3. PRE Climatology
3.1 Statistical Climatology
3.1.1 Overview
As stated in section 1.1, 47 PREs were identified downstream of 21
Atlantic basin TCs during 1998–2006. The average PRE-producing TC (PPTC)
thus spawned 2.2 PREs, implying that the production of multiple PREs
downstream of a single TC is common. Seventeen of the 47 (36%) Atlantic basin
TCs that made landfall in the U.S. during 1998–2006 produced at least one PRE.
Three PPTCs that did not make landfall in the U.S. [Alex (2004), Irene (2005),
and Ophelia (2005)] recurved off the East Coast, and one [Emily (2005)] made
landfall in northern Mexico. While not all PPTCs made landfall in the U.S., they
all were located northwest of 20°N, 65°W when they produced PREs; thus, all
TCs crossing this point were deemed capable of producing PREs. Twenty one of
the 83 (25%) TCs in the study period crossing this point produced at least one
PRE.
Figure 3.1 shows a histogram of the occurrence of Atlantic basin PPTCs
and a line graph of the occurrence of all Atlantic basin TCs during 10- and 11-day
periods encompassing the dates for which Atlantic basin TCs were officially
recognized by NHC during 1998–2006. Atlantic TC occurrence peaked during 1–
10 September, but PRE occurrence peaked during 21–31 August and 11–20
September. The PRE minimum during 1−10 September corresponds to a relative
minimum in U.S. landfalling TCs (Fig. 3.2), but both of these minima are likely
statistical artifacts resulting from the limited nine-year TC sample. PREs also can
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occur early or late in the tropical season, but Fig. 3.2 indicates that they were
more likely to form in June and July than in October and November during 1998–
2006.
3.1.2 TC Tracks Favorable for PRE Development
The procedure and motivation for separating TCs into different categories
based on similarity of TC track is discussed in section 2.2.2. The tracks of all SR
TCs listed in Table II are shown in Fig. 3.3a so the tracks taken by TCs in this
category can be visualized. Seven of the 11 (64%) TCs represented in Fig. 3.3a
produced at least one PRE, which is the highest percentage of PPTCs in any
track category containing at least 10 TCs. The tracks of the seven SR PPTCs are
displayed in Fig. 3.3b on a map containing dots that represent PRE formation
locations. Each dot is colored to match the track color of the TC that helped
produce it. Sixteen PREs formed in association with the seven SR PPTCs, which
is an average of 2.3 PREs per PPTC. Nearly all of the PREs occurring
downstream of SR PPTCs formed in a southwest–northeast band from the Gulf
Coast to southern New England. Comparison of Fig. 3.3b with Fig. 3.4 shows
that PRE formation locations were generally along or south and east of the
Appalachian ridge line and in proximity to the Gulf of Mexico and/or Atlantic
Ocean.
Six of the 15 (40%) AR TCs whose tracks are shown in Fig. 3.5a produced
at least one PRE, which is the second highest percentage of PPTCs in any track
category containing at least 10 TCs. The tracks of the six AR PPTCs and the
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formation locations of the PREs that occurred downstream of them are illustrated
in Fig. 3.5b. The three nonlandfalling PPTCs that recurved off the East Coast
(see section 3.1.1) are represented by the green, red, and blue tracks,
respectively. Twelve PREs formed in association with the six AR PPTCs, which
is an average of 2.0 PREs per PPTC. Figure 3.5b shows that the PREs occurring
downstream of AR PPTCs formed along the immediate mid-Atlantic or southern
New England coastline or near elevated terrain of the Northeast (refer to Fig.
3.4).
Three of the 10 (30%) CG TCs whose tracks are shown in Fig. 3.6a
produced at least one PRE, which is the third highest percentage of PPTCs in
any track category containing at least 10 TCs. The three CG PPTCs depicted in
Fig. 3.6b tracked west of the Appalachian ridge line (refer to Fig. 3.4). Thirteen
PREs formed in association with the three CG PPTCs, which is an average of 4.3
PREs per PPTC—about twice as many as formed in association with SR or AR
PPTCs. Figure 3.6b shows that PRE formation locations downstream of CG
PPTCs spanned the distance from the Upper Midwest to the East Coast during
1998–2006.
Only six of the 47 (13%) PREs occurring during 1998–2006 formed
downstream of TCs following tracks other than the three already described. Two
TCs [Bret (1999) and Emily (2005)] out of 11 (18%) that made landfall southwest
of the Texas/Louisiana border produced at least one PRE. Two TCs [Harvey
(1999) and Wilma (2005)] out of six making landfall in Florida, but passing
southeast of SR TCs (see section 2.2.2 for exact definition) produced at least
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one PRE. Finally, Isabel (2003) and Fran (1996) both made landfall in the midAtlantic and produced PREs, but were not classified as AR cases because they
continued moving northwestward into the U.S. after landfall. No null cases
following tracks similar to Isabel and Fran have been identified since 1996.
3.1.3 PRE Locations Relative to TC Track
Anticipating the likely location of PRE formation relative to the eventual TC
track can help forecasters assess the potential impacts of PRE and TC rainfall. In
particular, areas affected by AT PREs may experience extreme flooding because
of the occurrence of heavy TC rainfall after the predecessor rain. Areas affected
by LOT and ROT PREs do not receive heavy TC rainfall, but the PRE rainfall in
those areas still may be significant enough to cause flooding.
Figure 3.7 illustrates that 26 of the 47 (55%) PREs in the study period
occurred LOT. In comparison, 12 of the 47 (26%) PREs were AT and 9 of the 47
(19%) PREs were ROT. The AT percentage implies that, given a PRE will form,
there is approximately a one-in-four chance that the rainfall directly associated
with a TC will subsequently fall over the same area affected by a PRE.
3.1.4 Other Statistical Properties of PREs
Box plots were constructed for the distributions of PRE SDs, time lags,
durations, 24-h rainfall rates, and speeds. Four box plots will be shown for each
set of data: one showing the distribution for all PREs that occurred during 1998–
2006, and one each for the distributions of LOT, AT, and ROT PREs. According
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to Wilks (2005), box plots are a concise way of displaying the five-number
summary of a data distribution, which includes the smallest and largest data
points, the lower and upper quartiles, and the median. In the box plots described
below, the box defines the bounds of the interquartile range (IQR), which
contains the middle 50% of the data and provides a measure of statistical
variability. The whiskers extend to values within a distance of 1.5 times the IQR
from the upper and lower quartiles. The median is represented by a black dot
within the IQR, and outliers are plotted individually outside the IQR. Positive
(negative) skewness can be inferred from the box plots in the figures that follow if
the top (bottom) whisker is longer than the bottom (top) whisker and if the median
is less (greater) than the mean.
The median SD between all PPTCs and PREs is 935 km. Positive
skewness can be inferred in the all-PRE SD distribution shown in Fig. 3.8
because the mean SD is approximately 150 km greater than the median. The
LOT PRE SD distribution is similar to the all-PRE SD distribution, in part because
LOT PREs comprise more than half the PRE database. The narrow IQR of the
AT PRE SD distribution indicates that SDs vary less with AT PREs than with LOT
or ROT PREs. The ROT PRE SD distribution demonstrates a notably higher
mean and median than the other two PRE categories and exhibits a slight
negative skewness.
The median time lag for all PREs is 36 h, but time lags could not be
calculated for one LOT and two ROT PREs because the centers of their parent
TCs never reached the latitudes of the PRE centroids. Figure 3.9 shows that the
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all-PRE time lag distribution is positively skewed because the mean of 45 h is 9 h
greater than the median. The extreme outlier in the all-PRE time lag distribution
occurred when the remnants of Hurricane Dennis (2005) stalled for several days
over the lower Mississippi Valley before reaching the latitude of the PRE. The
LOT PRE time lag distribution in Fig. 3.9 shows a greater mean and median and
greater variability than the all-PRE time lag distribution. The AT and ROT PRE
time lag distributions exhibit much less variability than the LOT PRE time lag
distribution. The mean and median time lags for ROT PREs are less than for
LOT or AT PREs. Given the expectation that small time lags would be associated
with small SDs, it is counterintuitive that ROT PREs have a larger median SD
than LOT or AT PREs (refer to Fig. 3.8). However, calculation of TC speeds
during PREs provides no statistical evidence that the large SDs and short time
lags associated with ROT PREs are attributable to faster TC movement in ROT
PRE cases than LOT or AT PRE cases.
The four box plots summarizing the data distributions of PRE durations
(Fig. 3.10) all have means and medians between 12 and 15 h, suggesting that
the location of a PRE relative to the track of its parent TC has no effect on how
long it lasts. However, each duration distribution, except for the one associated
with AT PREs, contains an outlier of 36 h, suggesting that long-lasting PREs can
occur. The AT and ROT PRE duration distributions exhibit the least variability of
the four distributions because they have the smallest range and IQR,
respectively.
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Figure 3.11 reflects the median rainfall rate of all PREs during 2001–06 of
203 mm (24 h)−1 and the lower bound of 100 mm (24 h)−1 required for
classification as a PRE. Comparison of the AT and ROT PRE rainfall distributions
with the LOT PRE rainfall distribution in Fig. 3.12 reveals that the AT and ROT
PRE rainfall distributions have larger IQRs and greater mean and median rainfall
rates than LOT PREs. Specifically, the mean 24-h rainfall rates of AT and ROT
PREs are approximately 60 mm greater the mean rainfall rate of LOT PREs.
Despite the smaller samples of AT and ROT PREs compared to LOT PREs, the
highest 24-h rainfall rates of all PREs occurred with AT and ROT PREs.
The box plots shown in Fig. 3.12 reveal that PRE speeds can range from
nearly zero to greater than 20 m s−1. The speed distribution of all PREs is
positively skewed with mean and median values of 9.5 and 7.4 m s −1,
respectively. The LOT and AT PRE speed distributions are similar. Despite one
extreme outlier, the mean ROT PRE speed is 5.8 m s −1, which is ~4–6 m s−1
slower than the mean LOT and AT PRE speeds. The IQR of the ROT PRE speed
distribution is considerably smaller than that of the all-PRE speed distribution,
indicating that ROT PRE speeds have much less variability. Comparison of Fig.
3.11 with Fig. 3.12 shows that high ROT PRE rainfall rates correspond to slow
ROT PRE speeds, but that high AT PRE rainfall rates are associated with fast AT
PRE speeds.
3.2 Composite Climatology
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Synoptic-scale geography-relative composite plots spanning the 24-h
period centered on PRE initiation time were constructed using 11 of the 16 PREs
forming downstream of SR PPTCs. The dates and times included in the
composites are listed in Table III alongside the TCs associated with the PREs.
Two types of composite plots will be shown every 12 h to emphasize important
synoptic-scale signatures: 1) 700-hPa geopotential height overlaid on upward
vertical motion to diagnose the geostrophic flow pattern in which PREs form
downstream of SR PPTCs and 2) 925-hPa geopotential height and θe overlaid on
200-hPa wind speed to diagnose the low-level temperature and moisture fields
and upper-level jet structure.
Figure 3.13a shows that the composite TC (represented by the green star)
is located just west of the Florida peninsula 12 h prior to PRE initiation. Upward
vertical motion is maximized northeast of the TC center along the Georgia coast,
within a confluence zone created by a 700-hPa trough northwest of the TC and a
700-hPa ridge east of the TC. A 200-hPa jet streak with peak winds greater than
40 m s−1 is oriented from southwest to northeast over the northeastern U.S. and
is collocated with a 925-hPa θe gradient downstream of a trough axis in
southeastern Canada (Fig. 3.13b). Southwesterly geostrophic flow at 925 hPa is
oriented parallel to a θe-ridge axis extending northeastward from the TC (Fig.
3.13b), implying poleward transport of warmer and moister air.
The synoptic-scale pattern changes little during the next 12 h. Figure
3.14a shows that the composite PRE forms along the North Carolina/Virginia
border downstream of the nearly stationary 700-hPa trough and north of the
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maximum upward vertical motion just prior to TC landfall along the western
Florida coast. The PRE also forms near the equatorward entrance region of the
200-hPa jet streak, just to the west of the 925-hPa θe-ridge axis (Fig. 3.14b).
Twelve hours after it forms, the composite PRE is located in northeastern
Virginia and is accompanied by a second composite PRE that forms in close
proximity to the first PRE (Fig. 3.15a). While it may seem contradictory that two
PREs are located in virtually the same place at the same time, the composite
position of the second PRE at this time indicates that the subsequent PRE
occurring downstream of a SR PPTC forms 12 h after the first PRE forms and
within a synoptic-scale environment similar to that of the first PRE. Figure 3.15a
shows that the composite TC and the composite PREs are located downstream
of the 700-hPa trough. A significant change from 12 h earlier, however, is that
there now are two maxima in the upward vertical motion field—one near the TC,
and one near the two PREs northeast of the TC. Figure 3.15b indicates that the
PREs are located on the anticyclonic shear side of the 200-hPa jet streak and
near an implied axis of dilatation at 925 hPa between ridges to their west and
east.
Although composites of PREs in other TC track categories have not been
constructed, the synoptic-scale environments in which the PREs formed have
been examined subjectively. Nine of the 12 PREs occurring downstream of AR
PPTCs and three of the 13 PREs occurring downstream of CG PPTCs formed in
association with the four synoptic-scale signatures evident in the SR
composites—namely, ahead of a midlevel trough, near the equatorward entrance
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region of an upper-level jet, within a low-level θe ridge or gradient, and in an area
receiving warm, moist air from the TC. Including the 11 PREs in the SR
composites, 23 of the 47 (49%) PREs identified during 1998–2006 formed in
environments characterized by all the synoptic-scale signatures identified by the
SR composites. The synoptic-scale environments of the remaining PREs varied
more widely and featured only some of the signatures seen in the SR
composites. Possible explanations of this observation are that: 1) some of the
four synoptic-scale signatures seen in the SR composites may play greater roles
than others in producing PREs and 2) mesoscale and physiographic processes
below the 2.5° × 2.5° resolution of the composites may have a greater influence
in producing PREs than synoptic-scale processes.
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