A nearly stationary frontal zone was located just to the north of the area over far southern Ontario, Canada. A weak wave of low pressure tied to a cluster of thunderstorms moved east along this stalled frontal zone during the late morning and early afternoon. Thunderstorms strengthened across southern Ontario during the late morning. These storms further intensified as they moved into Western NY during the early afternoon. The southernmost storm of the cluster quickly evolved into a long lived cyclic supercell, going through numerous cycles of weakening and strengthening. This storm was responsible for all four tornadoes across Western NY.
The storms moved onshore from Lake Erie with damage beginning in Hamburg before moving across Orchard Park. Windows of hundreds of car windows were blown out at the Hamburg Fairgrounds where trees were downed and several buildings including the Grandstand sustained damage. The tornado continued across the Chestnut Ridge Park and the Town of Orchard Park causing more structural and tree damage.
About ten miles further down along the thunderstorm's path, a second tornado touched down in the Town of Holland. Several structures were damaged with a significant amount of tree damage. Roads were blocked and wires were downed. Tracks of the Norfolk Southern Railway were blocked in several places by large downed trees.
The storm briefly re-intensified near Rushford spinning up a weak tornado over a mainly wooded area. A small shed was destroyed and drone video shows a path of downed and twisted trees.
Finally, based on radar and spotter reports two thunderstorms merged and intensified and produced a tornado that extended from the Town of Allen across the Town of Angelica to the Town of West Almond. Several homes were damaged. A barn was severely damaged. Several sheds or small outbuildings were tipped and moved. Numerous large trees were shredded and downed. Some fell causing additional structural damage.
These thunderstorms later evolved into a line, producing a corridor of wind damage extending through the Southern Tier of New York.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 12:35:00 to 20-JUL-17 12:35:00 (EST-5)
Location: HOUGHTON
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds on Council House Road.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 12:42:00 to 20-JUL-17 12:42:00 (EST-5)
Location: HOUGHTON
Magnitude: 50.00 EG
Property damage: 12.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds on Holiday Road.

Thunderstorm Wind in CATTARAUGUS, NEW YORK
From 20-JUL-17 13:02:00 to 20-JUL-17 13:02:00 (EST-5)
Location: RICEVILLE
Magnitude: 50.00 EG
Property damage: 12.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds at Beach Tree Road and Highway 240.

Thunderstorm Wind in CATTARAUGUS, NEW YORK
From 20-JUL-17 13:04:00 to 20-JUL-17 13:04:00 (EST-5)
Location: YORKSHIRE
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds on Creek Road.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 13:35:00 to 20-JUL-17 13:35:00 (EST-5)
Location: SHORT TRACT
Magnitude: 50.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 13:47:00 to 20-JUL-17 13:47:00 (EST-5)
Location: ANGELICA
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported thunderstorm winds knocked down a tree onto a car.

Tornado in ALLEGANY, NEW YORK
From 20-JUL-17 12:29:00 to 20-JUL-17 12:30:00 (EST-5)
Location: HOUGHTON COLLEGE ARP
Tornado F-scale: EF0
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Drone video provided by the county emergency manager confirmed an EF0 tornado touched down near Rushford.

Tornado in ERIE, NEW YORK
From 20-JUL-17 11:55:00 to 20-JUL-17 11:56:00 (EST-5)
Location: SOUTH WALES
Tornado F-scale: EF0
Property damage: 100.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

A NWS storm survey confirmed an EF1 tornado touched down in Holland.

Tornado in ALLEGANY, NEW YORK
From 20-JUL-17 12:45:00 to 20-JUL-17 12:52:00 (EST-5)
Location: ANGELICA
Tornado F-scale: EF1
Property damage: 250.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

A NWS storm survey confirmed an EF1 tornado touched down in Angelica.

Thunderstorm Wind in NIAGARA, NEW YORK
From 20-JUL-17 10:50:00 to 20-JUL-17 10:50:00 (EST-5)
Location: NEWFANE HOLLANDS ARP
Magnitude: 50.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds.

Thunderstorm Wind in ERIE, NEW YORK
From 20-JUL-17 11:28:00 to 20-JUL-17 11:28:00 (EST-5)
Location: MT VERNON
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Law enforcement reported trees and wires down by thunderstorm winds near St. Mary's on the Lake Church.

Thunderstorm Wind in LIVINGSTON, NEW YORK
From 20-JUL-17 12:24:00 to 20-JUL-17 12:24:00 (EST-5)
Location: LIMA
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 12:27:00 to 20-JUL-17 12:27:00 (EST-5)
Location: HUME
Magnitude: 50.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds on Mills Road.

Thunderstorm Wind in ALLEGANY, NEW YORK
From 20-JUL-17 12:35:00 to 20-JUL-17 12:35:00 (EST-5)
Location: CANEADEA
Magnitude: 50.00 EG
Property damage: 12.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reported trees and wires down by thunderstorm winds.

Tornado in ERIE, NEW YORK
From 20-JUL-17 11:30:00 to 20-JUL-17 11:40:00 (EST-5)
Location: LOCKSLEY PARK
Tornado F-scale: EF2
Property damage: 3.50K
Crop damage: 0.00K
Reported by: NWS Storm Survey

A NWS storm survey confirmed an EF2 tornado touched down in Hamburg.