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COMET CASE STUDY
FLORIDA WILDFIRES:
19-22 JUNE 1998
Arlene Laing* and Charlie Paxton#
* University of South Florida, Tampa, FL
# NWS Tampa Bay, Ruskin, FL
The wildfires that occurred in late spring and early summer of 1998 in the state of Florida were unprecedented in scale and impact on the state. Over 2,300 fires burned approximately half a million acres at a cost of over $620 million. This case study focuses on the period 19-21 June during which the daily fire starts increased dramatically to the seasonal peak frequency and the total acres burned made an almost nine-fold increase during one 24-hr period.
Across the state, lightning was the largest primary ignition source (31%) and lightning fires covered 79% of the total acres burned during 1998. On the 19th of June lightning ignited 81 new fires compared with 46 the previous day and was the start of two-week period during which daily fire starts were an order of magnitude higher than the previous weeks. The 19th was also noteworthy for a large thunderstorm complex that developed over the northwestern peninsula Fig. 1 . The western sea breeze convergence and outflow from previous convection and fires interacted to create the thunderstorm complex which in turn generated more lightning.
While the number of new fires
increased to a daily frequency maximum of 85 on 20 June, it was the nearly 900%
increase in total acres burnt across the state that garnered the most
attention. Number of acres burnt increased
from 26,000 on 19 June to 178,000 acres on 20June. The largest single lightning-initiated fire (61,500 acres) was in
Volusia County close to the border with Flager. By the end of 21 June when 72 new fires had started, several
counties in northeast Florida had been declared federal disaster areas.
Following
this dramatic outbreak of fires, 22 June was so marked by heavy smoke that the Volusia
County Health Department
issued a health advisory; warning all residents to stay indoors. By this time, the fires had been burning in
some counties since 31 May, with some 100 homes destroyed. According to a CNN report filed from Ormond
County on 23 June, the fires destroyed more than 100 homes and structures,
damaged an estimated $10 million in commercial timber, and caused $100 million
in crop damage.
The synoptic environment provided the first
indicator for wildfires across Florida.
An atypical, predominant surface high-pressure area Fig. 2 had settled
over the southeastern US and the Gulf of Mexico causing rather dry airflow into
the fire areas from the north and northwest.
The synoptic environment led to a sharp decrease
in moisture in the mid-lower atmosphere and, with each dry surge, progressively
less moisture to modify the subsiding air.
The result was a period of low relative humidity, high temperatures and
record drought particularly in north-central and northeastern FL. Six of the seven Florida climate divisions
were among the top ten driest in 104 years (NCDC).
Prevailing westerlies drove the western sea
breeze far inland while the eastern convergence zone hugged the coastline Fig. 1 . This was in contrast to the
predominant Florida summer climate regime, under which predominant easterlies
propel the eastern sea breeze front toward the west sea breeze front, where the
two converge to create a thunderstorm and flash density maximum over west
central Florida.
Convective Initiation, Lightning, and Downdrafts:
The tendency for the eastern sea breeze front to
remain nearly stationary led to initiation of convection, lightning strikes,
and fires just inland of the coast.
Across the state, lightning was the primary
(31%) ignition source and lightning fires covered 79% of the total acres burned
during 1998. Although dry conditions
prevailed across much of the state, fires and lightning were highly varied in
time and space with the most numerous and largest fires occurring in
northeastern Florida. The largest
single fire was 61,500 acres on the border of Volusia and St. John’s counties.
With weak surface pressure gradients, weak
vertical wind shear, and CAPE (averaged for three north Florida stations)
exceeding 1500JKg-1 on 19 June, it was not surprising that deep
convection developed. Lightning strikes
increased by an order of magnitude (to more than ten thousand flashes per day)
and signaled the start of a two-week period with the most frequent fire starts
and largest fires. Thunderstorm
downdrafts played a significant role in spread of wildfires. This was particularly evident on 20 June
when thunderstorm gusts (20-28 kts reported at Jacksonville, KCRG) were the
major contributors to the tremendous increase in fire area even while the
numbers of new fires and lightning strikes remained similar to the previous
day. JAX WSR-88D velocity analyses,
during the fires indicate downburst areas near fire areas (Pat Welsh, SOO JAX,
personal communication). Direct effects
were suppression in rain areas and flare ups outside of rain areas.
The major outbreak was characterized by high levels of instability and moisture for deep convection, weak surface pressure gradients, quasi-stationary eastern sea breeze converging with opposing sea breeze, and weak vertical wind shear. Copious lightning was the single largest cause of fires and fire spread was closely tied to movement of thunderstorm downdrafts.
Supporting Documentation:
NCDC
Florida Wild Fires and Climate Extremes
http://lwf.ncdc.noaa.gov/oa/climate/research/1998/fla/florida.html
Florida Division of Forestry
ECOLOGICAL AND ECONOMIC CONSEQUENCES OF THE 1998 FLORIDA WILDFIRES
http://flame.fl-dof.com/joint_fire_sciences/
Volusia County Fire Reports
http://www.volusia.org/firenews.htm
FEMA 1998 Disasters
http://www.fema.gov/library/diz98.shtm
CNN Features
http://www.cnn.com/US/9806/23/florida.fires.02/index.html
http://www.cnn.com/US/9806/24/florida.fires.01/
St. Petersburg Times: Florida on Fire
http://www.sptimes.com/Floridafires/index.html
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