Transcript A Case Study of the August 25, 2007 Severe Weather Event in Eastern New York and Western New England Brian J.

A Case Study of the August 25,
2007 Severe Weather Event in
Eastern New York and Western
New England
Brian J. Frugis
NWS WFO Albany, New York
Brief Introduction of Event
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17 confirmed
individual severe
weather events
One of the largest
events of the
summer of 2007
for ALY CWA
Storms generally
occurred between
19Z-01Z (3pm9pm EDT)
Surface Setup
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Tropical air surged
northward along
the coast
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Temps 32-35°C
(lower to mid 90s
°F)
Dewpoints 23-25
°C (mid 70s °F)!
Frontal boundary
over eastern
Great Lakes

Pre-frontal trough
was the focus for
convection
Special 18Z Sounding
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Massive amounts of
CAPE (nearly 5000 J/kg)
Dry layer between 700500 mb
Very moist low levels
Modest wind shear
0-6 km shear: 30 kts
0-3 km helicity: 116 m²/s²
Freezing level rather high
 Located at 15.5 kft
Convective Available Potential
Energy (CAPE)

Impressive surface
based CAPEs up and
down Hudson Valley
 19Z LAPS analysis
shows values over
4000 J/kg common
 Previous ALY study
by LaPenta &
Maglaras (2002) of
“major” severe
weather events (10+
reports) had a mean
CAPE of 2272 J/kg

Double the
average of other
major severe
events in the ALY
CWA
Helicity

18Z LAPS 0-1 km
analysis shows not
overly impressive
amounts
 Values
generally
80-150 m²/s²
 Indicated tornadic
threat was limited
Lifted Index (LIs)

19Z analysis shows
very low values of -6 to
-8°C across most areas
(exception was
northern Adirondacks)
 Lowest values in
mid Hudson Valley
would suggest best
area for explosive
convective
development if
storms can be
triggered
 Previous ALY case
studies by LaPenta
& Maglaras (1995)
showed “major”
severe events to
have a mean of -6
°C.
Mid-Level Lapse Rates
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19Z 850-500 mb level lapse
rates shown here
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Values not overly impressive
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LaPenta & Maglaras (2002)
found 800-600 mb level best
to look at, but unavailable
for analysis
Between 5.85 and 6.5°C
Generally values above 6.5
°C considered good for
severe weather according to
SPC
Showed convectively
unstable in most areas,
except extreme north
Values were higher in
morning, decreased
throughout the afternoon
Downdraft CAPE (DCAPE)
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DCAPE is an estimate of
kinetic energy available to
a downdraft parcel due to
negative buoyancy
Impressive amounts
indicated damaging wind
potential
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Lapenta & Maglaras (2002)
case study showed mean
value of major severe
events studied was 353
J/kg
Highest values of 900 J/kg
found in Hudson and
Mohawk Valleys at 18Z
Severe Thunderstorm Watch
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Mesoscale discussion
issued at 1630Z
(1230 PM EDT)
concerning watch
potential
Actual watch issued
at 1755Z (155pm
EDT), valid until
0100Z 8/26 (9 PM
EDT)
Storm #1 - Adirondacks
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First severe tstorm warning of
the day issued for
Herkimer County
at 1904Z (304 PM
EDT)
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Developed in a
pocket of higher
CAPE values as
depicted on
LAPS
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Around 3000
J/kg
VIL values up to
70 kg/m²
 Produced
significant tree
damage in
towns of Ohio
and Russia in
Herkimer
County
Storm # 1 - Adirondacks
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Storm continued
eastward through
Adirondacks
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Photo of damage from
town of Ohio in
Herkimer County
Warning for Hamilton
and northwestern
Fulton counties issued
at 322 PM and 342 PM
respectively
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Moved through
unpopulated area of
forest and lakes
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No verification from
this area
Storm #1 - Adirondacks
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Storm continued
into northern
Saratoga and
Warren counties
towards area of
CAPE values over
4000 J/kg
Nickel sized hail at
Hadley, Saratoga
County at 2030Z
(430 PM EDT)
 Only severe
hail report of
the day
 VIL values
over 60
kg/m²
Storm #1 - Adirondacks
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Storm reaches
the Lake
George &
Glens Falls
area
 VIL
image
taken at 2041Z
(441 PM EDT)
 Values
approaching 70
kg/m² on
eastern shore
of Lake George
Storm # 1 - Adirondacks
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4 Panel reflectivity
from 2041Z (441
PM EDT)
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55 dBZ up to
35,000 feet
Extensive wind
damage
countywide in
Warren County
13,000 people
were without
power at one point
Many trees and
power lines down
across roads near
Lake George
Elevated core
passed over
Glens Falls
 Little damage
within the City
of Glens Falls
Storm # 1 – Adirondacks
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Core finally falls
across northern
Washington
County
0.5 radial velocity
image taken at
2058Z
Extensive tree
and wire damage
in towns of
Kingsbury, Fort
Ann and
Whitehall
Storm #2: Capital
Region/Taconics/Berkshires
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Storm developing
over Helderbergs
in western
Albany County
around 2147Z
(537 PM EDT)
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4 panel image
from
Binghamton
(BGM) radar
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Storm had a
few pixels of
~50 dBZ in
mid-levels,
but
considered
marginal for
warning
Storm #2: Capital
Region/Taconics/Berkshires
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Storm was headed
towards the axis of
very high CAPE
values
 Values at 21Z
(last available
time during
reanalysis) were
still over 4000
J/kg in the
Hudson Valley
Decided to warn
based on the track
of storm towards
very unstable air
even though dBZ
was marginal
Warning for Albany
and Rensselaer
counties issued at
2216Z (616 PM
EDT)
Storm #2: Capital
Region/Taconics/Berkshires
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Widespread tree and
wire damage across
the town of
Bethlehem in
extreme eastern
Albany County at
2216Z (616 PM
EDT)
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Less than a minute
lead time
Storm continued to
maintain strength as
it headed across
Rensselaer County
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Warning issued for
Berkshire, Mass. at
2253Z (653 PM
EDT)
Storm #2: Capital
Region/Taconics/Berkshires
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0.5 radial velocity
image taken at
2306Z (706 PM
EDT)
 45
kts pixel right
over KAQW (North
Adams, Mass.) at
4000 feet AGL
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Wind gust at
KAQW to 70 MPH
(60 Knots) at 708
PM
Storm #2: Capital
Region/Taconics/Berkshires
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Pea and marble
size hail
reported at
North Adams as
well
Storm
continued
across northern
Berkshire
County
Trees down in
Savoy, Mass. at
2340Z (740 PM
EDT)
Conclusions of Study
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Thunderstorms developed and were prolonged due to
extreme instability and high moisture
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LAPS data useful in determining best areas of
explosive thunderstorm development
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Lots of CAPE can go a long way, despite modest shear
Both storms examined were long-lived, despite not having
supercell structures
Bethlehem wind damage was an example of this
Lead time can be improved by warning on marginal storms if
they are headed towards areas of extremely high CAPE
Anomalously large amount of DCAPE was supportive of
wind damage potential
Conclusions of Study
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18Z RAOB observation was very useful
 Sounding
was representative of mesoscale
environment
 Actual convective parameters measured
helped determine severe threat was real
 More useful than modifying 12Z sounding
since it accurately depicts if the cap is
definitely broken
Acknowledgments
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Much thanks to ALY
forecasters Tom
Wasula, Neil Stuart,
and Joe Villani for
their help with this
study!
Questions?
Mammatus Clouds over
Dolgeville, New York on
August 25, 2007, courtesy
of Matt Lanza, WKTV-Utica