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A Case Study of the August 25, 2007 Severe Weather Event in Eastern New York and Western New England
Brian J. FrugisNWS WFO Albany, New York
Brief Introduction of Event 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 (3pm-9pm EDT)
Surface Setup Tropical air surged
northward along the coast 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 Massive amounts of
CAPE (nearly 5000 J/kg) Dry layer between 700-
500 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 19Z 850-500 mb level lapse
rates shown here LaPenta & Maglaras (2002)
found 800-600 mb level best to look at, but unavailable for analysis
Values not overly impressive 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) DCAPE is an estimate of
kinetic energy available to a downdraft parcel due to negative buoyancy
Impressive amounts indicated damaging wind potential 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
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 First severe t-
storm warning of the day issued for Herkimer County at 1904Z (304 PM EDT) Developed in a
pocket of higher CAPE values as depicted on LAPS
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 Storm continued
eastward through Adirondacks 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 Moved through
unpopulated area of forest and lakes
No verification from this area
Storm #1 - Adirondacks 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 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 4 Panel reflectivity
from 2041Z (441 PM EDT) 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 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
Storm developing over Helderbergs in western Albany County around 2147Z (537 PM EDT) 4 panel image
from Binghamton (BGM) radar
Storm had a few pixels of ~50 dBZ in mid-levels, but considered marginal for warning
Storm #2: Capital Region/Taconics/Berkshires
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
Widespread tree and wire damage across the town of Bethlehem in extreme eastern Albany County at 2216Z (616 PM EDT) Less than a minute
lead time Storm continued to
maintain strength as it headed across Rensselaer County Warning issued for
Berkshire, Mass. at 2253Z (653 PM EDT)
Storm #2: Capital Region/Taconics/Berkshires
0.5 radial velocity image taken at 2306Z (706 PM EDT) 45 kts pixel right
over KAQW (North Adams, Mass.) at 4000 feet AGL
Wind gust at KAQW to 70 MPH (60 Knots) at 708 PM
Storm #2: Capital Region/Taconics/Berkshires
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 Thunderstorms developed and were prolonged due to
extreme instability and high moisture Lots of CAPE can go a long way, despite modest shear Both storms examined were long-lived, despite not having
supercell structures
LAPS data useful in determining best areas of explosive thunderstorm development 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
18Z RAOB observation was very usefulSounding was representative of mesoscale
environmentActual convective parameters measured
helped determine severe threat was realMore useful than modifying 12Z sounding
since it accurately depicts if the cap is definitely broken
Acknowledgments
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
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