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