 0-1 km SRH increases over the southern FL peninsula in juxtaposition with 40 kt 850-300 mb shear vectors spreading southeast. Eventually, conditions at MIA and other south FL stations are supportive of supercellular structures, including sufficient veering at mid to upper levels; slight lingering CINH; weakening cap; widespread Tds into the mid/upper 60s (central FL) and upper 60s/low to mid 70s (south FL); SBCAPE values of 900-1,000 (portions of central FL) to 1,500 in several areas of southern FL; peak MUCAPE of 1,100 (central FL) and 1,650 (south FL), respectively; widespread SB LI values of -4 to -5, especially southward from south-central FL; and lapse rates comparable to the MIA sounding on February 2, 2008.

1200Z



The warm front continues to lift north before eventually stalling along a line from Saint Augustine to Cedar Key, FL after dawn. Low level winds back slightly along the west and southwest coast of the FL peninsula. SBCAPE values have spread further north to north-central Florida, where isolated values of around 1,000 J/KG are present near the warm front. Further south, values of 1,350 to 1,700 are present, especially from south-central Florida southward. MUCAPE values increase to 1,400 (central FL) and 1,850 (south FL). A few supercells develop along the warm front in north-central and central FL, aided by 50 kt 850-300 mb shear and lower LCL values that foster lower cloud heights. Some of the cells become long tracked tornadic supercells. From south-central FL southward, 850-300 mb shear has increased to 40 kt, and an isolated supercell develops just north and west of Lake Okeechobee with unobstructed low-level thermodynamics. It quickly becomes tornadic and produces a long tracked tornado in Okeechobee County that moves northeast into Saint Lucie County. Stronger forcing gradually spreads across the central and southern peninsula, increasing the threat further south. SB LI values of -4 to -5 spread to portions of north-central FL, while values of -5 to -6 appear from south-central FL southward. 0-1 km SRH increases over the southern peninsula. The Saint Lucie cell and additional supercells further north also leave residual outflow boundaries over the central and south-central peninsula, which gradually affects locations further south and southwest. This supports further initiation of supercells over the southern peninsula from south-central FL southward.

1800Z



Surface low over eastern AL gradually weakens as the surface low over the eastern Gulf of Mexico slowly deepens and becomes the dominant surface low. Afternoon surface heating over the southern FL peninsula combines with increasing instability and forcing, leading to further development of supercells. Strengthening mid to upper level wind fields and high 0-1 km SRH aids discrete supercells, juxtaposed with the gradually intensifying backed LLJ over the peninsula. Backed low level winds over the southern FL peninsula combine with low LCL values and strong thermodynamics to support tornadic supercells. Approaching cold front also acts as a forcing mechanism over the Gulf of Mexico, where slightly veered low level winds support the development of discrete supercells that congeal into a broken squall line along a N/S line from west of Tampa to well west of Cape Sable. Further east, backed low level winds over the peninsula and portions of southwestern FL support discrete supercells. Multiple supercells affect southwestern FL from Cape Coral to Everglades NP; some produce tornadoes, including a tornadic supercell at Everglades City. Several supercells form over the Everglades region and affect the tri-county area of southeastern FL, many of which produce tornadoes during the afternoon. Overall, multiple long tracked supercells occur over southern peninsular FL.

0000Z



Low level winds gradually veer over southern FL as the primary surface low over the eastern Gulf of Mexico moves northeast. Low level winds remain slightly backed over the central and southern peninsula, excluding the west coast. Pre-existing and new supercells continue to produce isolated tornadoes as the nocturnal LLJ strengthens. Supercells eventually evolve to bowing or linear segments as the cold front and squall line approaches and enters the peninsula overnight. Strong mid to upper level shear, the stronger and slowly veering nocturnal LLJ, 0-1 km SRH, and LCL values support isolated embedded tornadoes/BEWRs and damaging winds.

Total tornadoes within Miami or southern MLB CWAs: 10
EF1: 2
EF2: 2
EF3: 5
EF4: 1
EF5: 0

Please feel free to correct me or offer instruction if any science is false or mistakes were made in the analysis/surface charts.

wx247 wrote:Your research is impressive. I just wanted to compliment your work. I am not familiar with local meteorological intricacies that I will leave to someone else to dissect.

Thanks! In a nutshell, I believe the tornado threat and subsequent potential for damages and considerable loss of life in southern FL is highly underestimated. In fact, my research indicates that there is a greater threat from tornadoes (including a scenario like the hypothetical one above) than a strong <940 mb landfalling tropical cyclone in south Florida. Significant outbreaks can occur here, including from tropical cyclones. The Agnes 1972 tornado outbreak over the Keys and FL peninsula is a classic example. It originated from the effects of a tropical storm that intensified to a low end hurricane west of the peninsula.

HURAKAN wrote:When compared to the plains of the US, what Miami lacks are the strong jetstreams and arctic air that plagues that region. Therefore, it will be very rare for a severe outbreak of tornadoes to affect the Miami area. Nonetheless, it not impossible during a strong El Niño to have a minor outbreak. Still, I'm more worried about hurricanes than tornadoes not associated to hurricanes!

Thanks for bringing this up. Very interesting.

Thanks again. Personally, I disagree with the bolded statement. Most people do not realize that although Florida has received the most TC landfalls, from 1851 through 2007, only five tropical cyclones have struck the state with central pressures below 940 mb (1926, 1928, 1935, Donna 1960, and Andrew 1992). Even Texas' huge coastline has only been struck by intense tropical cyclones with sub-940 mb pressures on four occasions in more than 100 years of recorded history. In my view, May 5, 2007, May 3, 1999, Super Tuesday 2008, the upper Midwest in May 2008, and other countless tornado events and outbreaks across the nation have demonstrated that severe wx may be a greater threat than the winds of the strongest tropical cyclones. Heavy precip and tornado outbreaks (including those associated with weak tropical cyclones) are a greater threat.

How many strong tornadoes have impacted Miami? How many strong hurricanes have impacted Miami? That's why I worry more about hurricanes. Even if tornadoes were a lot more common in out area, there impact is limited by their size and time on land.

HURAKAN wrote:How many strong tornadoes have impacted Miami? How many strong hurricanes have impacted Miami? That's why I worry more about hurricanes. Even if tornadoes were a lot more common in out area, there impact is limited by their size and time on land.

In regards to south Florida severe wx/tornado climatology within the NWS Miami CWA, the following link leads to an excellent paper by Dan Gregoria of the Miami office. Official SPC Fujita scale records began in 1950, though the first tornadoes to operationally receive a Fujita scale classification based on structural damage occurred in 1971. Keep in mind that many tornadoes have in southern FL have remained over the Everglades or never impacted structures, so they never received a rating above F0/EF0. However, if they actually impacted structures, damages could have been as high as F2-F3/EF2-EF3 or stronger.

http://www.srh.noaa.gov/mfl/files/MFLSVRCLIMO.pdf

"Tornadoes across South Florida are typically “weaker” than those of the plains states,
though strong tornadoes have occurred. One such tornado occurred in Miami on April 5,
1925, killing 5 people and injuring 35. The estimated property loss was between
$200,000 and $300,000 (Gray, 1925). The strength of the 1925 Miami tornado is
unknown, since tornado ratings did not begin until 1971. However, damage pictures
from the 1925 tornado suggest that this tornado could have been an F3. It was T.
Theodore Fujita who developed a tornado rating scale in 1971, called the Fujita Scale, or
F-Scale [Figure 6]. Tornado intensities are assigned rankings based on damage
assessments by National Weather Service personnel. The F-Scale distribution of
tornadoes across the Miami CWA is provided in Figure 7. Since 1950, ninety three
percent of observed and rated tornadoes, or 388, were rated F0 or F1. Seven percent, or
29, were rated F2 or F3. Thirty tornadoes were not rated. There have been no observed
tornadoes rated either F4 or F5 across the Miami CWA since 1950."

21 intense hurricanes (at least Cat 3) have struck the east and west coasts of southern FL from 1851 through 2007, respectively.

http://www.aoml.noaa.gov/hrd/hurdat/ushurrlist18512007.txt