Overview of Dec. 11-12, 2002 Snow and Ice Event

A mixed bag of precipitation developed across central New York and northeastern Pennsylvania during the late morning and early afternoon hours of the 11^th. Precipitation took the form of sleet and freezing rain at the onset. Freezing precipitation began to mix with and change over to snow at times from late afternoon through the evening hours. Some rain was even reported through early evening in valley locales and more urbanized places like Wilkes-Barre/Scranton and Syracuse. A transition to all snow occurred during the overnight hours and into the early morning of the 12^th.

The main focus of this discussion will be on the late afternoon and evening period of the 11^th across northeastern Pennsylvania and the Catskills when and where the heaviest precipitation fell. 24 hour liquid equivalent totals from 12z on the 11^th to 12z on the 12^th were commonly over an inch across this region, with a narrow band of greater than 1.5 inches. A stripe of 6-10” snowfall totals also accompanied the heaviest liquid equivalent amounts.

A deep, nearly isothermal layer near 0 degrees Celsius made precipitation type quite variant as outlined above and difficult to pinpoint at any given time. The Albany, NY (ALB) sounding from 00z, 12/12 illustrates this.

Figure 1 – ALB Sounding from 00Z, 12/12.

Radar imagery (displayed in Figures 2 through 5) indicated that heavy bands of precipitation rotated across much of northeastern Pennsylvania and into the Catskills during the evening hours. It is theorized that dynamic cooling associated with these heavier bands was just enough to tip the scales towards snow and result in the higher accumulation totals outlined above.

Figure 2 – Radar composite image for 0100Z, 12/12. Note the east to west oriented band with enhanced returns that developed from the southern Poconos across the lower Hudson Valley and into southern New England. Subsequent radar images shown below depict further intensification of the banded feature as it lifted slowly northward through the Poconos and western Catskills later that evening.

Figure 3 (Above) – Radar composite image for 0200Z, 12/12.

Figure 4 (Below) – Radar composite image for 0300Z, 12/12.

Figure 5 – Radar composite image for 0400Z, 12/12.

Several factors contributed to an environment conducive to large scale lift. First, a well developed short wave trough in the mid-levels pivoted across eastern Pennsylvania and ultimately into southern New England as the following figures show.

Figure 6 – ETA 500 mb heights and vorticity for 18Z, 12/11.

Figure 7 – ETA 500 mb heights and vorticity for 00Z, 12/12.

Secondly, mid and upper level jet configurations placed northeastern Pennsylvania underneath a diffluent wind pattern. This fostered a deep layer of upward motion.

Figure 8 – ETA 300 mb isotachs for 00Z, 12/12.

A coupled jet structure was evident at upper levels, as shown above. Northeastern Pennsylvania was within the entrance circulation of the northern speed maximum over the Canadian Maritimes and near the exit region of the southern maximum across the Carolinas. Thirdly, a pronounced conveyor belt of moisture was pointed towards the Poconos at 850 mb during the evening hours with convergent flow at this layer.

Figure 9 – ETA depiction of 850 mb winds and temperatures at 00Z, 12/12.

Within the broader area of synoptic scale forcing, atmospheric parameters appeared to favor the development of smaller scale, more intense banded features across the Catskills and Poconos, particularly between 00Z and 06Z on the 12^th.

Figure 10 – ETA model cross section from Newark, NJ (EWR) to Rochester, NY (ROC)

Of 2-D Frontogenesis (light blue) and Equivalent potential vorticity (EPV) (purple) at 00Z, 12/12.

As depicted in Fig. 10, a sloped front was present roughly between 850 and 800 mb over the Poconos and Catskills. The frontogenetical circulation appeared to be enhanced across this region by both an unstable layer and divergence directly above at mid-levels.

The next figure adds divergence to the fields shown in Fig. 10.

Figure 11 – ETA cross section from EWR to ROC with frontogenesis (purple), divergence (light blue), and EPV (black).

Figure 12 – Same ETA cross section with geostrophic momentum surfaces (purple) and equivalent potential temperatures (black) at 00Z, 12/12. The steeper slope of the theta contours further suggests the presence of instability above 600 mb over northeast PA.

By looking at Fig. 11 in particular, one could envision a situation where a nice convergence/divergence couplet in roughly the 700-600 mb layer above the elevated front would accelerate parcels upward towards the zone of negative EPV starting near 550 mb. This would, in turn, release the potential instability and help produce localized and intense banded structures. A look back at Fig. 1 also showed the instability aloft with fairly steep lapse rates from about 550 mb upward.

Observed precipitation and snowfall totals (Figures 13 and 14), as well as radar imagery (some displayed above in Figures 2-5), showed that fairly intense bands did propagate across sections of northeastern Pennsylvania and the Catskills for a period during the evening of the 11^th. The main axis stretched from eastern Luzerne county across central and northern Lackawanna county, northern Wayne county, and then across the higher terrain of northern Sullivan and southern Delaware counties.

Figure 13 – Event snowfall totals for 12/11 and 12/12.

Figure 14 – Liquid Equivalent totals for 12/11 and 12/12.

Later that night into the early morning of the 12^th, precipitation type became mainly snow as colder air wrapped into the system. However, precipitation intensity also slackened off as the best forcing mechanisms translated towards the Hudson Valley and New England. The last two figures (15 and 16) show coastal transformation taking place at

the surface, helping to cool the lower levels. At the same time, though, the short wave is ready to kick out into southern New England, with NVA forming locally.

Figure 15 – ETA MSLP depiction at 06Z, 12/12.

Figure 16 – ETA 500 mb heights and vorticity for 06Z, 12/12.