26-27 December 2012 snowstorm

An intense low pressure system developed over Texas by Christmas morning. This system moved eastward during the day into the Deep South resulting in a widespread severe weather outbreak that extended from eastern Texas across Louisiana, Mississippi and Alabama. A large number tornadoes occurred with over 2 dozen people reported injured.

This system moved northward into the Mississippi Valley Christmas night with signs of secondary development occurring along the southeast coast Wednesday morning as the northern and southern streams began to phase. By Wednesday evening the coastal low became the dominant low along the mid-Atlantic coast. Snow began to overspread the region during the mid to late afternoon hours.

This system was vertically stacked by Thursday morning. The surface low moved off the New Jersey coast early Thursday morning and tracked over eastern Long Island and southeastern New England/Cape Cod during the late morning into the afternoon. This track is favorable for heavy snow across the ALY CWA. Snowfall totals ranged from just under 3 inches to over 2 feet. By 11 pm, mixed precipitation was reported at Poughkeepsie with freezing rain being reported during the overnight hours. A quarter of an inch of ice accretion was reported in Hyde Park in Dutchess County. Sleet mixed in with snow only briefly at times overnight here at the office.

Despite the spread in the plumes, both the GEFS and SREF were consistent indicating high QPF amounts of an inch or greater with this storm. There were two main concerns: how much mixing/changeover would occurred and would the low-level easterly flow result in downsloping effecting the precipitation amounts and intensities in portions of the Hudson River Valley eastward into the western slopes of the Taconics, Berkshires and southern Green Mountains. The NAM was consistently the warmer of the models. For p-type used a blend of the NAM and GFS which seem to work out OK.

The "outlier" warmest thermal profiles of the NAM were ignored on the day shift prior to the event. A blend of the GFS/ECMWF was used for the thermal profiles on the 26 DEC 2012. These colder profiles advocated more snow and frozen precipitation, and less mix, especially from the Capital Region north and west. The NAM had ridiculous snow map output files that had little over no snow in the Hudson River Valley (including most or all of the tri cities). It is advocated to NOT look at the snow charts with a lot of confidence. The NAM seemed to overcompensate for the downsloping, which did occur, but still snow accumulations were respectable and just missed 24-hr warning criteria over counties such as Rensselaer, Columbia and Washington. (TAW)

MODEL TRENDS:

As early as Thursday December 20th, 00Z models were hinting at a significant storm potentially impacting the region for late Wednesday December 26th into Thursday December 27th. However, track differences were present, with the 00Z/20 ECMWF indicating a more western track, implying a possible mixed precipitation event changing to heavy rain for much of the CWA, while the 00Z/20 GFS and GEFS hinted at a track farther east, and hence colder, implying more of a significant snow and/or mixed precipitation threat for most of the CWA. By December 21st, both the 00Z/21 GFS and ECMWF hinted at a colder solution, tracking the potential storm along the east coast rather than up the Appalachians to the west. The 12Z/21 ECMWF and GFS, along with the GEFS, UKMET and JMA continued to suggest an increasing potential for significant snowfall, although the 12Z/GGEM tracked the storm much farther east and offshore, reducing potential impacts to the region. By Saturday December 22nd, the main difference among medium range guidance was the speed, rather than track of the potential storm, with the 00Z/22 GFS indicating a faster track, while the 00Z/22 ECMWF and GGEM hinted at a slower track, implying even heavier snowfall potential for the region. Track differences once again developed with the 00Z/23 model suite, with the 00Z/23 GFS hinting at a track close to Albany as it tracked north northeast, implying a potential transition of snow to mixed precipitation or plain rain for at least central and southern portions of the CWA, while the 00Z/23 ECMWF and GGEM remained consistently colder and continued to imply more of a significant snow threat for a large portion of the area. Forecasters, noting the excellent performance of the ECMWF, trended toward a colder solution with the extended portion of the forecast, which proved correct.

By Monday December 24th, 00Z/24 models began to narrow down the potential path and evolution of the storm system, suggesting that an initial surface low track from the western Gulf Coast region into the Tennessee Valley region for Wednesday afternoon, before redeveloping east of the Appalachians, somewhere near or just northeast of Washington D.C. This new low center was then expected to track northeastward, to near or just east of NYC Thursday morning, to near Cape Cod by late Thursday. As noted in the AFD issued at 355 AM EST December 24th, this potential track would favor a potentially moderate to heavy snowfall for at least areas north and west of the immediate Capital Region, particularly the southern Adirondacks, Mohawk Valley, Lake George/Saratoga Region, and southern VT, with the possibility of less snowfall further south and east due to a possible warm nose intrusion within the 900-800 mb layer. The 00Z/24 ECMWF remained the coldest and farthest southeast of tracks amongst the models, along with the 00Z/24 GEFS. It should be noted that the 00Z/24 GFS remained farther north and west of all the models at this point, and actually was among the farthest northwest among its Ensemble members, with the GEFS mean surface low track very close to the ECMWF track. It should also be noted that the GEFS indicated an easterly component of 5-6 SD, strongly supporting the potential for a heavy QPF event.

Models remained fairly similar on Tuesday December 25th in favoring widespread winter storm for much of the region, although the 00Z/25 NAM was indicating a slightly warmer solution. In addition, significant downsloping and precipitation shadowing was depicted in the 00Z/25 NAM12 due to the strong low level easterly flow. In fact, the "total snowfall accumulation" graphic from the NAM12 depicted sub-advisory level snowfall accumulations for much of the Hudson River Valley region, which proved incorrect.

By Wednesday morning December 26th, support continued amongst the 00Z/26 models for a major winter storm across much of the region. The only possible caveats would be the possibility of a dry intrusion above 700 mb from the south, as hinted at by the 00Z/26 GFS, and also the continued suggestion of strong downsloping and precipitation shadowing within the 00Z/26 NAM 12. Forecasters continued to favor a colder solution as implied by a combination of the 00Z/26 ECMWF and GFS, and therefore, in collaboration with surrounding offices, winter storm warnings were expanded farther south and east to include the entire Capital Region, central Taconics, and Berkshires. Also, there was some concern that strong easterly winds within the 925-850 mb layer, which were forecast to reach 50-60 KT, strongest as depicted by the 00Z/26 NAM, could potentially interact with the terrain and mix down along the west slopes of the southern Greens, Taconics, Berkshires, and Litchfield Hills. A high wind watch was consequently issued, since confidence levels were not quite sufficient for a warning at this time.

The guidance that came in at 12Z and 18Z 26 DEC 2012 increased forecaster confidence for the cancellation of the High Wind Watch. It was decided to include wind advisory levels wind gusts over portions of the forecast area, where gusts of 46-57 mph were possible. These winds were used in the WSW's. I do not recall any wind issues, except one rogue gusts around 05Z-06Z at KDDH. This was one the core of the low-level jet was moving across the region. The gust was advisory level.

Above:  18Z 27 December GEFS Mean Sea Level Pressure.

Above:  850 hPa temperatures from (left to right), the SREF, GFS and NAM.

Above:  Loops of 850 hPa winds from the (left to right) 09Z 25 December SREF and 21Z 25 December SREF.

Above:  Loops of 850 hPa winds from the (left to right) 00Z 25 December GEFS, 00Z 26 December GEFS.

Above:  850 hPa winds from the (left to right) 12Z 27 December GFS, 12Z 27 December NAM.

Above:  Loops of probability of 1 inch liquid equivalent precipitation in 24 hours from (left to right) GEFS and SREF.

Above:  Forecasted 250 hPa winds from the (left to right) GEFS and SREF.

Above:  Looping plume forecasts from the SREF for (left to right) Albany, NY, Watertown, NY, Binghamton, NY, Bradley Field, CT, and Monticello, NY.

Above:  Radar loop from IEM of the snow storm.

Above:  Snowfall plot from Google Earth.