Friday May 5, 2017
Moderate to Heavy Rain Event
Late Afternoon Localized Damaging Wind Event - Hudson Valley and Western New England
(Gravity Wave Induced)

Large uproot in East Greenbush, Rensselaer County, Friday afternoon May 5, 2017

Uprooted tree in East Greenbush, Friday afternoon May 5, 2017

What was a fairly routine rain event for the region ended with a short period of strong damaging wind gusts in pockets across Columbia, Rensselaer, Washington, Rutland, Bennington, and Berkshires counties between roughly 3:30 pm and 5 pm. Gusts from 45-55 mph on average occurred in localized areas which effectively downed numerous trees, with those trees a little easier to be blown down than typical due to the very wet ground conditions from the 1/2" to 1" of rain which had fallen through the day. The apparent initiation of the strong wind gusts was the passage of an atmospheric gravity wave.

Zone where pockets of strong damaging wind gusts occurred, Friday May 5, 2017

AN ATMOSPHERIC GRAVITY WAVE is generated by a region of rapidly rising air forced up by a front, low pressure system or even by terrain. At some point the rising motion of the air is abruptly halted due to changes in atmospheric stability which in turn causes the air to then rapidly sink forming an up and down motion in the atmosphere over a short horizontal distance, much like a wave on a body of water. The wave, comprised of rising air on the forward side and sinking air on the downward side, causes a rapid change in air pressure from low to high which can generate strong surface winds if the gravity wave is strong enough to effect surface conditions, which in most cases they are not. In this case, however, it was with pressure traces over western New England indicating a surface pressure rise and fall couplet. Additionally, radar images support the idea of a gravity wave as the data shows a zone of heavy rain (the yellows areas in the images below) which indicated strongly upward moving air, followed quickly by a sharp back edge with no rain, indicating rapidly sinking air, with the periods of strong wind gusts corresponding very closely to the passage of the back edge of the rain in the region of descending air.

Albany Doppler radar base reflectivity image at approximately 3:30pm Friday May 5, 2017. Strong wind gusts were just beginning in Columbia County on the back edge of the heavy rain zone, in an area of strongly sinking air on the west side of the passing gravity wave.

Albany Doppler base reflectivity radar image at approximately 3:30pm Friday May 5, 2017 showing the back edge of the heavy rain/gravity wave crossing the mid Hudson valley into western New England 

Albany Doppler radar reflectivity image at approximately 4:30pm Friday May 5, 2017. Pockets of strong wind gusts had translated north across Rensselaer, Washington, Berkshire and Bennington counties as the gravity wave moved quickly to the Northeast with a rapid shut down of the rain in the area of sinking air.

Albany Doppler base reflectivity radar image at approximately 4:30pm showing the heavy rain zone/gravity wave moving om the process of clearing Vermont and Berkshire County, MA

The gravity wave is believed to have been the overall trigger to the wind event. But, the scattered nature to where strong gusts occurred tends to suggest that the local terrain had some influence in where damaging gusts occurred and where they did not. (Note: Pittsfield, MA, near to areas where strong wind occurred had a peak gust of only 25 mph.) And an additional important player in the set-up included pre-existing strong low level easterly winds a couple of thousand feet off the ground blowing in a region just above a temperature inversion which was situated just above the ridge tops. These easterlies were caused by the circulation around the storm which produced the day's rain, and strong high pressure off shore.

So, what likely occurred was this...the passing gravity wave, with its zone of strongly sinking air on its west side, effectively interacted with what was likely a standing wave over the mountains causing it to amplify which effectively pushed the pre-existing strong easterly winds down through the inversion toward the ground. In areas where that fast flow was descending, cuts in the local terrain likely acted to channel and further accelerate the wind, aided by some downsloping, resulting in the scattered and localized bursts of damaging wind gusts which occurred in especially Columbia, Rensselaer, parts of southern Washington, Bennington, Rutland, and northern Berkshire counties.

Had the gravity wave not occurred, the easterlies would have remained aloft, above the inversion, allowing surface winds to remain light, which had characterized the winds throughout the region through the duration of the rain event during the day. So the event was quite unique in that it took the combination of factors, the gravity wave, the pre-existing strong easterlies a couple of thousand feet off the ground, and the local terrain channeling effects through the southern Green mountains, the Berkshires, and the Taconics, to create the short duration period of 45-55 mph gusts (locally stronger bursts of 60+ mph) responsible for many downed trees and power lines and up to 23,000 electrical customers in the Hudson valley without power at the peak.

It's important to note that predictability on gravity wave induced surface weather events is zero. The wave only becomes apparent as the effects are unfolding and the pressure rise and fall couplets are measured along with winds, in cases when wind is an issue, which is only sometimes. Sometimes you will hear about large gravity waves during big winter storms when in extreme cases, the sinking air motion side of the wave can be so strong that it can completely dry up a storm's snow shield greatly reducing the amount of snow which falls vs. the original expectation.

NWS Damage and Wind Gusts Reports: Note, these reports do not necessarily represent an accounting for all of the wind damage which may have occurred, only what was reported to the Albany NWS.

Town County Wind Damage and Gust Reports Time
Ashland Greene Downed wires 2:30pm
Claverack Columbia Tree blown down onto a house and car 3:40pm
Stockport Columbia Tree blown down onto an occupied vehicle 3:40pm
Claverack Columbia Trees and wires blown down 3:40pm
Livingston Columbia Tree blown down 3:40pm
Castleton-on-Hudson Rensselaer Trees blown down 3:40pm
Hudson Columbia Trees and wires blown down on Green Street 3:40pm
Greenport Center Columbia Trees and wires down on Fairview Ave. and on Route 9 3:40pm
Schodack Center Columbia Two trees blown down 3:40pm
East Greenbush Rensselaer Several trees blown down 3:45pm
Chatham Columbia Tree blown down on Route 32 3:45pm
Kinderhook Columbia Tree blown down on Route 21 3:45pm
Claverack Columbia Tree blown down on SR 23 3:45pm
North Adams, MA Berkshire Power outages due to downed trees and wires 3:50pm
Cambridge Washington Multiple trees blown down 3:50pm
Bennington, VT Bennington Trees blown down 3:50pm
East Greenbush Rensselaer Trees and power lines blown down 4:00pm
West Sand Lake Rensselaer Approximately 30 trees blown down, power lines down in a region from Johnsonville to West Sand Lake to South Schodack 4:15pm
Cambridge Washington Tree blown down 4:20pm
Salem Washington Several trees blown down in the Salem area starting around 4:20pm 4:20pm
East Schodack Rensselaer Several small trees blown down 4:37pm
East Hebron Washington Multiple trees blown down 5:00pm
3 Miles NNW of Dorset, VT Bennington Large tree limbs and branches down blocking Danby Mountain Road 5:04pm
Old Bennington, VT Bennington Measured 68 mph wind gust 4:45pm
Cornwall Dutchess Measured 55 mph wind gust 3:43pm
5 Miles ESE of Waterford Rensselaer Measured 56 mph wind gust 4:15pm
Wynantsill Rensselaer Measure sd 54 mph wind gust 4:02pm
Bennington, VT Bennington Measured 53 mph wind gust 3:22pm
Hoosick Falls Rensselaer Measured 49 mph wind gust 4:05pm
Castelon-on-Hudson Rensselaer Measured 46 mph wind gust 4:05pm
North Adams, MA Berkshire Measured 47 mph wind gust 4:46pm
Copake Columbia Measured 47 mph wind gust 4:05pm

WeatherNet 6 Rainfall Reports - Friday May 5, 2017 - Reports are valid through 11pm and do not include additional light rainfall which fell in some areas into the morning of the 6th

Rainfall Report
Rainfall Report
Savoy, MA Berkshire 1.25" Pittsfield, MA Berkshire 1.43"
Cohoes Albany 0.80"      
Livingston Columbia 0.95"      
Arkville Delaware 0.52" Roxbury Delaware 0.25"
Red Hook Dutchess 0.92"      
Broadalbin Fulton 1,30" Perth Fulton 1.26"
Fish House Fulton 1.10"      
Halcott Greene 0.43" Cairo Greene 0.90"
Catskill Greene 0.87"      
Wells Hamilton 1.31"      
Florida Montgomery 0.83" Amsterdam Montgomery 1.02"
Fonda Montgomery 1.12" Glen Montgomery 0.93"
Oneonta Otsego 0.60" to 0.82" East Worcester Otsego 0.70"
Worcester Otsego 0.70"      
Speigletown Rensselaer 1.00" Center Brunswick Rensselaer 0.62"
Saratoga Springs Saratoga 0.87" Lake Desolation Saratoga 0.87"
Malta Saratoga 0.73" Milton Saratoga 0.91"
Stillwater Saratoga 0.91" Corinth Saratoga 0.58"
Clifton Park (Oaks) Saratoga 0.98"      
Glenville Schenectady 0.86" Rotterdam Schenectady 0.89"
Delanson Schenectady 1.14"      
Jefferson Schoharie 0.35"      
Whiteport Ulster 0.94" Saugerties Ulster 0.98"
Esopus Ulster 1.05"      
Warrensburg Warren 0.90"      
Manchester, VT Bennington 0.73" Danby, VT Rutland 0.64"
West Rutland, VT Rutland 0.62"