On July 21st, a developing surface low across the Great Lakes traveled along a stationary boundary draped across the North Country. Surface conditions became increasingly unstable during the afternoon with temperatures in the 80s and dewpoints in the 60s and lower 70s. More importantly, an unseasonably strong mid-atmospheric shortwave and winds aloft tracked across this region as well, which allowed for thunderstorms to develop rapidly, intensify and maintain longevity.
During the afternoon, scattered to numerous thunderstorms developed and/or traveled across northern New York. Several storms strengthened into supercells that produced widespread wind damage to trees, power poles and structures as well as large hail upwards to golf ball size in diameter.
Some of the communities largely affected were Potsdam, Ogdensburg, Bangor, Brushton and Plattsburgh.

Thunderstorm Wind in ESSEX, NEW YORK
From 21-JUL-10 14:30:00 to 21-JUL-10 14:30:00 (EST-5)
Location: LAKE PLACID
Magnitude: 50.00 EG
Property damage: 20.00K
Crop damage: 0.00K
Reported by: Newspaper

Several large limbs and a few trees downed by thunderstorm winds along Oneida, Grandview and Wesvalley roads.

Heavy Rain in CLINTON, NEW YORK
From 21-JUL-10 13:58:00 to 21-JUL-10 17:00:00 (EST-5)
Location: PLATTSBURGH
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Heavy rain fell in the city of Plattsburgh NY. There were reports of widespread street flooding as storm drains were overwhelmed by the runoff.

Thunderstorm Wind in CLINTON, NEW YORK
From 21-JUL-10 12:40:00 to 21-JUL-10 12:40:00 (EST-5)
Location: LYON MTN
Magnitude: 60.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Trees down throughout the town and along Route 374 toward Merrill.

Thunderstorm Wind in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:40:00 to 21-JUL-10 14:45:00 (EST-5)
Location: POTSDAM
Magnitude: 55.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Numerous trees and power lines reported down in Potsdam as well as neighboring communities. Some isolated structural damage from fallen trees reported as well.

Thunderstorm Wind in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:45:00 to 21-JUL-10 14:45:00 (EST-5)
Location: WADDINGTON
Magnitude: 55.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Powerlines and trees down along Route 37 as well as County Road 28 Waddington, Lisbon and Ogdensburg.

Thunderstorm Wind in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:45:00 to 21-JUL-10 14:50:00 (EST-5)
Location: OGDENSBURG
Magnitude: 55.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Trees and Powerlines down throughout Ogdensburg and Route 37 closed to Waddington.

Hail in ST. LAWRENCE, NEW YORK
From 21-JUL-10 15:15:00 to 21-JUL-10 15:15:00 (EST-5)
Location: FT JACKSON
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Reports of hail upwards to quarter size in diameter reported.

Thunderstorm Wind in FRANKLIN, NEW YORK
From 21-JUL-10 12:35:00 to 21-JUL-10 12:35:00 (EST-5)
Location: BANNERHOUSE
Magnitude: 60.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Trees and power lines downed by thunderstorm winds on Blanch road and other roads near Upper Chateguay Lake. In addition half inch diameter hail reported.

Thunderstorm Wind in FRANKLIN, NEW YORK
From 21-JUL-10 12:00:00 to 21-JUL-10 12:05:00 (EST-5)
Location: BRUSHTON
Magnitude: 75.00 EG
Property damage: 200.00K
Crop damage: 50.00K
Reported by: NWS Storm Survey

Roof torn off working garage/shed in West Bangor. Numerous (hundreds) trees uprooted, snapped across Franklin county between Moria and Malone with greatest concentration in a 2-3 mile swath across Brushton, West Bangor with at least a dozen structures that sustained damage. NWS Storm survey suggests combination of straight-line winds and possible microburst with winds estimated at 85 mph.

Thunderstorm Wind in FRANKLIN, NEW YORK
From 21-JUL-10 11:55:00 to 21-JUL-10 12:00:00 (EST-5)
Location: MOIRA
Magnitude: 60.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: COOP Observer

Numerous trees down in Moira and east along Routes 11 and 11B toward Brushton, Bangor and Malone.

Hail in FRANKLIN, NEW YORK
From 21-JUL-10 12:05:00 to 21-JUL-10 12:10:00 (EST-5)
Location: BRUSHTON
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 25.00K
Reported by: Trained Spotter

Numerous reports of half to one inch size diameter hail falling for five minutes or longer that accumulated several inches but drifted to one or two feet in spots near storm drains.
Corn crops as well as some other agricultural crops were affected at peak harvest.

Hail in FRANKLIN, NEW YORK
From 21-JUL-10 12:15:00 to 21-JUL-10 12:15:00 (EST-5)
Location: MALONE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Dime to nickel size hail reported in Malone, which lasted several minutes.

Hail in CLINTON, NEW YORK
From 21-JUL-10 13:08:00 to 21-JUL-10 13:08:00 (EST-5)
Location: CADYVILLE
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Dime size hail reported.

Hail in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:40:00 to 21-JUL-10 14:45:00 (EST-5)
Location: POTSDAM
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 25.00K
Reported by: Public

Estimated Golf ball size hail reported, as well as numerous reports of large hail ranging from nickel, quarter and half dollar size throughout Potsdam and immediate vicinity.

Hail in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:48:00 to 21-JUL-10 14:50:00 (EST-5)
Location: OGDENSBURG
Magnitude: 1.25 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several reports of nickel to quarter to slightly larger size diameter hail reported in and near Ogdensburg.

Thunderstorm Wind in CLINTON, NEW YORK
From 21-JUL-10 12:55:00 to 21-JUL-10 13:00:00 (EST-5)
Location: LEDGER CORNERS
Magnitude: 60.00 EG
Property damage: 35.00K
Crop damage: 0.00K
Reported by: Public

Numerous trees and power lines down across Dannemora as well as along Routes 374 and 26 headed toward Merrill and Plattsburgh respectively.

Thunderstorm Wind in ST. LAWRENCE, NEW YORK
From 21-JUL-10 14:50:00 to 21-JUL-10 14:50:00 (EST-5)
Location: LISBON
Magnitude: 55.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Powerlines and trees down in Lisbon and along County Road 28 toward Waddington and  Ogdensburg.

Thunderstorm Wind in CLINTON, NEW YORK
From 21-JUL-10 13:10:00 to 21-JUL-10 13:15:00 (EST-5)
Location: PLATTSBURGH
Magnitude: 55.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Public

Numerous trees reported down throughout Plattsburgh as well as power lines. Very heavy rainfall caused some localized street flooding in poor drainage areas.