# THE MARCH 9-10 SERIAL DERECHO

By Mike Evans

A squall line of showers with embedded thunder and lightning moved
rapidly east across central New York and northeastern Pennsylvania
during the evening of March 9^th^ and the early morning hours of March
10. The line developed several bow echo segments as it moved east across
the area, with wind gusts estimated at 60 mph. Downed trees and wires
were reported in nearly every county across the BGM county warning area.

## A REVIEW OF DERECHOS

A derecho is defined as a convective wind-storm that occurs along a path
length of at least 400 km (Johns and Hirt 1987). The damage reports must
be continuous and non-random; ie widespread damage. A serial derecho is
defined as a derecho associated with a strong, migratory low-pressure
system. These types of derechos typically occur during the winter or
early spring. A progressive derecho is defined as a system that occurs
in west or northwest flow aloft and is frequently associated with weaker
synoptic scale forcing. This type of convective system typically
develops near or north of a warm front, then races off to the southeast
through a very unstable warm sector. The favored time of year for a
progressive derecho is late spring through summer.

Studies of warm season, weakly forced (progressive) derechos indicate
that important factors for development include very high instability
south of the warm front, and strong shear, especially in the lowest 2 km
of the atmosphere. A source of mid-level dry air have also been found to
be important for the development of a weakly forced, warm season
derecho. Mid-level dry air leads to stronger (colder) cold pools, which
can lead to the fast movement of the storms, even in the absence of
strong mid-level winds.

Studies of cold season, strongly forced (serial) derechos indicate that
those types of systems can occur with very limited instability. Shear
and mean wind values are typically very large, and hodographs can often
appear to be supportive of tornaodes (long, strongly curved hodographs).
The presence of mid-level dry air is not as important in a strongly
forced event as with a weakly forced event, but dry air can still play a
role in strengthening the cold pool and increasing the wind speeds
associated with the convection (Evans and Doswell 2001).

The tendency for hodographs to look similar for strongly forced derecho
environments and tornado environments can often make forecasting the
most likely dominant mode of convection difficult in situations with
long, curved hodographs. Some modeling studies indicate that derechos
are most favored when the strong shear is confined to the lowest 1 or 2
km of the atmosphere, while supercels are more favored when the shear is
deeper (Weisman 1993). The hypothesis for this finding is that strong
shear extending through the mid-levels of a storm results in strong
rear-to-front storm relative wind flow increasing with height at
mid-levels of the storm. This flow results in large amounts of
precipitation being blown downstream away from the downdraft of the
storm, which decreases the amount of cooling associated with the
downdraft and decreases the amount of outflow at the surface. Storms
that are less outflow-dominated at low-levels have a better chance to
develop tornadoes, since strong outflows tend to stabilize the area
underneath the updraft of the storm (Brooks et al. 1994).

Another helpful discriminator between derecho environments and tornadic
environments may be the energy helicity index (EHI). Observational
studies have shown that the EHI ((shear x cape) / 160000) is
significantly larger in tornadic environments than in environments that
produce derechos. A cut-off value of around 1.0 has been informally
found to be a good value for our area. Recent observational studies also
indicate that high boundary layer relative humidity is also a good
indicator that the environment may be more favorable for tornadoes than
strongly forced derechos (Evans and Doswell 2001).

**March 9^th^ and 10^th^.**

A quick look at the weather maps on March 9^th^ indicates that the
severe convective event of March 9^th^ and 10^th^ was a strongly forced
Derecho. The figures below show 12 hour Eta forecasts of the 300 mb wind
speed and heights, 500 mb heights, surface pressure and 850 mb
temperatures verifying at 00 UTC on March 10^th^. A major upper-level
trough is moving east across the Great Lakes, with a very strong cold
front moving east across the eastern Great Lakes.

![](media/image1.png){width="5.995138888888889in"
height="4.678472222222222in"}

12 hr Eta forecast 300 mb wind speed and heights verifying at 3/10 00z.
Note the strong upper-level trough moving east across the Great Lakes,
with two 64 m/s speed maxima, one at the base of the trough, and one on
the west side of the trough.

![](media/image2.png){width="5.995138888888889in"
height="4.678472222222222in"}

12 hr Eta forecast 500 mb heights verifying 3/10 00z. A strong trough is
moving east across the Great Lakes.

![](media/image3.png){width="5.995138888888889in"
height="4.678472222222222in"}

12 hr Eta forecast pmsl and 850 mb temperatures verifying at 3/10 00z. A
strong cold front is moving east across western New York and western
Pennsylvania.

The following figures show IR and water vapor satellite imagery from
March 9^th^. Both images indicate a strong trough moving east across the
Great Lakes area. The water vapor imagery in particular shows strong
mid-level drying behind the squall line over western Pennsylvania and
western New York at 03z on the 10^th^.

![](media/image4.png){width="5.996527777777778in"
height="3.3402777777777777in"}

*IR satellite imagery at 0015 UTC on March 10^th^.*

![](media/image5.png){width="5.996527777777778in"
height="3.3402777777777777in"}

*Water Vapor satellite imagery at 0315 UTC on March 10^th^.*

The following figure shows the 600 mb Eta forecast advection of relative
humidity verifying at 00 UTC on the 10^th^. Note the strong advection of
dry air into the back-side of the squall line.

![](media/image6.png){width="5.995138888888889in"
height="4.678472222222222in"}

*12 hr Eta forecast advection of relative humidity at 600 mb verifying
3/10 00z.*

The next figure shows the Eta 12 hr forecast 6-hour pressure tendencies
across the Great Lakes and northeast verifying at 06z on the 10^th^.
Note the strong rise-fall couplet associated with the front as it moves
east across the eastern Great Lakes. Strong rise-fall couplets of
surface pressure tendency are a good indication of strong winds, as they
lead to strong isallobaric wind components pointing in the direction
from the rises toward the falls.

![](media/image7.png){width="5.995138888888889in"
height="4.678472222222222in"}

*18 hr Eta forecast 6 hour surface pressure change verifying at 3/10
06z.*

The following figure shows the observed sounding at Buffalo, NY at 3/10
00z. Note the very strong winds just above the surface indicated by this
sounding (40 kts at 925 mb and 60 kts at 850 mb). The strong winds just
off the surface of the ground were available to mix to the surface with
any convection that developed. Model forecast soundings (shown later in
this document), indicated that 60 kt winds may have been present as
close as 1000 feet above the surface. Another feature of note on this
sounding was the inverted V signature evident in the lowest 3 km of the
sounding, and the layer of dry air evident in the mid-levels of the
sounding. Both of those features would have been favorable for
evaporational cooling within the downdrafts of any convection, and
therefore would have been favorable for the production of enhanced wind
gusts. In this case however, the production of enhanced winds by
evaporational cooling within convective downdrafts was probably of
secondary importance compared to the convective mixing of the strong
environmental winds to the surface.

![](media/image8.png){width="5.9944444444444445in"
height="3.4298611111111112in"}

*Observed Buffalo sounding at 3/10 00z.*

The following figures show composite radar imagery over the Great Lakes
and northeast as the squall line moved east across the area. Clearly,
convection occurred and would have been able to mix the strong winds
shown in the previous figure down to the surface. This mixing of very
strong low-level winds to the surface was likely the primary mechanism
for the production of the damaging winds. The squall line appeared to
weaken slightly as it moved across Lake Erie, then strengthen as it
continued to move east across western New York and western Pennsylvania.

![](media/image9.png){width="5.995833333333334in"
height="3.2847222222222223in"}

*Composite radar reflectivity at 2200 UTC, March 9^th^, 2002.*

![](media/image10.png){width="5.995833333333334in"
height="3.2847222222222223in"}

*Composite radar reflectivity at 2330 UTC, March 9^th^, 2002.*

![](media/image11.png){width="5.995833333333334in"
height="3.2847222222222223in"}

*Composite radar reflectivity 0200 UTC, March 10^th^, 2002.*

![](media/image12.png){width="5.995833333333334in"
height="3.2847222222222223in"}

*Composite radar reflectivity 0400 UTC, March 10^th^, 2002.*

![](media/image13.png){width="5.995833333333334in"
height="3.2847222222222223in"}

*Composite radar reflectivity at 0535 UTC, March 10^th^ 2002.*

A close look at the last couple of radar images, indicates that the line
developed several small-scale bow echoes as it moved across central New
York and northern Pennsylvania. The bow echoes can be identified as
areas where the higher reflectivity bowed out ahead of the main line,
with channels of weaker reflectivity developing behind the bow. A good
example can be seen at 0535 UTC on the 10^th^, over southeastern Otsego
county. Breaks in the line can also be seen in some areas, as the weak
echo channels punch through the line. These bow echoes and breaks in the
line were all locations where enhanced wind gusts likely developed, and
caused damage.

The squall line shown in the last few figures was moving toward the
east-northeast at approximately 50 kts. The following figure shows a
BUFKIT forecast sounding taken at BGM from the 18 UTC run of the Eta
model. The MBE (meso-beta element) velocity vector indicated by the
figure is toward the east at 3 kts. (Recall that MBE vectors are the
vector sum of the mid-level flow (advection of individual cells within
the convective system), and the negative of the low-level flow (an
indication of where new cells are likely to form). The resulting MBE
vector is supposed to give a first guess estimate of system motion).
Clearly, this is a classic example of why MBE vectors do not always give
a good estimate of storm motion, especially in uni-directional flow in
outflow-dominated storms. In this case, with strong, nearly
uni-directional flow, MBE vectors will be small, as the mid-level flow
and the negative of the low-level flow will nearly cancel.

![](media/image14.png){width="5.997222222222222in"
height="4.5055555555555555in"}

BUFKIT sounding at BGM from the 18/09 run of the Eta, verifying at 04
UTC, 3/10. The MBE vector calculation in this case is not accounting for
a forward propagating storm.

In cases with strong uni-directional flow, when storms are expected to
injest dry-air in the mid-levels and develop strong cold pools, they
typically become "forward propagating" storms, in the sense that the
best storm relative inflow typically develops on the forward, or
downwind, side of the cold pool. In that case, use of the MBE vectors in
BUFKIT to estimate storm motion will be seriously misleading. An option
is available in BUFKIT to estimate the storm motion for a forward
propagating storm. The figure below shows the storm motion estimate from
BUFKIT, after using the forward propagating option. Note that the
estimate is now 75 kts toward the northeast, quite a difference from the
3 kts shown on the previous figure! Note also that the indicated
movement toward the northeast was still not a very good estimate of
system movement, as the system in this case moved almost due east. The
reason for the more eastward movement of the system in this case may
have been that the synoptic-scale forcing for upward motion was moving
toward the east.

![](media/image15.png){width="5.997222222222222in"
height="4.5055555555555555in"}

*BUFKIT sounding at BGM from the 18/09 run of the Eta, verifying at 04
UTC, 3/10. The MBE vector calculation in this figure is now accounting
for a forward propagating storm.*

Clearly, this event developed as a well-organized squall line, or serial
derecho, with embedded bow echoes, in the presence of strong forcing.
The question then becomes, how well did the environment with this event
match environments typically associated with strongly forced derechos,
as specified in the introduction of this study?

The next two figures show BUFKIT forecast soundings, with hodographs, at
BGM verifying at 11 pm, on March 9^th^. In the first figure, the
hodograph shown is for the lowest 2 km of the atmosphere. Note the
strong shear (43 m/s) indicated in the lowest 2 km. The second figure
shows a hodograph for the lowest 6 km of the atmosphere. Note that most
of the shear with this event was in the lowest 2 km, as the hodograph
becomes much shorter and disorganized above 2 km. Recall that this may
be favorable for weak storm relative inflow in the mid-levels of the
storm, which would then be favorable for outflow-dominated storms. Note
also that BUFKIT is estimating a storm relative inflow for this case of
23 m/s at the surface, and just 14 m/s at 6 km. Storms with strong storm
relative inflow at low-levels and weak storm relative flow at mid-levels
are typically outflow dominated, forward propagating storms. That would
be the typical storm expected with a derecho.

![](media/image16.png){width="5.997222222222222in"
height="4.5055555555555555in"}

*BUFKIT forecast sounding for BGM from the 18/09 UTC Eta, verifying at
04 UTC, March 10^th^. Hodograph is from the surface through 2 km.*

![](media/image17.png){width="5.997222222222222in"
height="4.5055555555555555in"}

*BUFKIT forecast sounding for BGM from the 18/09 UTC Eta, verifying at
04 UTC, March 10^th^. Hodograph is from the surface through 6 km.*

SUMMARY

The severe convective squall line on the evening of March 9^th^ can be
classified as a classic strongly-forced serial derecho. The strong
forcing was supplied by a pronounced mid and upper level trough, and its
associated surface cold front. The large-scale winds associated with
this event were produced by a combination of the strong pressure
gradient associated with the front, and a strong westerly isallobaric
wind component associated with a pronounced fall-rise pressure couplet
along the front. Wind damage occurred when more localized, damaging
gusts were produced by convection associated with numerous fast-moving
bow echoes along the squall line. The convection was able to tap the
very strong wind fields located just above the ground, mixing those
winds down to the surface. In addition, large amounts of dry air were
advected into the back edge of the squall line by the large-scale wind
field, which could have led to enhanced evaporation in the downdrafts of
the storm, and enhanced winds. Some dry air was also evident in the
low-levels of the environment, which would also have acted to increase
cooling and wind production associated with thunderstorm downdrafts.

The environment associated with this event was typical of environments
associated with strongly forced derechos. The event was associated with
very strong wind shear, with the majority of the shear located in the
lowest 2 km of the atmosphere. The instability was weak. The
environmental storm-relative inflow was strong in the low-levels of the
storm and weak in the mid-levels, resulting in a forward propagating,
outflow dominated convective system. The result was widespread
convective straight-line wind damage, but no tornado reports.