7-8 December Challenging RainSnow

This was a challenging event in terms of forecasting when rain would change to snow due to a complex thermal profile and uncertainty as to when colder air would filter in. Forecasts for snowfall in the Hudson Valley including the Capital Region were overdone due to most of the precip falling as rain vs. snow. Some higher elevation areas did receive a plowable snowfall, as elevation was an important factor. Also, there was a max snowfall in portions of the Taconics, where mesoscale banding was significant.

A cold front slowly sagged southward across the forecast area during the afternoon and evening of 7 December. A weak wave of low pressure developed along the front across the Southern Appalachians and Mid-Atlantic region and tracked northeastward. A negatively tilting shortwave trough spawned cyclogenesis as it tracked just north of the Delmarva. The deepening cyclone produced significant bands of precip, with strong frontogenesis developing associated with the frontal boundary. A low-level southerly flow brought significant moisture into the region as well.

Model guidance performed poorly with regards to forecasting too much snow accumulation in the valleys. Operational guidance as well as Ensemble Plumes depicted rain changing to snow at Albany much faster than what actually occurred. The colder air gradually sagged southward across the area, while also descending in elevation rather slowly as well.

The heaviest QPF fell south and east of Albany, with some river flooding occurring in the CT portion of the Housatonic River basin. Elsewhere, snowfall ranged from around an inch in the Capital Region, to as much as 4 to 8 inches across portions of the Eastern Catskills, Northern Taconics and Bekshires, as well as Southern VT.

Click on images to enlarge and/or loop.

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Above: Total snowfall reports from local PNS. Note the absence of snowfall in the Capital Region.

 

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Above: Loop of 925mb temps from the NAM and GFS show how gradual the cooling process was. Note the 0C line does not reach Albany until 03Z Dec 8th.

 

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Above: MAV and MET MOS forecasts for KALB. Note the 0 forecast for the snowfall category. MOS equations latched on to warm boundary layer. This example shows how MOS snow forecasts should not necessarily be dismissed.

 

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Above: NAM Bufkit shows rain changing to snow too quickly at both ALB and GFL. The result was snow accumulations that were well overdone. The changeover occurred later than expected, but even when it began to snow in the Capital Region the ground was too warm and wet for any accumulation to occur in the first hour or two.

 

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Above: Operational models were in good agreement showing the heaviest QPF south and east of Albany. Minor flooding occurred along the Housatonic Basin in CT at Stevenson Dam, Gaylordsville, and Brookfield.

 

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Above: Loop of GEFS plumes indicated way too much snow at ALB, especially closer to the event time, which is unsettling. Loop of SREF plumes show similar trends, but at least had rain lasting longer before the changeover to snow occurred. Still, too much snow forecast by both the GEFS and SREF.

 

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Above: At 850 mb wind dominated by anomalous v (southerly) component.

 

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Above: Four days preceding the event models depicted only weak low pressure developing along the Mid Atlantic coast. Within the days 2-3 time frame a deeper cyclone was predicted and positioned farther northeast. The ensembles latched on to a much deeper cyclone as the event drew closer.

 

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Above: The cyclone center tracked just east of Long Island and eventually over Cape Cod. Deepened from 996 mb along New Jersey shore at 02Z on 08 December, to 985 mb by 10Z on 08 December southeast of Bangor, ME.

 

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Above: Radar reflectivity indicated moderate to heavy precip moving into the area during the evening of 07 December. The high reflectivity values of 45-50+ dBZ indicate the changeover to snow.

 

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Above: Big surge of 50+ dBZ echoes from the Capital Region south and east during the late evening. This was when the changeover to snow finally occurred.

 

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Above: Heavy precip moved south and east towards the Taconics, Berkshires, and Litchfield Hills right before midnight.

 

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Above: Moderate to occasionally heavy north-south oriented snow bands set up east of the Capital Region, which produced the 4-8 snowfall amounts.

 

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Above: Steeply sloped and deep layer frontogenesis aided in formation of the intense mesoscale banding.

 

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Above: Water vapor imagery shows a strong upper level jet core punching northward into Long Island and Southern New England overnight with convection around 0730-0800Z on 08 December. Winds gusted over 60 mph with this line of convection over Long Island.