Remembering the December 11-12, 2008 Ice Storm

Major Ice Storm, Thursday-Friday December 11-12, 2008

The atmosphere unleashed an onslaught of ice on the Catskills, Capital Region, mid Hudson valley, and portions of Bennington and Berkshire counties as well as much of the rest of central and northern Massachusetts, southern New Hampshire and coastal Maine, resulting in massive damage to trees and power lines resulting in an estimated 1.5 million utility customers in the Northeast being plunged into a cold darkness which would last longer than a week in some of the hardest hit communities. States of emergency were declared throughout Schenectady, Rensselaer, Greene, and Columbia counties with states of emergency also declared in the towns of Stillwater Clifton Park, Rotterdam, Guilderland, Halfmoon, Schodack, Troy, Schenectady, Glenville, Nassau, Coeymans, Cohoes, Berne, and Bethlehem, as well as for the entire state of Massachusetts. Storm total rainfall amounts ranging from 2" to 3.5" on average with locally higher amounts in the Catskills, mid Hudson valley to Berkshire and Litchfield counties caused widespread urban and poor drainage flooding, which also affected the Capital Region. Minor to in some places moderate river flooding also occurred along the Housatonic and Hoosic rivers as well as the Walloomsac @ Bennington, VT, and along the Battenkill at Arlington, VT with small streams and creeks out of their banks throughout Greene, Columbia, Ulster, Dutchess, Washington, Berkshire, and Litchfield counties during the height of the storm. It was a storm that will be talked about for years and used as a benchmark for comparisons to future storms of the same type due to the widespread devastation to trees and the power infrastructure which occurred from the 1/2" to 1.25" of icing (radial measurements.)

Set-Up
Mid fifty degree weather early on Wednesday December 10 came to an end with the passage of a cold front which stalled south of the region by the late afternoon. A shallow cold air mass gradually seeped into the Northeast on a light surface flow from the north setting the stage for the ice storm on Thursday December 11. The flow at the jet stream level, originating in the Gulf of Mexico and blowing from southwest to northeast up and over New York and New England, was instrumental in holding a deeper cold air mass, one that would have produced a snow storm, over the Midwest and far northern New England. The jet orientation also allowed a strong upper air low pressure system to plunge from the Plain's states into the northern Gulf of Mexico, a track that allowed snow to fall in New Orleans the morning of Thursday December 11, but also a track that favored the formation of a huge moisture channel which when caught in the steering currents flowed north up the spine of the Appalachians into New York and New England. A significant tropical connection out of the Atlantic also developed ahead of the storm which flowed up the eastern seaboard pouring additional moisture into the local region resulting in excessive amounts of precipitation that when falling as rain onto subfreezing surfaces froze on contact resulting in the damaging ice storm for a narrow zone that extended from the Catskills to the coast of Maine.

Ice Storm Zone Thursday Night Into Friday Morning December 11-12, 2008

Storm total combined snow and sleet accumulations reported by the CBS6 WeatherNet 6 spotters as well as National Weather Service Cooperative Observers for the December 11-12, 2008 Storm

Snow, Sleet, and Freezing Rain - How These Types of Precipitation Form

Snow: Snow falls, in general, when the atmosphere from the top down is below freezing. Precipitation forms as snow and falls to the ground as snow. There are cases when a very shallow layer of above freezing air near the ground can exist but is of insufficient depth to melt the snow before it hits the ground.

Sleet: Sleet occurs when a fairly narrow wedge of above freezing air develops in the mid levels of the atmosphere. This mid level warming often occurs when a storm's circulation draws mild air northward aloft. In this case precipitation forms as snow, falls into the above freezing layer and melts into rain. The rain then freezes into ice pellets (sleet) before making it down to the ground.

Freezing Rain: Freezing rain occurs when the mid level warm layer is fairly deep, leaving only a very shallow layer of sub-freezing air near the ground. In this case precipitation forms as snow then melts into rain as it falls through the above freezing layer. The rain, however, has insufficient time to freeze into ice pellets before making it down to the ground and therefore freezes on contact with surfaces causing glazing. In the case of an ICE STORM, a protracted period of freezing rain results in large accumulations of ice, greater than 1/2", which adds so much weight to trees and power lines that they eventually fail, resulting in significant damage.

Storm TimeLine
Thursday Morning
Areas of light snow fell throughout the Mohawk valley to Saratoga Springs into the Capital Region southern Vermont and northern Berkshire County with light freezing drizzle or freezing rain in the Catskills, mid Hudson valley and from central and southern Berkshire County on south in New England. Nuisance type snowfall accumulations through 2pm generally ranged from a coating to 2". This precipitation resulted from an overrunning flow over the Northeast as the strong SW to NE jet stream flow propelled warm moist air up and over the cold air hugging the boundary layer of the atmosphere.

Thursday Afternoon and Evening
Precipitation intensity steadily increased through the afternoon and the evening as mid level warming was simultaneously occurring along and south of the Mohawk river to the Capital Region to southern Bennington County on south as the main surface storm began moving up the Appalachian mountain chain, but at this time was still located over eastern Tennessee. Moderate snow fell in the Mohawk valley and southern Adirondacks, to Saratoga Springs to northern Bennington County on north where combined accumulations of snow and sleet ranged from 2"-5" by midnight. A mixture of sleet and freezing rain fell in the Capital Region with moderate to heavy freezing rain in the Catskills through the mid Hudson valley and portions of Berkshire County. (Downsloping ENE winds in western Berkshire and Bennington counties benefited some lower elevation communities such as Pittsfield, MA, North Adams, MA, and Bennington, VT as the warming associated with the sinking air nudged temperatures to just above freezing, mitigating some of the icing in those specific locations. Higher elevations in the Berkshires as well as throughout the Capital Region, mid Hudson valley, and Catskills, however, remained colder with temperatures ranging from 28 to 31 degrees, due to a subtle cold air northerly drainage flow originating from a moderately cold high pressure system located across southern Canada.

Thursday Night-Friday Morning
The low pressure center tracked from eastern Tennessee to just south of New England during this period, pushing an enormous amount of moisture through the Northeast resulting in continuous moderate to at times heavy freezing rain from Schenectady, southern Saratoga, southern Washington, and southern Bennington counties on south. Temperature profiles were such that a sleet, freezing rain mixture remained fairly steady state in the Mohawk valley to Saratoga Springs and Manchester, VT on north, with mostly snow in the Adirondacks. The scale tipping in favor of sleet in the Mohawk valley to Saratoga Springs spared those regions damaging accumulations of ice as was the case in the Adirondacks where the air remained cold enough through a deep enough layer in the atmosphere to support predominantly snow through the event where storm total accumulations generally ranged from six to eight inches with up to twelve inches in northern Herkimer and Hamilton counties.

By 11pm Thursday night, an average of 1/2" of ice had accumulated on elevated surfaces in the ice storm zone pushing tree limbs and power lines to the breaking point with the power outage numbers climbing from an estimated 14,000 local National Grid customers out at 11pm to approximately 200,000 by daybreak. It was a night filled with the sound of cracking and snapping as tree limbs broke away, crashing down to the ground. It was a night of eerie blue flashes as thousands of overloaded transformers blew plunging much of the region into darkness.

Friday
The steady freezing rain tapered down to a freezing drizzle by daybreak Friday with the storm pulling out in a quick flurry of snow during the mid to late morning. The ending snow burst left anywhere from a coating to 2" on top of ice accumulations which ranged from 1/2" to 1" on average. Gusty winds followed the storm through the afternoon causing additional trees and power lines to come crashing down and a subsequent jump in the number of local utility customers without electricity to approximately 250,000.

This Table Lists the Reported WeatherNet 6 and National Weather Service Cooperative Observer Storm Total Ice Accumulations for the December 11-12, 2008 Storm (Many of the reports are estimates as ice accumulations are often difficult to measure...significant tree damage begins with radial ice accumulations of 1/2" with severe tree damage as accumulations approach 3/4")

Ice Storm Photographs From Around The Region