18-20 December 2009 East Coast Blizzard

(Click on images for larger display)

 

This historical storm produced a widespread 18+ snowfall from Virginia, through the mid-Atlantic region into Long Island, NY and extreme southeastern New England. Major metropolitan areas from Washington, D.C. through Baltimore, Philadelphia, Delmarva to NJ and central/eastern Long Island received the most snow. There were predictability issues with the precipitation transition zone south of the main snow axis, and with the northern extent of the precipitation shield. There was considerable cold and dry air north of the storm, and there were questions whether the heavier snow would extend from New York City to interior southern New England and points north and west. New York City to Boston did receive near 12 of snow, but the amounts rapidly diminished 20-40 miles north of the major metropolitan areas.

 

The signals in the long term guidance prior to 17 December were unclear, with large spreads seen in the ensembles and in the deterministic operations runs. This high level of uncertainty was most prevalent for the mid-Atlantic and northeastern U.S. Once the storm developed and began to track through and off the southeastern U.S. coast, there was much less spread in the guidance, but still enough to result in low confidence and short lead time watches and warnings for areas along the northern periphery of the precipitation shield.

 

Low level U wind anomalies were dampened out by the large spread in ensemble guidance in long range forecasts, but consistently exceeded -4 SD and at times exceeded -5 SD during the entire event. Low level U wind anomalies in the deterministic NAM and GFS exceeded -6 SD since there were no parameters to dampen out the values. Upper level U wind anomalies did not exceed -2 SD in the ensembles, suggesting a relatively progressive closed upper low, not necessarily a cut-off upper low. Upper level U wind anomalies peaked between -2SD and -2.5 SD in the deterministic NAM and GFS, but for the most part did not reach -2.5 SD, again, implying a relatively progressive closed upper low, but not cut off.

 

Boundary layer frontogenesis was extreme, seen in the thermal gradient at 850 hPa and 925 hPa, which implied unusually strong low level forcing and mesoscale band potential. Agestrophic wind flow at 925 hPa was north to west, implying that cold and dry air would be anchored to the north and the northern edge of the precipitation field would be very sharp. Successive runs of the ensembles and deterministic models showed significant shifting of the northern edge of the precipitation shield with each successive run, adding to the high levels of uncertainty.

 

The aspects of the storm that were relatively certain, were the maximum snowfall amounts where all snow would fall, in the region of best low level forcing and upper dynamics. The low level wind anomalies and frontogenesis suggested widespread 18+ within the region of maximum snowfall. There was relatively little uncertainty about where the location of the band of maximum snowfall would fall based on the regions of peak U wind anomalies and probabilities for 1.00 liquid equivalent in the ensembles. It should be noted that the SREF and GEFS showed their respective biases, and areal extent of probabilities for heavy precipitation were reduced, as is typical, due to resolution of individual ensemble members and consensus from all ensemble members. It has been found subjectively that regions of precipitation often extend beyond the areas outlined in ensemble probabilities for various precipitation values.

 

The following graphics are grouped by guidance source, and groups are separated by bold blue text headlines. Click on images for larger images. Labels for images are black text below the images. Click here for e-mail discussions from the UAlbany MAP and NE CSTAR lists.

 

Long Range Guidance 15 December through 17 December model runs

 

 

Above: 500 hPa heights from successive runs of the GFS (upper left), ECMWF (upper right), GGEM (lower left) and GFSEnsemble (lower right).

 

 

Above: 500 hPa heights from successive runs of the NAM80 (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

 

 

Above: Mean sea level pressure (MSLP) from successive runs of the GFS (upper left), ECMWF (upper right), GGEM (lower left) and GFSEnsemble (lower right).

 

 

Above: Mean sea level pressure (MSLP) from successive runs of the NAM80 (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

 

 

Above: 850 hPa temperatures from successive runs of the GFS (upper left), ECMWF (upper right), GGEM (lower left) and GFSEnsemble (lower right).

 

 

Above: 850 hPa temperatures from the NAM80 (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

 

 

Above: 925 hPa ageostrophic winds from the 12Z December 18 NAM80 (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

 

 

Above: Liquid equivalent precipitation from successive runs of the GFS (upper left), ECMWF (upper right), GGEM (lower left) and GFSEnsemble probability of 1 inch (lower right).

 

 

Above: Liquid equivalent precipitation from successive runs of the NAM80 (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

 

 

Above: GFSEnsemble successive runs of 500 hPa heights (upper left), probability of 1 inch (upper right), MSLP (lower left) and 850 hPa temperatures (lower right).

 

BUFKIT NAM selected precipitation forecasts from 00Z 18 to 06Z 19 December for Poughkeepsie, NY (KPOU) and New Canaan, CT (CAN)

 

 

BUFKIT GFS precipitation forecasts from 06Z 18 and 00Z 19 December for Poughkeepsie, NY (KPOU) and New Canaan, CT (CAN)

 

 

GFS Loops

 

 

Above: GFS MSLP and precipitation loops from successive runs illustrating the large spatial shifts in each set of guidance.

 

Above: GFS 850 hPa wind anomaly loops from 3 consecutive runs.

 

 

Above: GFS 250 hPa winds.

 

GEFS Loops

 

 

Above: 850 hPa wind anomalies for multiple runs 12z 17 December through 00Z 19 December.

 

 

Above: 250 hPa wind anomalies for multiple runs 12z 17 December through 00Z 19 December.

 

 

Above: Probability for 1 inch liquid equivalent in 24 hours for multiple runs 12z 17 December through 00Z 19 December.

 

 

Above: Probability for 2 inches liquid equivalent in 36 hours from 00z and 12Z 18 December guidance.

 

ECMWF Loops

 

 

Above: MSLP and precipitation for successive runs illustrating the spatial shifting with each run.

 

 

NAM loops

 

 

Above: NAM80 MSLP and precipitation loops for successive runs note the spatial shifting with each run, also note the greater detail from the NAM12 during intermediate 06Z and 18Z runs.

 

 

Above: NAM 850 hPa wind anomalies from successive runs.

 

 

Above: NAM 250 hPa wind anomalies from successive runs.

 

 

SREF Loops

 

 

Above: Successive runs of 500 hPa heights (left), MSLP (middle), and precipitation (right). Note the spatial shifting with each run.

 

 

Above: Winds and anomalies at 850 hPa for selected runs of the SREF 15Z December 17 through 15Z 19 December.

 

 

Above: Probability for 1 inch liquid equivalent precipitation in 24 hours for selected runs of the SREF 15Z December 17 through 15Z 19 December.

 

 

Above: Probability for 2 inches liquid equivalent precipitation in 36 hours for selected runs of the SREF on 18 December.

 

Water vapor loops

 

 

Above: Each loop is roughly every 12 hours, with the first loop beginning at 00Z 12/18.

 

Visible loops

 

 

Above: Loops during daylight on 12/19 and 12/20.

 

Radar Loops

 

 

Above: First 2 from OKX each in 3 hour segments beginning at 0300 UTC and 0600 UTC and the last from BOX beginning at 0900 UTC.

 

Preliminary NESIS rank

 

 

Above: Preliminary rank of category 3 on the NESIS scale.